Power Plate Research and Studies
Influence of vibration on delayed onset of muscle soreness after eccentric exercise
Power Plate Studies
The first detailed description of delayed onset muscle soreness (DOMS) was given by Hough in 1902.1 DOMS is often precipitated predominantly by eccentric exercise, such as downhill running, plyometrics and resistance training. It has been explained2 3 that the injury itself is a result of eccentric exercise causing damage to the muscle cell membrane, which sets off an inflammatory response. In other words, shocking the muscles during the eccentric range ofmotion is probably the leading factor in producing DOMS. It has been claimed that the type of force development during eccentric exercise may cause sarcoma disruption and consequently an inflammatory response within the muscle.4–6 DOMS is typically experienced by all individuals regardless of fitness level, and is a normal physiological response to increased exertion and the introduction of unfamiliar physical activities.7 The pain and discomfort associated with DOMS typically peaks 24–48 hours after an exercise bout, and resolves within 96 hours.8 Generally, an increased perception of soreness occurs with greater intensity and a higher degree of unfamiliar activities.9 Other factors that play a role in DOMS are muscle stiffness, contraction velocity, fatigue, and angle of contraction.2 Due to the sensation of pain and discomfort, which can impair physical training and performance, prevention and treatment of DOMS is of great concern to coaches, trainers and therapists.10 Although DOMS is experienced widely, science has not established a sound and consistent treatment for it.

Recent research has shown that vibration training (VT) may improve muscle performance.11 12 By considering this mechanism, we designed this study to find out if VT before eccentric exercise may prevent DOMS by improving muscular strength and power development strategy,13–17 improving kinesthetic awareness,18 and providing insights into the effects of fatigue, 19 20 within the vibrated muscles.

MATERIALS AND METHODS
 
The study was approved by the ethical committee of Semnan University of Medical Sciences. Fifty healthy non-athletic volunteers (25 females, mean (SD) age 21.1 (0.2) years and 25 males mean (SD) age 20.1 (0.5) years) were assigned randomly into two experimental, VT and non-VT groups. Exclusion criteria included a history of cardiac and neuromuscular diseases, undertaking severe sport activity or having received an intramuscular injection during the last week.
 
A computer generated randomisation list was drawn up by the statistician for each group. It was given to the physiotherapy department in sealed numbered envelopes. When the subjects qualified to enter the study and had signed their informed consent forms, the appropriate numbered envelope was opened at the reception; the card inside indicated the subject’s allocation to one of the VT or non-VT groups. This information was then given to the physiotherapist to administer the appropriate intervention.
 
Intervention
 
Both experimental groups walked downhill on a 10° declined treadmill at a speed of 4 km per hour for 30 min. In the VT group, a 50 Hz vibrator apparatus (model VR-7N, ITO, Tokyo, Japan) was used to apply vibration on the middle line of each of the left and right quadriceps, hamstring and calf muscles for 1 min before downhill treadmill walking, while the subjects in the non-VT group did not receive any vibration before downhill treadmill walking.
 
Measurements
 
Measurements were performed before and 24 h after treadmill walking and included isometric maximum voluntary contraction. Figure 1 Mean change of maximum isometric voluntary contraction force of quadricepsThe IMVC force of the left and right quadriceps muscles in 100°of knee flexion was measured in the sitting position. The subject was asked to sit on a back-supported quadriceps table and a load cell was connected to the distal end of her/his leg by means of a tight sling. The output of the load cell was connected to a digital monitor so it was possible to record and save the maximum tension on the load cell. After bringing to zero the output of the load cell, the subject was encouraged to perform IMVC by pulling the sling tight as hard as she/he could, three times, with 30-second intervals between each pull. The best attempt was recorded and considered as the quadriceps IMVC force in newtons.
 
Pressure pain threshold (PPT) on the 5, 10 and 15 cm above the left and right patellae and also on the middle line of calf muscles was measured by a 20 ml syringe with a spring inside which was scaled from 0 to 10. The rounded tip of syringe was placed at the above points in a vertical position and the piston was pressed down. The subject was asked to announce any unpleasant sensation (pain), and then the indicating number on the syringe was recorded as the PPT.
 
The level of muscle soreness was evaluated by mean of a Visual Analogue Scale (VAS). The day after treadmill walking, the subject was asked to indicate her/his feel of the level of muscle soreness in each lower limb along a 10 cm line ranging from 0 (‘‘no muscle soreness at all’’) to 10 (‘‘the most severe muscle soreness that I can imagine).
 
The serum level of creatine kinase (CK) was measured 24 hours after treadmill walking by taking 3 cc blood samples from the brachial artery in the front of the elbow and then the level of CK enzyme was measured in the laboratory.
 
Statistics
 
To compare the possible effect of VT on DOMS, an intention to treat analysis was used which involved all subjects who were randomly assigned to their group. Student’s t tests were used to compare the mean changes in the IMVC force, PPT values and the mean of CK level and muscle soreness between the experimental groups.
 
RESULTS
 
Fifty healthy subjects were randomly assigned into two experimental VT (n=25, 12 male and 13 female) and non-VT (n=25, 13 male and 12 female,) groups and the study was then completed. The mean age was 20.6 (1.9) years (mean (SD)) for the VT group, and 20.6 (2.1) years (mean (SD)) for non-VT group, without any significant differences between the two groups.
 
Isometric maximum voluntary contraction force
 
A comparison of the mean change in the IMVC force in the right quadriceps showed a significantly higher decrease (P=0.001) in the non-VT group (-39.6 (46.6) years, mean (SD)) compared with the VT group (37.2 (100.1) years, mean (SD)). This reduction was also seen in the left quadriceps (non- VT group -16.5 (77.6) years vs VT group 56.8 (100.9) years, P=0.006), fig 1.

Pain pressure threshold
 
Table 1 shows the mean changes in PPT at 5, 10 and 15 cm above the right and left patella. Comparison of these values from non-VT and VT groups showed significant reduction of PPT in the non-VT group (P=0.0001). The same significant reduction (P=0.0001) of PPT was seen in the calf muscles of the non-VT group (right -1.1 (1.3) and left -1.3 (1.4)) compared with the VT group (right 0.4 (0.8) and left 0.4 (1.2)).
 
Level of muscle soreness
 
The comparison of mean level of muscle soreness recorded the day after treadmill walking showed higher soreness in the non- VT group (right 2.3 (1.9) and left 2.3 (2.1)) vs VT group (right 0.4 (1.1) and left 0.5 (1.1)). These differences were significant in both lower limbs (P=0.0001), fig 2.
 
Level of CK enzyme
 
The higher mean of the CK enzyme was found in the non-VT group (195.2 (109.2)) compared with the VT group (116.1 (27.8)) which was statistically significant (P=0.001), fig 3.

 
Figure 2 Mean change of delayed onset muscle soreness at 24 h after eccentric exercises.

 
Mean changes in the pain pressure threshold of the right and left quadriceps at 5, 10 and 15 cm above the patella
DISCUSSION
 
Delayed onset muscle soreness and reduced muscle strength after eccentric exercise5 21 may decrease functional activities in athletics.7 22 Because of this, different methods have been investigated and recommended to prevent these symptoms.2 This study was designed to find the possible effects of VT to control and prevent DOMS after eccentric exercise.
 
A previous study showed muscle strength reductionFigure 3 Mean serum level of creatine kinase enzyme at 24 h after eccentric exercise. after eccentric activities,9 while our findings showed no muscle strength reduction in the IMVC force of quadriceps in the VT group, which may be due to the establishment of optimum neuromuscular function in the quadriceps muscles by applying VT. This has been reported by other researchers,12 15 who showed better muscle performance after vibratory stimulation. Thompson and Belanger (2002) also showed that VT may increase muscle spindle activities and establish motor unit activity synchronisation that may optimise neuromuscular function.23 By contrast, it has been shown that muscle spindle stimulation by vibration may increase the afferent activities of muscle spindles which may increase background tension in the vibrated muscles.24 25 This increased background tension and motor unit activity synchronisation in the vibrated muscle may prevent sarcoma disruption or damage to excitation–contraction coupling, which may happen due to tension development during eccentric exercise.4 Therefore, this optimised muscle performance may control and prevent muscle damage and so reduce DOMS. This reduction in DOMS was seen in our study, as we found increased PPT in the right and left quadriceps and calf muscles, lower muscle soreness, and lower levels of CK enzyme in the VT group compared with the non-VT group.
 
The CK enzyme has been defined as an index for muscle damage and its level will be increased within 24 to 48 hours after eccentric activities,5 22 which is a sign of eccentric muscle damage. However this increase was seen only in the non-VT group, and not in the VT group. In fact, the lower CK level in the VT group may indicate lower muscle damage in this group, while the non-VT group showed a higher CK level and so higher muscle damage, which was accompanied by higher muscle soreness.
 
These findings may indicate that vibration training before eccentric exercise may help the muscles to build up a background tension and optimal neuromuscular activity to overcome the increased passive tension within the exercised muscles during eccentric activities. Thus, vibration training could be used before eccentric activities to control and prevent delayed onset muscle soreness and it might be a useful method for athletes to prevent any DOMS in their sports activities.
 
CONCLUSION
 
DOMS is a major complication faced by athletes after eccentric activities, which may compel them to postpone their sports activities, thus prevention and treatment of DOMS is of great concern to coaches, trainers, and therapists. In this study, we investigated the effect of vibration on muscle before downhill treadmill walking and our results showed that applying vibration before eccentric activities may prevent DOMS and so it may help non-athletic people to follow and complete their sport activities without any delay. Further studies are needed to investigate these results to find the possible application in athletics.
 
 
The acute effect of whole-body vibration on the hoffmann reflex
Power Plate Studies
ABSTRACT
 
The extent to which motoneuron pool excitability, as measured by the Hoffmann reflex (H-reflex), is affected by an acute bout of whole-body vibration (WBV) was recorded in 19 college-aged subjects (8 male and 11 female; mean age 19 ± 1 years) after tibial nerve stimulation. H/M recruitment curves were mapped for the soleus muscle by increasing stimulus intensity in 0.2- to 1.0-volt increments with 10-second rest intervals between stimuli, until the maximal M-wave and H-reflex were obtained. After determination of Hmax and Mmax, the intensity necessary to generate an H-reflex approximately 30% ofMmax (mean 31.5%6 4.1%) was determined and used for all subsequent measurements. Fatigue was then induced by 1 minute of WBV at 40 Hz and low amplitude (2–4 mm). Successive measurements of the H-reflex were recorded at the test intensity every 30 seconds for 30 minutes post fatigue. All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV; however, four distinct recovery patterns were observed among the participants (œ = 0.50). There were no significant differences between genders across time (P = 0.401). The differences observed in this study cannot be explained by level or type training. One plausible interpretation of these data is that the multiple patterns of recovery may display variation of muscle fiber content among subjects. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response, and the effect of WBV on specific performance measures should be interpreted with the understanding that there may be considerable variability among individuals. Recovery times and sample size should be adjusted accordingly.
 
KEY WORDS H-reflex, potentiation
 
INTRODUCTION
 
Whole-body vibration (WBV) is being examined for use in rehabilitation and sport training (6,7,9,23,30–32). Investigators have noted improvements in response to WBV training in such physiological measures as neuromuscular performance (2), force output (10,12,22,33), flexibility (36), and hormone concentrations (2). Not all investigators, however, have noted positive benefits (9,13,14,30), particularly in the short term. Rittweger and co-workers (31) examined the effects of squatting with and without vibration (26 Hz). They concluded that WBV may enhance neuromuscular excitability. This was measured, however, using the patellar reflex, and the duration of the effect was not determined. Still, little is known about the exact physiological adaptations responsible for these changes or what is the most appropriate combination of frequency (of vibration) and amplitude.
 
The Hoffmann reflex (H-reflex) is widely established as a measure of motoneuron excitability (18,20,21,28,29,34). The H-reflex is analogous to the spinal stretch reflex induced via electrical stimulation, thus bypassing the muscle spindle. Progressive stimulation of a percutaneous mixed nerve, such as the tibial nerve, reveals the H-reflex, which appears first on the electromyographic trace (EMG) as the threshold of the Ia afferents is attained, and is followed by the muscle response (M-wave) as the œ-motoneurons (œMNs) reach threshold. The peak-to-peak amplitude of the H-reflex increases until maximum and subsequently diminishes. As more aMNs are recruited, the M-wave increases until it plateaus at Mmax. The disappearance of the H-reflex as the M-wave approaches Mmax results from the collision of the reflexive action potential traveling down the motoneuron (orthodromic) with the action potential traveling up the motoneuron toward the spinal cord (antidromic), which blocks the H-reflex. For a review of the H-reflex and its application to sports medicine, the reader is referred to Palmieri et al. (29).
 
The H-reflex may be normalized using a stimulus intensity that produces a percentage of Mmax (29). Theoretically, the H-reflex represents a given percentage of the total motoneuron (MN) pool (i.e., a peak-to-peak amplitude equal to 30% of Mmax represents 30% of the MN pool). This method permits an investigator to measure changes in theMN pool after some intervention. Thus, changes in the peak-to-peak amplitude of the H-wave after a bout ofWBVmay be used to determine the extent to which WBV may affect neuromuscular activity and indicate the degree to which fatigue, recovery, and, possibly, potentiation of the reflex response occur.
 
METHODS
 
Experimental Approach to the Problem
 
The present study is the first in a series of studies involving WBVconducted in our laboratory. Exploratory in design, this study was designed to determine the time course of changes in the H-reflex after a single 1-minute bout of WBV at a moderate frequency (40 Hz) and low amplitude (2–4 mm). In particular, it was hoped that the results would allow the investigators to determine whether random order of treatment (e.g., frequency and amplitude levels) may be assigned and, if so, how much recover time is required between treatments.
 
Subjects
 
Nineteen subjects (8 male and 11 female, mean age 19 ±1 years [range: 18–21 years]) with measurable H-reflexes were recruited from the student population at Hope College by word of mouth. No effort was made to control for training specificity, but current physical activity was self-reported on a medical history questionnaire. Subjects ranged from sedentary to NCAA Division III athletes (football, soccer, swimming, and track sprinters) and were overall heterogeneous. Subjects had no indicated neurological defects, no history of lower extremity surgery, and no lower extremity injury for 12 months before the start of the study as reported by the medical history questionnaire. Informed consent was obtained, and all procedures were approved by the Hope College human subjects review board.
 
Procedures
 
H-reflex and M-wave measurements were collected at a rate of 2000 Hz using surface EMG (MP150; Biopac Systems Inc., Santa Barbara, CA). Signals were amplified (EMG100C; Biopac Systems Inc.) from disposable, 10-mm pregelled Ag- AgCl electrodes (EL503; Biopac Systems Inc.). A stimulator module (STM100C; Biopac Systems Inc.) with a 200-V (maximum) stimulus adaptor (STMISOC; Biopac Systems Inc.) using a disc electrode (EL548; Biopac Systems Inc.) and a 4-cm dispersive pad were used to stimulate the muscle contraction.
 
Subjects reported to the Athletic Training Room at Hope College’s DeVos Fieldhouse after at least 12 hours’ rest from exercise and 12-hour abstention from alcohol, caffeine, and any medication that affects the central nervous system.With the subject lying prone, Subject position for H-reflex. Insert, Stimulating electrode in posterior popliteal fossa.two areas were abraded with fine sandpaper and cleaned with alcohol for placement of the EMG electrodes. Surface electrodes were placed on the posterior leg, centered on the soleus muscle, approximately 2 cm distal to the medial head of gastrocnemius and spaced 2 cmapart, parallel to the muscle fibers. The ground electrode was placed on the lateral malleolus of the ipsilateral leg.
 
The stimulating electrode was fixed in the medial portion of the popliteal fossa behind the knee over the tibial branch of the sciatic nerve (Figure 1). The corresponding anode (dispersive pad) was placed on the anterior thigh superior to the patella. The stimulating electrodes were then wrapped with 2-inch PowerFlexTM tape (Andover, MA) to maintain constant pressure on the electrodes. Subjects were subsequently positioned supine with the hands positioned at the sides, knees supported at approximately 10–15 degrees, and the feet secured against a footplate to maintain neutral foot position. Participants wore noise-canceling headphones (Phillips HN110) and listened to ocean sounds to minimize the effects of extraneous sound on the H-reflex. Subjects were asked to remain still, and the same body positioning was used throughout testing.
 
H/M recruitment curves were mapped for the soleus muscle by increasing the stimulus intensity in 0.2- to 1.0-V increments, with a 10-second rest interval between stimuli, until the Mmax was obtained. Peak-to-peak amplitudes of the H-reflex and M-wave were determined for all test stimulations. Five measurements of the H-reflex were made using a stimulus sufficient to produce an H-wave 30% of Mmax pre- WBV (T0), and a single measurement was made immediately post-WBV (approximately 1 minute post-WBV; T010) and every 30 seconds for 30 minutes. In addition, because previous studies have indicated a conditioning effect of the electrical stimulation on the H-response, three conditioning stimuli (approximately 100 V) were applied at onset of the recruitment curve. H-reflex measurements using the described protocol have been found to be reliable (r = 0.9953, 0.9514, and 0.9747 for Hmax, Mmax, and H/M ratio, respectively) (28).

 
Whole-body vibration (Next Generation Power Plate; Power Plate North America, Inc., Northbrook, Ill.).
Subjects stood with the feet shoulder-width apart and the knees flexed approximately 10 degrees on a Next Generation Power Plate (Power Plate North America, Inc., Northbrook, IL) for 1 minute with the frequency and amplitude settings at 40 Hz and 2–4 mm, respectively (Figure 2).
 
 
Statistical Analyses
 
Multiple recovery patterns were observed among the subjects. Therefore, data were separated into four patterns according to the rate at which the subject returned to baseline and whether potentiation of the H-reflex was observed. Comparisons of changes in the H-reflex over time and between groups were made using repeated-measures ANOVA. One-way ANOVA was also used to compare differences between groups and between genders at key time points where notable changes in the H-reflex occurred, i.e., T0, T010, 3-min post-WBV(T030), 15-min post-WBV (T150), and 27.5-min post-WBV (T275), and Bonferroni post hoc comparisons were made to determine where significant differences between groups occurred. In addition, the intraclass correlation coefficient was determined the five pre-WBV H-reflex measurements at the test stimulus intensity. These values were used to determine internal consistency because these were the only multiple measurements taken at a given intensity. The level of significance was set at œ= 0.05.
 
RESULTS
 
Four distinct recovery patterns were observed among the participants (Figure 3). All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV. Recovery to baseline occurred in three of the groups, whereas the fourth group (G4) showed a nearly complete suppression (peak-to-peak amplitude <10% of Mmax) for the duration of recovery. One group (G1) returned to baseline within 3 minutes, and the H-reflex subsequently increased to above baseline. Group 2 (G2) returned to and remained at baseline after 7 minutes. Group 3 (G3) was suppressed until approximately 15 minutes post-WBVand gradually increased to baseline by 30 minutes.

 
Figure 1. Subject position for H-reflex. Insert, Stimulating electrode in posterior popliteal fossa.
 
There was a significant effect for time for all groups (P < 0.001) and for group by time (P <0.001). There were no significant differences between genders across time (P = 0.410). There was not, however, sufficient gender representation in all groups to perform time xgroup xgender comparisons. Pre-WBV, G1 displayed a baseline H-reflex significantly higher than G3 and G4 (P = 0.014 and 0.019, respectively) but not significantly different from G2 (P = 0.07). At T010, there was no significant difference between groups on the H-reflex (P >0.34). At T030, there was a significant between-group interaction (P = 0.002). G1 was significantly higher than G3 and G4 (P >0.35), but G2 did not differ from the other groups (P >0.16). At T150 and T275, there was a significant between-group interaction (P , 0.001). At T150, G1 was higher but not significantly different than G2 (P = 0.083); G3 and G4 were similar (P >0.99); and G1 and G2 were significantly higher than G3 and G4 (P < 0.001). At T275, G1 was significantly higher than the other groups (P <0.04), and G2 and G3 were similar (P >0.99) and significantly greater than G4 (P <0.004). Group means for T0, T010, T030, T150, and T275 are reported in Table 1.
 
TABLE 1. H-reflex at 40 Hz and low amplitude (2–4 mm).
Intraclass correlations were performed on the five pre- WBV H-reflex measurements at the stimulus test intensity. The observed intraclass correlations value was r = 0.81.
 
DISCUSSION
 
The investigators sought to examine the recovery response of the H-reflex to a 1-minute bout of WBV exercise of moderately high intensity. The most significant finding of the present study is that the response of the H-reflex to a single bout of WBV is highly variable and may be associated with individual differences other than gender, training specificity, or conditioning status. One plausible explanation for these differences is muscle fiber type differences. The present study is limited in that the effects of training cannot be thoroughly eliminated. In addition, without information (i.e., biopsies of muscle samples) about muscle fiber content, these conclusions are speculative at best.
 
It has been reported that acute bouts of cycling or jogging exercise reduce the H-reflex (3–5,15,26,27). deVries and coworkers (15) indicated a ‘‘tranquilizer effect’’ of 20 minutes of moderate aerobic exercise on H/Mratio. These investigators examined H-M recruitment curves in the gastrocnemius of 10 volunteers (2 ‘‘elderly’’ [66 and 80 years] and 8 ‘‘younger’’ [mean age 27.3 ±4.8 years]) for which training levels and muscle composition are not reported. Suppression of the H/M ratio ranged from 6% to 44%. Motl and Dishman (26) reported an attenuated soleus H-reflex 10 minutes after moderate-intensity leg cycling (60% V_ o2peak) but observed no change in the flexor carpi radialis H-reflex. In a similar study, Motl et al. (27) observed suppressed soleus H-waves 10 and 30 minutes after active and passive cycling. The mechanism by which this occurred is unclear. Bulbulian (3) concluded that endogenous opiods play no role in the suppression of H and M after exercise. No study was found that detailed the pattern of H-reflex response over time after fatiguing exercise.
 
The data in the present study support previous findings that there is a fatiguing effect of WBV and that, comparing intensities for research, a substantial recovery should be allowed between exercise bouts. At least some of the subjects showed a suppressed H-reflex for 30 minutes, whereas others showed some level of potentiation for this duration. In either case, not all subjects were fully recovered with the 30-minute period. Because there was a limit to the recovery time, there is no way to conclude how long a recovery is required. The minimal period of recovery may be anywhere between 3 hours (13) and 24 hours (4).
 
There were at least four clear response patterns revealed in the study. Similar patterns were observed in an earlier experiment in which the researchers studied the H-reflex for 15 minutes after two bouts of the Wingate test (1).
 
The role of training on the H-reflex has been clearly demonstrated (16,24). The differences observed in both the Wingate study (1) and the present study, however, cannot be explained by level or type of training. In the Wingate study (1), the sample size was too small to quantify significant difference in training. In the present study, observed variability was not consistent among individuals with similar training experience (e.g., similar sport team participation, endurance-trained vs. agility/power-trained). The sample in this study was heterogeneous, and no definitive analysis of training effects could be performed. Although there were similarities in the nature of training among some of the subjects, few subjects trained with the same specificity (i.e., whereas soccer goalies and football defensive backs both train for agility, the activities to accomplish this goal are not necessarily the same). Examination of individual differences should be performed using larger, homogeneous groups controlled for training specificity.
 
The soleus is composed largely of type I muscle fibers (25). The posture tested using WBVwould put stress on the soleus muscle to maintain balance, and these data confirm that the soleus was indeed fatigued by the protocol. One plausible interpretation of these data is that the multiple patterns of recovery may be a reflection of muscle fiber content variations among subjects. Postactivation potentiation may be affected by the muscle fiber characteristics (8,11,17–19, 35,37), although this hypothesis was not tested. Comparison of biopsied soleus muscle tissue with H-reflex recovery after fatiguing exercise is warranted. The postactivation response of the H-reflex may be dependent not only on fiber type and the nature of the stimulus (e.g., frequency and intensity), but also on the complex interaction between the muscle fiber fatigue characteristics and the nature of the fatiguing protocol (17,18). In addition, other muscles (e.g., flexor carpi radialis, quadriceps femoris, gastrocnemius) should be tested to compare the response of the H-reflex after vibration and other fatiguing exercise.
 
Intraclass correlations indicate that the H-reflex may have been more variable in the present study than noted by Palmieri and co-workers (28). This is likely the result of the prolonged time the subjects were required to lay motionless. H/M recruitment curves can take longer than 30 minutes, and finding a suitable testing stimulus intensity took more than 5 minutes in some individuals. The subjects were also asked to lie still for an additional 30 minutes post-WBV. Maintaining a stable level of alertness (i.e., keeping the subjects awake, focused, and in the same postural position) proved impossible. In addition, the rhythmicity of stimulus application may have caused some subjects to anticipate the stimulation, thereby affecting the H-reflex. Nevertheless, the frequency of measurements permitted the investigators to note observable patterns in the recovery H-reflex.
 
PRACTICAL APPLICATIONS
 
H-reflex is a viable measure ofMN excitability. After fatiguing exercise, this variable is notably suppressed. The rate at which the H-reflex recovers after an acute bout of WBVwas studied in the present study. The data indicate that the pattern of recovery is variable and may not be attributable to gender or type of training. Recovery from a single bout may be longer for some individuals. Therefore, when comparing the acute effects of WBV at varying intensities, investigators should consider designs that include subjects who are tested on multiple days rather than using designs that, although randomly ordered, may not permit adequate recovery of the motor unit (i.e., utilize short-duration recovery between tests).
 
These data may also indicate that a physiological difference other than training occurs among subjects. This, however, was not quantified in the present study. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response. In addition, the effect of WBV on specific performance measures (e.g., flexibility, vertical jump, force output, balance) should be addressed and interpreted with the understanding that there may be considerable variability among individuals, and sample size should be adjusted accordingly.
 
 
Effect of whole body vibration training on lower limb performance
Power Plate Studies
ABSTRACT.
 
The aim of this study was to examine the effects of 8 weeks of whole body vibration (WBV) training on vertical jump ability (CMJ) and knee-extensor performance at selected external loads (50, 70, and 100 kg; leg-press exercise) in elite ballerinas. Twenty-two (age, 21.25 ±1.5 years) full-time ballerinas were assigned randomly to the experimental (E, n = 11) and control (C, n =11) groups. The experimental group was submitted to WBV training 3 times per week before ballet practice. During the training period, the E and C groups undertook the same amount of ballet practice. Posttraining CMJ performance significantly increased in E group (6.3 ±3.8%, p <0.001). Furthermore, E group showed significant (p <0.05–0.001) posttraining average leg-press power and velocity improvements at all the external loads considered. Consequently, the force-velocity and power-velocity relationship shifted to the right after WBV training in the E group. The results of the present study show that WBV training is an effective short-term training methodology for inducing improvements in knee-extensor explosiveness in elite ballerinas.
KEY WORDS. strength, power, vertical jump, force-velocity, ballerina
 
INTRODUCTION
 
Ballet is considered mainly as an aesthetic activity (17, 22). Recent findings, however, have shown dance to be a physically demanding exercise mode (17). Due to its highly complex, multidirectional movement requirements, dance may be considered as a high-intensity intermittent exercise (26)
 
Body composition, joint mobility, cardiovascular fitness, and muscular strength have been reported as limiting factors in ballet performance (19). Dance requires quick bursts of energy interspersed with low-intensity activities (9), and strength level is an important component of ballet performance (19, 27). Additionally, strength has been suggested as an effective strategy for injury prevention in ballet (16, 22). Nevertheless, dancers see themselves as artists rather than athletes and only rarely undertake structured physical conditioning, particularly strength training (16, 17, 19, 27, 32, 33).
 
The few studies that have addressed the issue of strength training in ballet reported significant improvements from pretraining strength levels, without any detrimental effect on actual and perceived physical appearance (19, 27).
 
Despite the reported encouraging effect of strength training on ballet performance, the protocols that have been suggested are time-consuming and require moderate to high skill to be performed. Furthermore, they necessitate training facilities that are very often difficult to implement in a ballet academy due to economic or logistical reasons. Consequently, these protocols, usually involving free-weight exercises or weight-training machines, may have little applicability to the ballet population.
 
Recently, whole body vibrations (WBV) have been proposed as a training intervention to develop strength and flexibility (7, 15). The major documented benefit of WBV is explosive strength performance, usually considered as vertical jump height (2, 12, 29–31). This result is of particular interest, because vertical jump performance is considered to be related to ballet performance (19, 27). Additionally, WBV training protocols require only a limited amount of time and very limited, if any, specific skills to be performed (7).
 
The aim of this study was to examine the effects of a short-term WBV training intervention on dynamic strength and vertical jump performance in a selected population of elite-level classical ballerinas. It was hypothesized that WBV intervention may result in explosive strength and dynamic strength improvements in professional ballerinas if provided as a supplement to the usual dance training routine.
 
METHODS
 
Experimental Approach to the Problem
 
The strength training protocols that used ballerinas as subjects considered weight-machine and free-weight exercises (19, 27). Furthermore, the strength training protocols involved progressive loads exceeding 70% of 1 repetition maximum (1RM) during 3–4 exercises in 5–6 sets with up to 8 repetitions each (19, 27). Although these training protocols are widely used by athletes of different sports, they require proper facilities, good weight-training apparatus, and professional supervision (10). Additionally, those successful protocols require extensive resistance training over 50–90 minutes (19), an amount of time that may be intolerable given the often-busy dance practice routines of professional dancers. Moreover, strength training is felt by ballerinas to have potentially detrimental effect on actual and perceived physical appearance (19, 27).
 
With respect to this, WBV may grant several advantages (7). In fact, WBV has been reported to exert a positive effect on both explosive and maximal strength, requiring only a small amount of time per session (2, 3, 12, 30, 31). Furthermore, the WBV training exercise needs very limited familiarization and supervision (12, 30, 31) and can be performed without frequenting well-equipped weight training facilities. Additionally, there is no need to progressively adjust training loads with each testing session, being time of treatment an effective way to implement increments into training stimulus (30, 31). Finally, unlike the reported successful strength protocols (19, 27), WBV does not involve exercising to failure, a condition that may exacerbate the likelihood of a pernicious condition of overreaching that, over a long training period, may lead to an overtraining status (18).
 
This study used a randomized, fully controlled experimental design to examine the effect of WBV on a group of classical ballerinas. Ballerinas’ countermovement vertical jump height (CMJ) and maximal strength and power at selected loads (50, 70, and 100 kg) were considered as dependent variables in this study. These variables had not been addressed previously in training studies despite being considered as limiting factors in ballet performance (17, 19, 27).
 
Recent research has shown that dance training routines provide few stimuli for physical performance improvement, suggesting the need for supplemental conditioning (17, 19, 27, 32, 33). However, usual ballet training routines involve plyometric-type exercises (17, 27) that may improve jumping ability as a synergistic effect of ballet practice. In this regard, Newton et al. (21) showed that if proper training stimuli are provided, vertical jump performance may be improved further in female athletes who are already highly trained in the jump. In this study we used WBV as a training stimulus for inducing improvement in vertical jump and dynamic strength, because such training has been shown to enhance this neuromuscular performance domain (12, 23).
 
The working hypothesis was that in non–strengthtrained but potentially highly trained individuals who jump, WBV may induce significant improvements in various aspects of functional strength performance using nonspecific movements (2, 7, 20).
 
 
Subjects
 
Twenty-two well-trained ballerinas (age, 21.25 ±1.25 years; body mass, 50.8 ±3.7 kg; height, 165.7 ±5.6 cm), full-time students of the National Ballet School, volunteered for the present study. At the time of the investigation, all of the subjects had at least 8 years of dance experience and no previous history of fractures or musclebone injuries. In order to be included in the study, participants had to possess official medical clearance at the beginning of the season, according to the law. None of the subjects were active smokers or suffered nutritional disturbances, and no medication or drug that would have been expected to affect physical performance was being taken by the ballerinas during the course of this investigation. Furthermore, none of the ballerinas had been involved in strength or explosive-power training prior to the commencement of the training study.

 
Mean +/- standard deviation of age, body height, and mass of subjects studied.
 
Ballerinas were assigned randomly into an experimental group (E, n =11) or control group (C, n =11). Throughout the 8-week training intervention, the E and C groups undertook the same amount of ballet training (5 session per week), equated in terms of intensity and volume. Ballet training was performed 5 times a week (Monday–Friday), each session lasting 60–90 minutes. The typical training session consisted of barre technical exercises, center choreography exercises, and ballet jump exercises.
 
Unlike the C group, the E group also undertook a specifically designed WBV training 3 times per week. No other form of conditioning, apart from those considered for this study, were allowed or were performed by participants during the study duration.
 
Full advisement was given to the volunteers regarding the possible risks and discomfort that might be associated with the testing and training procedures used in this study. Prior to the training study and after a detailed written and verbal explanation as to the nature of the procedure involved in this study, all the subjects gave their written informed consent. The study was approved by the Institutional Review Board of the Italian Society of Sport Science. Table 1 presents the physical characteristics of the subjects.
 
Testing Procedures
 
Testing procedures were administered at the beginning and the end of the training intervention using the same order. Posttraining assessments were accomplished 3 days after the last training session to avoid the acute effects of WBV and to allow recovery in both groups. Prior to the commencement of the training study, subjects became accustomed to the testing and training procedures with 2 familiarization sessions that took place during the preceding week. Familiarization procedures were performed on an individual basis until subjects appeared to be fully accustomed to the testing and training procedures involved with this study. The first 2 authors of this study supervised the familiarization sessions.
 
Before each testing session, the ballerinas’ body mass was measured to the nearest 0.5 kg (Seca Beam Balance 710, Hamburg, Germany) with subjects lightly dressed and barefooted, and standing height was measured to nearest 0.5 cm (Seca Stadiometer 208). After body mass and body height were measured, ballerinas undertook a standardized general warm-up protocol that preceded the jumping and mechanical power measurements. The general warm-up comprised stationary cycling (5 minutes) on a computerized cycle ergometer (Technogym, Gambettola, Italy) followed by 5 minutes of static stretching of quadriceps and calf muscles.
 
Countermovement Jump Height Testing
 
Countermovement jump height was assessed with subjects performing maximal vertical jumps (with hands on hips) on a switch mat connected to a computer (Ergojump; Boscosystem, Rieti, Italy), according to the procedures reported by Bosco et al. (6). Prior to testing, each subject performed 3 submaximal CMJs with 1 minute of recovery between repetitions, followed by 3 maximal CMJs paced with similar recovery time. During CMJs, knee flexion was standardized, allowing subject to bend their knee at approximately 90°. According to Bosco et al. (6), the rise of center of mass after the takeoff (h) was measured applying the following ballistic formula:
 
h = tf2 g 8-1,
 
Height of rise of center of gravity
where g is the gravity pool constant (9.81 m·s-2). The best h was used for statistical analysis. The reproducibility of the CMJ testing procedure here used has been reported to be high
 
Mechanical Power Measurements
 
Following CMJ testing, the mechanical power of subjects’ knee extensors was assessed using a horizontal leg-press machine (Technogym, Gambettola, Italy). After a brief, specific warm-up of approximately 10 minutes, consisting of 3 sets with increasing loads (40, 60, and 70 kg with 10, 6, and 3 repetitions, respectively) separated by 2–3 minutes of passive recovery, each subject performed maximal dynamic leg-press exercises with 50, 70, and 100 kg until exhaustion. During leg-press exercises, knee flexion was standardized at 90° and only the repetitions that matched the selected angular displacement were used for calculations. Knee flexion was monitored using an electronic goniometer (Muscle-Lab, Ergotest Technology, Langesund, Norway) worn by subjects on their nondominant leg and interfaced with a laptop computer. In order to improve measurement reliability, horizontal leg-press seat position was modified according to subjects’ lower limb length and was recorded for posttraining testing reconsideration. Magnitude and rate of load displacement were assessed with a computerized system (Muscle-Lab, Ergotest Technology, Langesund, Norway) according to Bosco and colleagues’ (1) procedures. As suggested by Bosco et al. (1), average knee-extensor power was considered as a variable representing lower limb mechanical power.
 
Subjects performed 3 trials of each load and the best measures were considered for calculations.
 
The reliability of this testing procedures and measurements reported as an intraclass correlation coefficient has been reported to be r =0.95
 
Vibration Treatment Procedures
 
Subjects were exposed to vertical sinusoidal WBVs using the Nemes LC device (Boscosystem, Rieti, Italy). Vibration frequency was set at 30 Hz according to Cardinale et al. (8) (5-cm displacement; magnitude =5 g) to maximize WBV training effects. During each training session, the subjects in the E group were exposed to WBV treatment before their usual ballet lesson with C group. Whole body vibration treatment consisted of five 40-second repetitions separated by 60 seconds of passive recovery and was provided 3 times a week (Monday, Wednesday, and Friday). The total training intervention comprised 24 WBV sessions over 8 weeks. Whole body vibrations were applied with the subject standing on the vibration platform in a half-squat position (approximately 100°) with feet and knee rotated externally (i.e., demi-plie´ position). In order to avoid bruises, E group ballerinas wore dancertype shoes throughout the WBV session.
 
Statistical Analyses
 
Data are shown as mean and standard deviation. Paired t-tests were used for within-group comparisons. The level of significance was set at p <=0.05. Unpaired t-tests with Bonferroni adjustment were used for between-group comparisons, with a resulting p value of 0.025. In order to assess meaningfulness of pre- to posttraining changes, the effect size (ES) was calculated according to Thomas et al. (28). Effect sizes of 0.8 or greater, around 0.5, and 0.2 or less were considered as large, moderate, and small, respectively (28).
 
RESULTS
 
The E and C groups differed at pretraining only for average velocity (AV) and average power (AP) during the 50-kg condition (p >0.05). The E group showed significant CMJ performance improvement after training (p < 0.001) (Table 2). Effect size for CMJ was medium (0.67).
 
Results of the mechanical variables of interest are showed in Table 3. The E group showed significant (p < 0.05–0.001) pre- to posttraining improvements in all the mechanical parameters (average force, AP, and AV) for the 70- and 100-kg conditions. Effect sizes ranged from medium to large (0.33–1.8). In the 50-kg condition, only AP and AV showed pre- to postintervention improvements in the E group (0.89 and 1.00, respectively). These results shifted the force-velocity and power-velocity curves to the right (Figure 1). No significant change in any of the mechanical variables was observed in the C group.
 
DISCUSSION
 
This is the first training study that investigated the effects of WBV on vertical jump and lower limb dynamic muscular strength and power in elite-level ballerinas.
 
The main finding of this investigation is that WBV training positively affected vertical jump performance in well-trained elite ballerinas. This result is of particular interest, because vertical jump performance is considered to be related to ballet performance (19, 27). The pre- to postintervention improvement in CMJ performance found in this study is similar (6.3 vs. 7.6%) to that reported by Delecluse et al. (12) for age-matched, untrained female subjects submitted to WBV treatment. However, in the Delecluse et al. (12) study, the WBV treatment was undertaken for 12 weeks using higher WBV frequencies (35–40 vs. 30 Hz) but with a similar vibration platform displacement (5 mm) and magnitude (5 g). Furthermore, WBV training loads were adjusted (vibration frequency, repetition number, and duration) over the intervention duration in order to provide progressive overloads (12).

 
Mean +/- SD of the average force (
 
The few studies that addressed the issue of strength training in ballerinas reported significant improvement in isometric and isokinetic strength (19, 27). Although those studies analyzed the effect of free-weight and weight-training machines on ballet performance using repeated-jumping ballet simulation protocols, they did not study CMJ performance. In this regard, Delecluse et al. (12) reported no CMJ improvement in a population of untrained young female subjects using strength-training Average power (AP) and average force (AF) developedprotocols similar to those used for ballerina training studies (19, 27).
 
Providing 40-Hz vibrations during squat training (6– 10RM), Rønnestad (24) reported significant improvement in CMJ (9%) and squat 1RM performance (32.4%) as a consequence of a 5-week WBV training in male resistance- trained subjects. Interestingly, the control group, which comprised resistance-trained individuals who performed the same strength training protocol without added vibration stimuli, reported no significant improvements in CMJ performance.
 
In light of these considerations, it may be argued that WBV or explosive strength training (14) should be considered for ballerinas in order to improve vertical leap performance. However, transfer of CMJ height improvements to dance performance should be assessed with sound performance analysis.
 
In this study, the WBV training determined significant improvement of AP. The major effects of WBV were observed at the 100-kg load with an average AP improvement of 25% (ES =0.62). This AP improvement was due mainly to a concomitant AV increment of 26% (ES =1.0).
 
As a consequence of WBV training, a general shift of force and power profiles to the right was observed (see Figure 1). Training-induced improvements (i.e., shift to the right) of force-power profiles (11, 13) may be caused by several factors such as motor unit synchronization, cocontraction of synergistic muscle, or increased inhibition of the antagonist muscles (25). From this we can infer that WBV training probably was effective in causing an alteration of the neural traffic that regulates muscle stiffness (2, 7). Interestingly, WBV intervention induced force-power profile improvements similar to those reported in studies involving heavy-load strength training (11, 13).
 
According to Bosco et al. (4), stimuli duration seems to be important in inducing neuromuscular adaptation. In the present study, the total duration of WBV intervention was short (only 24 minutes), with a constant gravitational perturbation of 5 g. This means that in order to induce similar training loads using leg-press or half-squat exercises, 150 repetitions with extra loads of approximately 3 times body mass should be provided over the same training period (4) twice a week.
 
The mechanisms mediating the effects of WBV on the neuromuscular system are not completely understood (7, 20). However, current knowledge of WBV effects determinism are considered to be related to neuromuscular, hormonal mechanisms, or both (5, 7). In the present study, no neural extra excitatory inflow or hormonal changes have been demonstrated, because neither electromyographic recordings nor blood samples were collected. Nevertheless, enhancement of mechanical power in leg press performance and the improvement in the CMJ height strongly suggest that a neural adaptation occurred in response to the vibration training in the subjects of the experimental group.
 
These findings are of particular interest because maximal anaerobic power is highly involved in ballet performance (17). However, as suggested for CMJ, the actual effects of AP and AV improvements on ballet performance should be investigated with proper performance analysis.
 
The successful WBV protocol used in this training study was devised specifically to be as nonintrusive as possible to the usual dance routine. However, use of this training protocol obtained very limited improvements in dynamic strength in the E group at the selected external loads (1–3%, p <0.05). It can be speculated that if periodized and progressive WBV training programs (12) are used, further improvements in strength, explosive strength, and CMJ performance could be achieved.

 
PRACTICAL APPLICATIONS
 
According to previous research (18, 19, 27), this study reported that ballet practice is ineffective in promoting significant strength improvements in well-trained ballerinas.
 
Whole body vibration training may be considered as an effective and safe training strategy for improving muscular power in well-trained ballerinas. In this regard, a 30-Hz vibration frequency should be used for inducing muscular adaptation throughout the muscular strengthpower spectrum. Whole body vibration stimuli should be provided in the form of 3–5 repetitions of 40 seconds. Recovery time between repetitions should be no less than 60 seconds.
 
In this study, no significant (p >0.05) variations in body mass were found to result from the WBV treatment. Furthermore, the WBV group reported a perceived increase in physical appearance and technical efficacy during ballet performance (a multi-response questionnaire was submitted at the end of the training intervention). Whole body vibration training studies that used similar or even higher training loads for a longer duration also reported no variation in body mass (12, 30, 31). Although this study did not provide detailed information about the variations in body composition that may have occurred as consequence of WBV, it can be suggested that WBV might increase muscular performance without significantly affecting body appearance. Finally, it is difficult to say if increasing dynamic and explosive muscular strength can increase ballet performance, but it can help the dancers be safer and prevent muscle injures related to their highly intense plyometric activity.
 
 
Six Weeks of Whole-Body Vibration Exercise Improves Pain and Fatigue in Women
Power Plate Studies
Abstract

Objective:
The aim of this study was to investigate the effectiveness of a 6-week traditional exercise program with supplementary whole-body vibration (WBV) in improving health status, physical functioning, and main symptoms of fibromyalgia (FM) in women with FM.

Methods:
Thirty-six (36) women with FM (mean ± standard error of the mean age 55.97 ±1.55) were randomized into 3 treatment groups: exercise and vibration (EVG), exercise (EG), and control (CG). Exercise therapy, consisting of aerobic activities, stretching, and relaxation techniques, was performed twice a week (90 min/day). Following each exercise session, the EVG underwent a protocol with WBV, whereas the EG performed the same protocol without vibratory stimulus. The Fibromyalgia Impact Questionnaire (FIQ) was administered at baseline and 6 weeks following the initiation of the treatments. Estimates of pain, fatigue, stiffness, and depression were also reported using the visual analogue scale.

Results:
A significant 3 x2 (group xtime)-repeated measures analysis of variance interaction was found for pain (p =0.018) and fatigue (p =0.002) but not for FIQ (p =0.069), stiffness (p =0.142), or depression (p = 0.654). Pain and fatigue scores were significantly reduced from baseline in the EVG, but not in the EG or CG. In addition, the EVG showed significantly lower pain and fatigue scores at week 6 compared to the CG, whereas no significant differences were found between the EG and CG (p >0.05).

Conclusion:
Results suggest that a 6-week traditional exercise program with supplementary WBV safely reduces pain and fatigue, whereas exercise alone fails to induce improvements.


Introduction

Fibromyalgia (FM) syndrome is considered a chronic rheumatic condition of unknown etiology characterized by widespread noninflammatory musculoskeletal pain with tenderness on palpation in a minimum of 11 of the 18 tender points for at least 3 months.1 Numerous symptoms may also be associated with this syndrome, including fatigue, anxiety, depression, nonrestorative sleep, muscular stiffness, or irritable bowel syndrome.

The management of FM is based on symptomatic multidisciplinary treatment through pharmacologic and nonpharmacologic strategies. Among all nonpharmacologic treatments, exercise, cognitive behavioral therapy, and education have the strongest evidence for efficacy.2 Numerous researchers support the benefits of aerobic exercise,2–7 strength training,2,6,8 health education,3,9 and relaxation techniques7 on health status, physical functioning, and main symptoms of FM.

Whole-body vibration (WBV) is a mode of exercise that has recently been utilized to improve muscle strength, bone density, and balance in healthy adults10 and aging populations.11,12 In this therapy, the subject performs exercises on a platform that generates vertical vibrations with a frequency and amplitude of 20–50 Hz and 2.0–10.5 mm, respectively.

The rationale for investigating the effects of WBV in patients with FM is based on the following evidence. First, vibratory stimulation has been shown to induce pain relief in acute and chronic pain.13 However, this was a local vibration applied to the forearm, face, and skull rather than a WBV, effects that have not been studied in patients with FM. Second, WBV has been shown to increase strength in untrained14 and postmenopausal women.15 Third, some studies have reported increased growth hormone secretion with WBV.16,17 It is possible that this WBV-induced endocrine effect could enhance the effectiveness of the exercise therapy.18 Therefore, it is reasonable to expect that patients with FM could benefit from the salutary effects of WBV by improving health status, physical functioning, and main symptoms of FM. However, the effects of WBV on patients with FM have not been previously studied.

The purpose of this study was to investigate the effectiveness of a 6-week traditional exercise program with supplementary WBV in improving health status, physical functioning, and main symptoms of FM in women with FM. It was hypothesized that women with FM undergoing the traditional exercise program with supplementary WBV would improve health status, physical functioning, and main symptoms of FM more so than women undergoing exercise only. We considered pain, fatigue, stiffness, and depression as the main symptoms of FM.

Patients and Methods

Subjects

Participants were recruited by referral from family physicians and through public announcements distributed in local associations of FM in Barcelona (Spain). One hundred and four (104) women were interested in the study, and those with diagnosis of FM, according to the American College of Rheumatology criteria,1 for at least 3 years were considered for the study. Women were excluded if they had any orthopedic limitation, cardiovascular, pulmonary, or metabolic disease that would preclude exercise, or when participating in any other study (Fig. 1). Written informed consent was obtained from each subject prior to participation in the study according to procedures approved by the Committee on Biomedical Ethics of the Jordi Gol Gurina Foundation (Spain).

Study Design

A 2-factor mixed experimental design was employed in this study. Women were randomized into 3 treatment groups: exercise and vibration (EVG), exercise (EG), and control (CG) (Fig. 1). Measurements were taken at baseline and at 6 weeks following the initiation of the treatments. To minimize the residual effects of the last bout of exercise, the sixthweek questionnaire was administered 48 hours following the last session of the treatment. Prior to the initiation of the study, an individualized interview was performed with each patient in order to obtain the following information: (1) complete medical history, (2) current medications, (3) physical activity habits, (4) preferred types of exercises, and (5) socioeconomic status. In addition, patients were instructed to report any changes in medication regimens.

Self-report Health Status, Physical Functioning, and Main Symptoms of FM

Health status, physical functioning, and main symptoms of FM were assessed using the validated Spanish version of the Fibromyalgia Impact Questionnaire (FIQ).19 The FIQ is a valid, reliable, and sensitive tool19–22 widely used in research and clinical settings. This questionnaire measures physical function (activities of daily living), work, and general well-being. It contains 100-mm visual analogue scales (VAS) for pain, sleep, fatigue, stiffness, anxiety, and depression.19,20 An overall FIQ score is obtained by normalization of physical function, work, and well-being into a 0–10 scale, which are added to the total scores of VAS. We chose to singly document VAS scores for pain, fatigue, stiffness, and depression, as these are among the most common symptoms related to FM.25 For all our dependent variables, a higher score indicates a greater level of difficulty or illness. Questionnaires were administered to all patients on the same day, time, and by the same investigator to ensure equal instructions and thus minimize potential confounding variables. In addition, administration and analysis of the questionnaires were performed by an investigator who was blind to the treatment groups.
 
FIG. 1. Study design. EVG, exercise and vibration group; EG, Exercise Group; CG, Control Group.
Treatment Groups

Both EVG and EG underwent the same traditional exercise program twice per week for a total of 6 weeks. Following each traditional exercise session, EVG and EG were separated into 2 different rooms. EVG underwent a protocol with WBV, whereas the EG performed the same protocol without vibratory stimulus. The vibratory apparatus in the EG was turned on yet did not produce vibrations, which is a strategy that has been utilized by other authors.13 We informed both EVG and EG that they would receive a perceptible and imperceptible vibratory stimulus, respectively, thus maintaining the potential of a placebo effect consistent in both groups. Women in the CG performed neither the traditional exercise program nor the protocol with WBV. All 3 groups were instructed to continue with their pharmacologic care.2 All exercise sessions were conducted by the same instructor, who had experience in working with FM. The protocol with WBV was supervised by experienced investigators. The 2 per week frequency of sessions was adopted to avoid the risk of exacerbating symptoms and to ensure adherence.

Although our traditional exercise program was similar to that from other studies,3–5,7,9 the inclusion of WBV in patients with FM is the unique aspect of this investigation. Six (6) weeks was chosen as the duration of our intervention because, based on previous research, this is sufficient time for WBV-induced adaptations to occur26–28 while inadequate duration for a traditional exercise program to improve pain, fatigue, stiffness, and depression in patients with FM.3,7 This approach allowed us to investigate the impact of WBV in enhancing the effectiveness of a traditional exercise program.

Traditional Exercise Program

The traditional exercise program was designed using the recommendations of Jones and Clark.29 The program consisted of 15 minutes of a warmup, 30 minutes of aerobic exercise, 25 minutes of stretching exercises, and 20 minutes of relaxation. Major emphasis was given to aerobic exercise because this is the most beneficial type of exercise for the population investigated.2 Aerobic activities consisted of 30 minutes of walking on flat ground at an intensity of 65%–85% of the theoretical maximal heart rate (220 – age (years)). The intensity was monitored by instructing the patients to take their pulse rate during the exercises. For adaptation purposes, during the first week, target heart rate was set at the lower range. Games and dances with low impact were presented at weeks 3 and 5 to avoid monotony. Patients danced salsa music for 30 minutes, performing two repetitions of 15 minutes each with a recovery of 3 minutes in between. Patients performed dancing tasks with a partner, whereas dancing games were performed in groups of 3–10 patients. Stretching exercises consisted of 5 different static wholebody stretches. Patients performed 5 repetitions of each whole-body stretch, holding the position for 30 seconds with a recovery of 30 seconds in between. Stretches involved (a) hamstrings, calves, Achilles tendons, back, and shoulders (downward-facing dog stretch); (b) gluteals, lower back, upper back, shoulders, arms, and chest (lying spinal twist stretch); (c) hamstrings, calves, shoulders, chest, and arms (forward-bend shoulder stretch); (d) Achilles tendons, calves, hamstrings, adductors, cervical spine, shoulders, and forearms (lying supine with open and straight legs with 90° of hip flexion and 100° of shoulder abduction with extended wrists); and (e) Achilles tendons, calves, hamstrings, back, and shoulders (sit-and-reach stretch). Stretches were individualized to teach each patient to locate her stop point and avoid overstretching. Relaxation exercises included diaphragmatic respiration, progressive muscular relaxation, contraction–relaxation, and imagery techniques. The total duration (over 12 sessions) of aerobic exercise, stretching, and relaxation was 9 hours, 6 hours, and 4 hours, respectively.

Whole-Body Vibration

The protocol, given to both EVG and EG, consisted of static and dynamic tasks while standing on a WBV platform (PowerPlate, ® Power Plate International B.V., Badhoevendorp, The Netherlands). Tasks mainly involved lower extremities and included (a) static squat at 100° of knee flexion; (b) dynamic squat between 90° and 130° of knee flexion; (c) maintained ankle plantar-flexion with legs in extension; (d) flexo-extension of the right leg between 100° and 130° of knee flexion; (e) flexo-extension of the left leg between 100° and 130° of knee flexion; and (f) squat at 100° of knee flexion shifting the body weight from 1 leg to the other. For all tasks, subjects held onto the supporting bar. The 6 exercises (30 seconds each) were repeated 6 times with a recovery of 3 minutes between repetitions. For adaptation purposes, only tasks (a), (b), and (c) (repeated 3 times) were performed during the first 2 sessions.

For the EVG, the WBV intensity was kept constant at 30 Hz of frequency and 2 mm of amplitude; whereas for the EG, the apparatus did not produce vibrations. The intensity of vibration was chosen based on previous literature. Thirty (30) Hz has been shown to induce maximal muscular electrical activity. 30 Lower frequencies (i.e., 20 Hz) were not used because they evoke muscular relaxation, whereas higher frequencies (i.e., 50 Hz) were not employed because they can generate severe soreness in untrained individuals.10,31 The duration of WBV was 4.5 minutes per session for the first 2 sessions, and 18 minutes for the remaining 10 sessions. Thus, the total duration after completion of 6 weeks (12 sessions) was 189 minutes.

Statistical Analysis

Descriptive statistics were used to summarize the demographic characteristics of the subjects and one-way analysis of variance (ANOVA) was used to compare the demographic variables among the 3 groups. To test the effects of the treat- ments on FIQ, pain, fatigue, stiffness, and depression, a 3 x 2 (group xtime) mixed-design repeated-measures ANOVA was performed for each dependent variable. Simple main effects were evaluated if a significant interaction was elicited. Tukey’s Honestly Significant Differences (HSD) procedure was used when a significant F-ratio with more than 1 degree of freedom was found. All data are presented as mean ± standard error of the mean). For all statistical tests, the (alpha) level was set at 0.05. Statistical analyses were performed with SPSS v.15.0. (SPSS, Inc. Chicago, IL).
 
TABLE 1. DEMOGRAPHIC CHARACTERISTICS OF THE SUBJECTS
Results

Of 104 women, 36 were recruited for this study, and randomly and evenly distributed to 3 treatment groups (Fig. 1). One (1) subject from the EVG and 2 from the CG did not complete the 6-week questionnaire due to a no-show on testing day. Since repeated-measures analysis only allows for inclusion of those subjects who completed all measurements, those subjects with missing values were excluded from the analysis. The demographics of the subjects in each group are summarized in Table 1. There were no differences (p >0.05) among the groups for any of the demographic variables.

Both experimental groups, EVG and EG, adhered to the treatments 93% and 92%, respectively (100% adherence =all sessions attended). None of the subjects dropped out of the study. All patients reported they maintained the same medication prescription throughout the study (Table 2). This program neither exacerbated FM-related symptoms nor resulted in musculoskeletal injuries; however, 1 patient exhibited a mild anxiety attack on the first session of WBV. This patient responded normally for the remainder of the sessions.

Figures 2 and 3 illustrate the change in FIQ and pain, fatigue, stiffness, and depression, respectively, across time and treatment groups. As shown, significant 3 x2 (group xtime) repeated-measures ANOVA interactions were found for pain (F(2,30) =4.60; p =0.018) and fatigue (F(2,30) = 7.50; p = 0.002) but not for FIQ (F(2,30) = 2.93; p = 0.069), stiffness (F(2,30) = 2.09; p =0.142), or depression (F(2,30) = 0.43; p = 0.654). When examining the simple main effects of the significant interactions, pain and fatigue scores were significantly reduced from baseline in the EVG, but not in the EG or CG (Fig. 3). In addition, pain and fatigue scores after 6 weeks of treatment were significantly lower in the EVG compared to the EG and CG; the differences between the EG and CG, however, were not statistically different (p >0.05) (Fig. 3).
 
TABLE 2. COMORBIDITIES AND MEDICATIONS OF THE SUBJECTS
Discussion

The purpose of this study was to investigate the effectiveness of a 6-week traditional exercise program with supplementary WBV in improving healthFIG. 2. Fibromyalgia Impact Questionnaire (FIQ) scores before and after 6 weeks of randomized treatments (mean +/- standard error of the mean). *Significant main effect of time status, physical functioning, and main symptoms of FM. Although WBV has been shown to produce positive results in a variety of populations, this is the first study to investigate the effects of WBV on women with FM. We found that a 6-week traditional exercise program with supplementary WBV safely reduced pain and fatigue, whereas the effect of exercise alone was not evident in any of the parameters.

The characteristics of our population are similar to these reported in other FM studies. The mean baseline FIQ score in our patients is slightly higher than that reported in several FM studies, but similar to others.6,7 Importantly, our 0% dropout and 92%–93% treatment adherence rates are highly unusual in interventions including patients with FM, and distinguish this investigation from other FM studies.Because subjects were not reimbursed, this high adherence rate may be suggestive of patient’s satisfaction with the intervention.

Our results clearly suggest the positive effects of the traditional exercise program plus supplementary WBV intervention on pain and fatigue, whereas the effect of exercise alone is not apparent in any of the variables. The results from the EG are consistent with those obtained by Gowans et al.,4 who found no improvements in estimates of pain, fatigue, stiffness, and depression using FIQ after 6 weeks of a similar exercise program plus education therapy. Similarly, Wiggers et al.7 showed no improvements in pain, fatigue, sleep, and depression at week 7 of aerobic exercise therapy, and King et al.9 reported no changes in FIQ scores after 12 weeks of a comparable exercise therapy. In contrast, Redondo et al.24 found improvements in FIQ and fatigue scores (but not in pain, stiffness, and depression) following an 8-week program consisting of cycling, strength, and stretching exercises (5 sessions per week). Our findings for EG do not differ from those of other studies utilizing similar intervention duration, exercise protocols, and the same assessment techniques.

Although differences between EVG and EG in pain and fatigue were not statistically different, the significant difference between EVG and CG, but not between EG and CG, could illustrate that exercise plus supplementary WBV is more effective in improving pain and fatigue than exercise alone. Furthermore, the EVG, but not the EG or the CG, exhibited an improvement in pain and fatigue from baseline to week 6.
 
Pain, Fatigue, Stiffness, and Depression scores before and after 6 weeks of randomized treatments
There are some limitations to this study. The small sample size can be considered a limitation of the study; however, the presence of statistically significant interactions for pain and fatigue with a large effect size are indicative of a meaningful treatment effect. Nevertheless, it is likely that with inclusion of more subjects, a statistically significant interaction for FIQ could have been found. Moreover, in terms of the experimental design, the absence of a fourth treatment group consisting of WBV without exercise impeded the examination of the WBV effect solely; thus, it is unknown to what extent an intervention exclusively based on WBV elicits improvements in health status, physical functioning, and main symptoms of FM. However, WBV alone was not considered as a single therapy, but instead in combination with exercise, because the salutary effects of exercise in this population are well recognized and should not be neglected. This study was attempted to improve the efficacy of exercise as opposed to devising a potential substitute for exercise. Nevertheless, whether WBV strengthens the effect of the exercise therapy, produces improvements independently, or both, needs further investigation.

The significance of this study is threefold. First, we showed that a traditional exercise program with supplementary WBV safely reduces pain and fatigue, the most important symptoms in patients with FM, whereas exercise alone fails to induce significant changes. Second, the results of WBV are observed after only 6 weeks of intervention. Because patients with FM typically report an insufficient available time to invest with an exercise therapy, or complain about not attaining positive results in a quick manner, the WBV may be considered a novel strategy in the management of FM. Third, the present study opens a new line of investigation; further research should be focused on exploring the mechanisms by which WBV produces these positive results, and determining other potential benefits associated with WBV (i.e., muscle strength, hormonal changes, sleep quality). Also, a longer study with greater sample size and objective measures appears timely. Since the present study did not describe the long-term effects or the rate of change, further investigations with multiple follow-ups are also warranted.
 
Whole-body vibration training compared with resistance training:
Power Plate Studies
Objective:
The aim of this study was to evaluate the effect on spasticity, muscle strength and motor performance after 8 weeks of whole-body vibration training compared with resistance training in adults with cerebral palsy.

Methods:
Fourteen persons with spastic diplegia (21-41 years) were randomized to intervention with either wholebody vibration training (n=/7) or resistance training (n=/7). Pre- and post-training measures of spasticity using the modified Ashworth scale, muscle strength using isokinetic dynamometry, walking ability using Six-Minute Walk Test, balance using Timed Up and Go test and gross motor performance using Gross Motor Function Measure were performed.

Results:
Spasticity decreased in knee extensors in the wholebody vibration group. Muscle strength increased in the resistance training group at the velocity 30degree/s and in both groups at 90degree/s. Six-Minute Walk Test and Timed Up and Go test did not change significantly. Gross Motor Function Measure increased in the whole-body vibration group.

Conclusion:
These data suggest that an 8-week intervention of whole-body vibration training or resistance training can increase muscle strength, without negative effect on spasticity, in adults with cerebral palsy.


Key words: adults, cerebral palsy, exercise, spasticity, strength training, vibration.

INTRODUCTION

Cerebral palsy (CP) is an umbrella term for a group of motor impairment syndromes secondary to brain lesions in early stages of its development (1). The most common form is spastic diplegia (2). In this form both legs are more involved than the arms so that walking ability is affected. Andersson & Mattsson (3) investigated walking ability in adults with CP and found that 79% of those with spastic diplegia were able to walk with or without walking aids, but in 51% this ability had gradually decreased and 9% had stopped walking. One factor that could explain the impaired walking ability characterized by flexion in the knees and hips, is weakness of the quadriceps muscles (4). Strengthening of these muscles is therefore often a goal in the treatment of adults with CP. In 2 studies, 10 weeks of progressive resistance training (RT) were found to increase muscle strength in adults with CP (5, 6). Other studies on children with CP also reported good effect on muscle strength after RT

Another method for muscle strengthening that recently has been used on healthy persons is whole-body vibration (WBV) training. It is practised on a vibrating platform where the person is standing in a static position or moving in dynamic movements. The vibrations stimulate the muscle spindles and the alphamotoneurons, which initiates a muscle contraction according to the tonic vibration reflex (9). This reflex muscle contraction has been suggested to increase the synchronization of the motor units when combined with a voluntary contraction (9).

Several studies have investigated the long-term effect on muscle strength in healthy persons after WBV training with a variety of results. In one study by Torvinen et al. (10) isometric muscle strength and vertical jump height increased after 4 months of WBV training, but after 8 months of WBV training only vertical jump height increased but not muscle strength (11). The reason to the lack of increase in muscle strength after 8 months could be that the vibration intensity was too low to get further neuromuscular adaptation and that the control group also performed better in the repeated strength test. In the first study (10) the main increase in strength was seen after the first 2 months. De Ruiter et al. (12) could not find any improvements in muscle strength after 11 weeks of WBV training in healthy physically active students.

More marked effect on muscle strength after WBV training was shown when the training intensity was progressive. Delecluse et al. (13) compared WBV training with RT in a placebocontrolled study in young healthy women and found that WBV training could increase muscle strength to the same extent as RT. This was later confirmed in several studies including postmenopausal women.

The fact that people with CP could have benefits by muscle strengthening makes it interesting to find out if WBV training could be appropriate for this group. The aim of this study was to evaluate the effect on spasticity, muscle strength and motor performance after 8 weeks of WBV training compared with RT in adults with CP. If WBV training has a similar effect as RT it could be an alternative training method.

METHODS

Design

A prospective, randomized clinical trail was conducted with the alternatives WBV training or RT. The criteria for participation were that the individuals were diagnosed with spastic diplegia, were able to walk with or without walking aids and could understand and follow instructions. Subjects who had practiced RT during the last 6 months, had problems with pain, were taking medicine for spasticity or were pregnant were excluded.

Thirty-eight patients who had contact with the habilitation unit for adults at the Danderyd Hospital received information about the study. Fourteen patients (6 women and 8 men) agreed to participate. One of the patients was diagnosed hereditary spastic paraplegia, but was not excluded because he had similar disabilities as persons diagnosed with spastic diplegia.

The 14 participants were randomized to intervention with either WBV training or RT. The WBV group consisted of 7 persons (4 men and 3 women) with a mean age of 32 years. The RT group consisted of 7 persons (4 men and 3 women) with a mean age of 30 years. Individual data are presented in Table I. All participants were instructed not to alter their normal physical activities during participation in this study.

The study was approved by the Ethic Committee at the Karolinska Institute, Huddinge, Sweden.

Measurements

The tests were performed before and after the 8-week training period. All tests except the isokinetic strength test were performed by a physiotherapist who was not involved in the training procedure.

Spasticity.

Spasticity was estimated according to the modified Ashworth scale (17), which has 6 degrees (0, 1, 1+, 2, 3, 4). The muscle groups estimated were hip flexors, hip adductors, knee extensors, knee flexors and plantar flexors of the foot. The intra- and inter-rater reliability of the modified Ashworth scale has been considered good (17), but the validity has been shown to be insufficient to be used as a 6-point ordinal scale to measure spasticity (18). Though it is still the most commonly used scale and nothing better is available.

Isokinetic muscle strength.

Concentric and eccentric work and peak torque in quadriceps muscles were measured bilaterally by an isokinetic dynamometer (KIN-COM®124E Plus CHATTECX Corporation) at 2 different angle speeds (30degree/s, 90degree/s). Similar tests on subjects with CP showed to be reliable when using these angle speeds (19 -21). One practice session was performed one week before the actual test, to reduce the effect of learning (22, 23). One participant was excluded from this test because of his length and one participant was only able to test one leg. The position was sitting with hips in 80degree of flexion. Chest, pelvis and thighs were secured with straps. The resistance pad was positioned over the distal part of the lower leg. The range of motion during the test was from 90degree of knee flexion to almost full extension. The start force was 20 N and the gravity correction feature was eliminated (24). After 3 submaximal contractions and 2 minutes of rest, the patients were verbally encouraged to perform a maximal concentric contraction, rest 5 s, perform a maximal eccentric contraction and rest about 20 s before repeating the procedure. When 3 maximal efforts had been recorded a mean curve was saved (25). Isokinetic work was defined as the product of the mean torque and the range of motion (radians), for the part of the record where the torque exceeded zero. Peak torque was defined as the maximal force during the movement.

Walking ability

Walking ability was tested using the Six-Minute Walk Test (6MWT) (26, 27). One practice session was performed some days before the actual test, to reduce the effect of learning (28). The participants were instructed to walk back and forth in a hallway as far as possible for 6 minutes. Instructions and comments during the test were standardized using guidelines from the American Thoracic Society (27). The reproducibility of this test has been found to be good in patients with chronic heart failure (29) and in adults with CP (28).

Balance.

Balance in basic mobility manoeuvres was tested with the Timed Up and Go test (TUG) (30). The participants sat on a standard armchair and were instructed to get up and walk in a comfortable and safe pace to a line on the floor 3 metres away, turn around, return to the chair and sit down again. The time required to complete the task was recorded. One practice session was performed once before the actual test. Intra- and inter-rater reliability of TUG has been found to be good in frail elderly persons (30), in persons with unilateral limb amputation (31) and in persons with Parkinson’s disease (32). Another study on elderly persons showed poor test-retest reliability.

Gross motor function.

The gross motor performance was tested with the Gross Motor Function Measure (GMFM) (34). It consists of 66 items within 5 dimensions: (A) lying and rolling; (B) sitting; (C) crawling and kneeling; (D) standing; (E) walking, running and jumping. The items are scored using a 4-point scale (0, 1, 2, 3) and the scores are presented in percentages. In this study only dimensions D and E were assessed. The reliability and validity of the GMFM has been shown to be good in children with CP (34, 35).
 
Table I. Gender, age, Six-Minute Walk Test (6MWT), gross motor function level by Gross Motor Function Classification System (GMFCS),
Interventions

The WBV group exercised 3 times weekly during 8 weeks. Each session consisted of 5 minutes warming up, approximately 6 minutes of WBV training (rest included) and finished with a short program of muscle stretching. The WBV training was performed in a static standing position with hips and knees in 50degree of flexion on a device called NEMES-LSC (Nemesis BV, Hengelo, The Netherlands). The participants were instructed to avoid holding on to the handles if possible and to focus on standing with equal weight on both legs. The WBV training program was progressive and consisted of 11 different levels of intensity with a frequency of 25 -40 Hz (Table II). The choice of level was depending on the participant’s rating of perceived exertion on the Borg CR-10 scale (36). The level of intensity when the rating of perceived exertion was ‘‘7/very strong’’ was considered being appropriate for the training session. Medians and ranges of the training levels used in each participant was 8 (1 -10), 3 (1 -3), 3 (1 -3), 7 (1 -9), 3 (1 -6), 2 (1 -2) and 3 (1-5) (Table II).
 
Like the WBV group, the RT group exercised 3 times weekly during 8 weeks. Each session consisted of the same type of warming up and stretching, but instead of WBV training this group performed RT in a leg press device. Three sets of 10 -15 repetitions were performed with 2 minutes of rest in between. The program was progressive and the load was after the first or second week about 70% of 1 RM (repetition maximum) which means that the participant was able to complete as a maximum 7 to 10 repetitions (37). In the beginning of the training period the load was lower until the participants were able to control the movement. With the lower load 15 repetitions were performed

Statistics

Results are presented as median and range. Wilcoxon’s signed rank test was used to analyse differences over time and Mann-Whitney U test to analyse differences between the 2 groups. The statistical program JMP 3.2 was used. The level of significance was 0.05.

RESULTS

There were no significant differences, in any of the presented variables, between the WBV group and the RT group before the intervention period. All participants were present in at least 75% of the 24 training sessions. Medians for the presence of the participants in the WBV group and the RT group were 96% (79-100) and 92% (75-100), respectively. One participant in the RT group was exercising with lower load the 3 last weeks because of back pain.

Spasticity

Medians and ranges for estimated spasticity before and after 8 weeks of WBV training and RT are presented in Table III. Of the 5 tested muscle groups there was a significant reduction of spasticity in the knee extensors of the stronger leg in the WBV group (pB/0.04). In the RT group there were no significant changes.

Isokinetic muscle strength

Medians and ranges for concentric and eccentric muscle strength in the knee extensors of the participants’ weaker and stronger leg are presented as work and peak torque at the angle speed 30degree/s and 90degree/s in Table IV. Individual results of concentric work in the weaker leg at 90degree/s are illustrated in Fig. 1.

Concentric and eccentric work and peak torque increased (p<0.04) in the RT group’s weaker leg at the angle speed 30degree/s (Table IV). In the RT group’s stronger leg concentric work and peak torque increased (p<0.05) at the angle speed 30degree/s. In the WBV group the muscle strength did not increase significantly at the angle speed 30degree/s. When comparing the groups the increase of eccentric work and concentric peak torque, were higher in the RT group’s weaker leg (p<0.05). The increase of concentric work was also higher, but not significant, in the RT group’s weaker leg (p=0.051).

At the angle speed 90degree/s there was an increase of concentric (p<0.02) and eccentric (p<0.03) work and eccentric peak torque (p<0.04) in the WBV group’s weaker leg (Table IV, Fig. 1). In theRTgroup there was an increase of concentricwork in the weaker leg (p<0.04) and of concentric peak torque in the stronger leg (p<0.04). When comparing the groups there were no significant differences in the changes of muscle strength.

Six-Minute Walk Test (6MWT)

Medians and ranges for 6MWT before and after 8 weeks of WBV training and RT are presented in Table V. Values for 6MWT did not change significantly in any group after training.

Timed Up and Go test (TUG)

Medians and ranges for TUG before and after 8 weeks of WBV training and RT are presented in Table V. Values for TUG did not change significantly in any group after training. Medians and ranges for the GMFM before and after 8 weeks of WBV training and RT are presented in Table V. The total value for dimensions D and E increased in the WBV group (pB/0.04), but there was no significant increase in the RT group. When comparing the groups there was no significant difference in training effect between the WBV and RT group.
 
Table II. Program for the whole-body vibration (WBV) training. Frequency (Hz), duration and rest(s) of WBV training for the 11 levels

III. Results of spasticity estimated with the modified Ashworth scale (0, 1, 1/, 2, 3, 4) before and after 8 weeks of whole-body vibration (WBV) training (n/7) or resistance training (RT) (n/7).
DISCUSSION

The aim of this study was to evaluate effect on spasticity, muscle strength and motor performance after 8 weeks of intervention with WBV training compared with RT. There was a significant decrease in spasticity in the knee extensors in the WBV group after the intervention period. The result of other estimated muscle groups showed no significant decrease in spasticity. It is important to consider the insufficient validity of the modified Ashworth scale. According to Pandyan et al. (18) it is not possible to discriminate between scores 1, 1+/ and 2. Because of this aspect, we chose to conclude that the spasticity did not increase after any of the interventions. This is in line with previous studies on RT and spasticity in persons with CP (5, 38) and stroke (39, 40).

Isokinetic muscle strength increased in both intervention groups, but not in every parameter that was tested. We tested concentric and eccentric muscle contraction on both legs in 2 different angle speeds. The results were presented as both work and peak torque. Accordingly, the strength test had 8 parameters on each angle speed. At the slow angle speed the RT group showed increasing muscle strength in 6 parameters while there was no increase in the WBV group. At the rapid angle speed the WBV group showed significantly increasing muscle strength in 3 parameters and in 2 that was almost significant (p<0.055) while the RT group showed increasing muscle strength in only 2 parameters. It is interesting that WBV training might have an effect on strength at rapid movements. This is in accordance with previous studies which have shown effects on explosive muscle strength in vertical jumps and in isokinetic testing at angle speeds >90degree/s

The WBV group performed static exercises and therefore it had been appropriate with a measure of isometric strength. It is interesting that the WBV group, however, showed an increase in isokinetic strength. It is possible that this increase had been more marked if the WBV training was performed with dynamic movements similar to movements used in the leg press device.
 
Table IV. Isokinetic concentric (conc) and eccentric (ecc) muscle strength, at the angle speed 308/s and 908/s, presented as work (J) and peak torque (Nm), before and after 8 weeks of whole-body vibration (WBV) training and resistance training (RT). The participants’ weaker and stronger legs are separated
Finally, we wanted to evaluate effects on motor performance, which were walking ability, balance and gross motor function. Walking distance in 6 minutes and balance in walking and turning did not change in any intervention group. The gross motor function increased significantly only in the WBV group. In previous studies on RT in persons with CP (5, 7) the same tests were used (6MWT, TUG and GMFM) and showed increased motor performance. In those studies the RT program consisted of 10 and 20 different exercises compared with only one in this study. It is probably necessary in both WBV training and RT that the training program is more extensive to obtain an effect on motor performance. It should be longer and consist of different types of exercises. Compared with 6 minutes of WBV training in one position in this study, other studies report increasing muscle strength after 20 minutes of WBV training in static and dynamic knee extensor exercises (13-16). In further studies it would be better with a more vigorous training program to get enough overload of the muscles to obtain a more marked effect on muscle strength and motor performance.
Fig. 1. Isokinetic concentric work (J) in the weaker leg, at the angle speed 908/s, before and after 8 weeks of whole body vibration (WBV) training (n/7) and resistance training (RT) (n/5).
Based on the subjective reports of the participants, negative side-effects did seldom occur in the WBV group. One participant was very stiff in her legs in the evening after training. In the RT group negative side-effects were more common. Muscle stiffness was on 3 occasions so bad that the participants chose to cancel the following exercise session. Another participant had problems with back pain at the beginning of the training period caused by uneven loading when performing the leg press exercise. Even if WBV training had few negative side-effects it was not free from risks. For some participants who had difficulties in standing with equal weight on both legs, the feet could slide off the vibration platform. It is therefore important that persons with motor disabilities have someone who is supervising and prepared to stabilize them if this is about to happen.
 
Table V. Median values of Six-MinuteWalk Test (6MWT), Timed Up and Go test (TUG) and Gross Motor Function Test (GMFM) dimensions D, E and total (T), before and after 8 weeks of whole-body vibration (WBV) training or resistance training (RT). Significant differences are marked with bold type
In conclusion, the data in this study suggest that 8 weeks of intervention with WBV can increase muscle strength during rapid movements and increase gross motor performance without negative effects on spasticity. The data also suggest that intervention with progressive RT can increase muscle strength at slow and rapid movements without negative effect on spasticity. Walking distance in 6 minutes and balance in basic mobility manoeuvres did not change significantly in any intervention group. When comparing the groups after the intervention period, there were no significant differences in changes of spasticity, muscle strength or gross motor performance.
 
Optimal frequency, displacement, duration, and recovery patterns
Power Plate Studies
ABSTRACT

Power is an important component of general health, fitness, and athletic performance. Traditional overload techniques require considerable time, intensity, and volume of training. Whole-body vibration (WBV) is a potentially less time-consuming method for increasing power performance than traditional training. However, the exact protocols that can maximize power output have not yet been identified. Eleven healthy men, aged 32.3 ±4.1 years, and 9 healthy women, aged 29.1 ±3.5 years, performed countermovement jumps (CMJs) of maximal volition to assess peak power pre and post (immediately and at 1, 5, and 10 minutes) randomized WBV stimuli set at different frequency (30, 35, 40, and 50 Hz), displacement (2–4 vs. 4–6 mm), and duration (30, 45, and 60 seconds) combinations. Repeated-measures analysis of variance on peak power normalized to initial power (nPP) revealed no significant effects attributable to duration of stimulus. However, high frequencies were more effective when combined with high displacements, and low frequencies were more effective in conjunction with low displacements (p <0.05). Additionally, the greatest improvements in nPP occurred at 1 minute posttreatment, with significant improvements lasting through 5 minutes posttreatment (p <0.05). Optimal acute effects can be attained using as little as 30 seconds ofWBV, and they are highest from 1 to 5 minutes posttreatment. Additionally, high frequencies were most effective when applied in conjunction with high displacements, whereas low frequencies were most effective when applied in conjunction with low displacements.

 
KEY WORDS
 
WBV, protocols, countermovement jumpimmediate effects
 
INTRODUCTION
 
Power is an important component of athletic performance. In fact, power training techniques are currently used to prepare athletes for such diverse sports as hockey (15), skiing (27), baseball (16), and football (14). Additionally, power is a major component of general health, fitness, and independence (2,18). Overload techniques such as weight training (24) and plyometrics (29) have traditionally been used to improve power (1,17,35), and specific protocols using thesemethods to optimize power have been studied extensively (1,17,19). Although the benefits of these traditional training methodologies are well established (14–16,21), they are often quite time consuming and require considerable training intensity and volume, making them unattractive to the majority of the population seeking to improve their health and fitness. In addition, time constraints placed on collegiate and professional athletes by controlling agencies and competitive schedules also argue for the development of more effective and efficient training techniques.
 
Within the past 25 years, a relatively new method of neuromuscular overload, whole-body vibration (WBV), has been slowly emerging that may address these concerns. Recent research suggests that mechanical vibrations that incorporate low amplitudes and frequencies are a safe and effective method of exercising the neuromuscular system (6). In fact, mechanical vibrations have been shown to induce nonvoluntary muscle contractions (20), so the application of vibration to a limb or entire body may be an appropriate treatment for sport training (20). Whole-body vibration is starting to be used as an alternative form of strength and power training (6,30,32). It elicits neuromuscular training in a short time period without a great deal of effort (30).
 
Many studies have reported acute (10,11,22,30,31,36) and chronic (6,9,12,26,32,37) increases in power after WBV training; however, a limited number of studies have reported no effect (9) or even a decline (30). The divergent results in these studies may have been attributable to the variations in the frequency, displacement, and duration of the WBV stimulus applied as well as variations in populations tested and WBVdevices. Target populations included men (10,33), women (32), and trained (3–5,10,12,21,22,27,30) and untrained (11,32) individuals. Because of the varied target populations, a specific group most sensitive to the acute effects of WBV has not yet been identified.
 
We have selected healthy untrained adults in a low-risk age group because we feel that this is an important target group, given their propensity to use exercise as a fitness and wellness tool.
 
Among the devices examined are triplanar devices that apply the vibratory stimulus in anterior/posterior, lateral, and vertical directions and uniplanar devices that either tilt on a central axis or move solely in a vertical direction.
 
Potential Mechanisms
 
The exact mechanism that regulates how the body reacts to vibratory stimulus is currently unknown; however, several potential mechanisms have been proposed.

Neuromuscular Facilitation
 
Initial strength and power gains during weight training are attributable to neuromuscular facilitation (32). The neuromuscular facilitation that maximizes muscular performance during weight training has also been shown with WBV training (6). Improvements in strength and power may be attributable to neural factors including increased recruitment, synchronization, muscular coordination, and proprioceptor response (6).
 
Tonic Vibration Reflex

Mechanical stimuli are transmitted from the vibration device through the body, where they stimulate sensory receptors, most likely muscle spindles. This activates alpha motoneurons, initiating reflexive muscle contractions—this is referred to as the tonic vibration reflex (6,13,32). Monosynaptic and polysynaptic pathways mediate this response, resulting in an increased activation of motor units (33). The tonic vibration reflex is probably dependent on the frequency of vibration, muscle length, and body position (23). Voluntary muscle contractions may be enhanced by the tonic vibration reflex when used in conjunction with strength-training protocols (23).
 
Increased Gravitational Forces on Muscle
 
Under normal gravitational conditions, muscles can maintain their performance. During conditions of decreased gravitational load, microgravity, a decrease in force capability, occurs. An increase in gravitational load, hypergravity, increases the cross-sectional area and force-generating capability of the muscles (6). Mechanical vibrations applied to the whole body can produce changes in gravitational conditions during the intervention (4,6). The vibrations produce fast and short changes in the length of the muscletendon complex. Sensory receptors, probably muscle spindles, call on a reflex muscular activity in an attempt to dampen the vibratory waves (6). Additionally, conditions of hypergravity have been shown to increase hormone levels, including androgens and growth hormones in the blood (6).
 
Types of Whole-Body Vibration Devices
 
The effects of WBV have been studied using vibrating plates that produce sinusoidal vibrations (7). During WBV training, the subject may stand or move on a platform that generates vibrations with frequencies ranging from 25 to 40 Hz and amplitudes from 2 to 10.5 mm (13,32). This study uses a triple-plane WBV device (Power Plate). The platform simultaneously oscillates up and down, left to right, and front to back. The potential amplitudes are low (2–4 mm) and high (4–6 mm). The frequency options for this device include 30, 30, 40, and 50 Hz.
 
A key reason for inconsistencies in scientific data regarding the effects of WBVmay be that protocols vary from study to study. Different frequencies and amplitudes have been applied to different populations with varying recovery periods. Each of these parameters has the potential to impact biological response to vibration training and, therefore, the effects of vibration training on strength and power performance.
 
In exercises geared toward increasing strength and power, the specific loading parameters must be carefully determined, applied, and controlled to ensure the desired training effect. Application of different training protocols would yield varying results, just as varying vibration training protocols is likely to yield variable physiological responses (23).
 
Whole-body vibration is currently used as an exercise program in many fitness and rehabilitation facilities, but the current knowledge on effective exercise protocols is limited (7). Currently, the sport industry is producing various devices for WBV; therefore, it is important to determine appropriate training protocols (20).
 
Although studies using WBV have been shown to elicit positive acute and chronic responses on strength and power, the exact intensity, duration, and postexercise recovery time that would optimize these benefits are not known. For WBV to be an effective and efficient tool for increasing muscular performance, the appropriate protocols for targeting muscular strength and muscular power must be established (10).
 
Once effective protocols are developed, WBVmay provide a less time-consuming alternative for improving power compared with traditional training methods (6,26). In addition, it has been shown that perceived exertion is lower during WBV compared with standard training techniques eliciting similar results 12,13,20,25). This may encourage sedentary individuals to participate in an exercise program and add to the options currently available to both recreationally active individuals and athletes seeking to improve their health, fitness, and performance.
 
To our knowledge, no work has examined the frequency, displacement, and work-recovery duty cycle that would maximize power development during WBV, and there has been little research examining the duration of the acute effects of WBV (10). Additionally, there is a paucity of information addressing the acute effects of triplanar WBV, such as that provided by the Power Plate vibration platform (Power Plate North America Inc., Northbrook, Ill), on the power of healthy untrained adults. Therefore, the objective of this study was to identify the WBVprotocols that would elicit the greatest improvement in power performance after a single exposure and to determine the duration of these effects.
 
METHODS

Experimental Approach to the Problem
 
Exposure to WBV has been shown to elicit variable power output changes immediately after WBV exposure. The divergent results in these studies may have been caused by the variations in the frequency, displacement, and duration of the WBVstimulus applied as well as variations in populations tested andWBVdevices. The goal of this study was to identify the immediate effects of specific combinations of frequency, displacement, and duration of WBV stimulus on the power output of a healthy, untrained, low-risk population. In accordance with Sayers et al. (34), we felt that a jump pad was appropriate to assess power because we were monitoring pre-post exposure change in power. Each subject served as his or her own control. Additionally, jump pads have been shown to be highly correlated with 3D jump height analysis, with a Pearson r of 0.967 (25).
 
Subjects
 
Twenty-two untrained individuals, 23–39 years of age, volunteered to participate in this study. Two subjects withdrew from the study—one because of time constraints, the other because of discomfort during WBV treatment. Characteristics of the 20 subjects who completed the study are presented in Table 1. Exclusion criteria included any chronic medical condition or medications that could affect skeletal muscle performance, or any contraindications to WBV use. Contraindications included unhealed fresh wounds, serious heart or vascular disease, recent hip or knee replacement, pregnancy, acute hernia, discopathy, spondylolysis, severe diabetes, epilepsy, tumors, acute inflammation, acute migraine, pacemaker or recently placed IUD, or fixation devices such as metal pins, bolts, or plates. This study was approved by the subcommittee for the use and protection of human subjects at the University of Miami.
 
Procedures
 
We developed specific protocols using different combinations of frequency (30, 35, 40, and 50 Hz), displacement (low, 2–4 mm; or high, 4–6 mm), and work duration (30, 45, and 60 seconds). Before testing, we randomized the protocols for each subject to reduce the potential for an order effect. Subjects came to the laboratory for 9 visits. During the first visit, each subject completed a health status questionnaire to confirm study eligibility. If inclusion criteria were met, procedures and risks were thoroughly explained to the subject, and his or her written informed consent was obtained. During this visit, the subject was also familiarized with WBV platform use. For this familiarization session, the subject stood on the plate in a half-squat position with the knees held at a 2.27-rad angle. The exposure time was 30 seconds at a frequency of 30 Hz and low displacement. The subject was also familiarized with the hands-on-hips countermovement jump (CMJ) used to assess power.
 
The treatment protocols used during days 2–9 are presented in Table 2. On each of the testing days, subjects completed the 3 randomized treatment protocols, which were determined during the initial randomization process. Subjects were instructed to stand in a half-squat position with the knees shoulder width apart. The knee angle was set at 2.27 rad using a handheld goniometer. A minimum of 24 hours and a maximum of 1 week separated testing days.
 
Testing
 
Subjects were instructed to refrain from training for 24 hours before testing. The impact of the specific WBV protocols on peak leg power was assessed using the CMJ to determine power (7,16). The starting position for the CMJ was feet shoulder width apart, hands on the hips, and knees at 2.27 rad. Subjects were instructed to keep hand position constant and were encouraged to give a maximal effort for each jump. On hearing the word ‘‘go,’’ the subject performed 3 jumps with a slight delay between jumps to reduce the potential impact of the previous jump on stored elastic energy. All jumps were performed on a pressure-sensitive mat interfaced with a laboratory computer containing an assessment program (Axon Bioingeneria Deportiva, version 2.01, 2005). The Bioingeneria program used time off the mat to compute jump height. We selected the CMJ because it measures the combined effects of contractile, neural, and elastic elements, each of which could have adapted acutely to the WBV stimulus.
 
TABLE 1. Subject characteristics (mean 6 SD).
 
Power for the 3 jumps was computed using the formula presented by Sayers et al. (34):
 
PP (W) = 51:9xCMJ height (cm) +48:9xBM(kg) -2007 where PP = peak power and BM = body mass.
 
This formula was specific to the CMJ and has been successfully used to compute power for the hands-on-hips jump technique employed in our study (8). The highest of the 3 jump heights recorded was used to calculate the power used for statistical analysis. Because each subject served as his or her own control to compare pre- vs. post-WBV jump heights, we determined jump technique consistency through visual observation.
 
 
Whole-body vibration (WBV) treatment protocols.
 
Because the effects of WBV are extremely time sensitive (3,10,36), CMJ data were collected immediately after and at 1, 5, and 10 minutes after each WBV bout. Subjects were instructed to remain seated after each testing session. This procedure allowed comparison of the effects of each protocol over time. All testing sessions were preceded by a 5-minute warm-up on a cycle ergometer at 50 W.

Statistical Analyses
 
The response variable for this study was the peak leg power achieved by the subject. This was calculated usingCMJ height and body mass (34). The peak power score was then normalized as a percentage of the pretest jump score (nPP) according to the method of Cormie et al. (10) to account for intersubject variation. This was necessary because our subject population consisted of men and women of varying athletic abilities.
 
Arepeated-measures analysis of variance was used to assess the impact of frequency, displacement, work duration, and recovery time on nPP. The criterion alpha level was set at p # 0.05. This analysis revealed that the greatest increase in power occurred at 1 minute posttreatment, no significant differences were identified as attributable to duration, and there were no interactions among other independent variables and duration; for these reasons, we performed a second repeated-measures analysis using frequency, displacement, and time (pretest and 1 minute posttreatment) as the independent variables. Bonferroni tests for multiple comparisons were used for all post hoc analyses.
 
RESULTS
 
Repeated-measures analyses revealed a significant time 3 displacement 3 frequency interaction (p <= 0.018). The graphs for the low-displacement (Figure 1) and highdisplacement (Figure 2) conditions illustrate that increases in nPP were greatest for the low-displacement conditions at lower frequencies and at high displacement for highfrequency conditions. Post hoc tests revealed significantly different patterns of change between pretest and immediately posttreatment and between immediately posttreatment and 1 minute posttreatment for low vs. high displacements (p = 0.015 and 0.044, respectively). Trends were also seen for patterns of change from immediately posttreatment to 1 minute posttreatment and from 5 minutes posttreatment to 10 minutes posttreatment for high vs. low displacements (p = 0.062 and 0.056, respectively). Actual PP values are provided in Table 3.
 
 
Figure 1. Countermovement jump (CMJ) peak power normalized to baseline values for various frequencies across time at low amplitude.
 
Our analysis also revealed a significant frequency 3 displacement interaction (p <= 0.012). Post hoc analysis identified that the source of this significant difference was the pattern of change from 40 to 50 Hz, with the highdisplacement condition showing an increase in performance and the low-displacement condition showing a decrease (p # 0.038) (see Figure 3).
 
Finally, the analysis showed a significant effect by time (p <= 0.001). Post hoc analyses showed significant differences at various time points. Figure 4 illustrates that significant increases in nPP were seen between pretest values and those recorded at 1 and 5 minutes posttreatment. The figure also shows that there was a significant decline in nPP by 10 minutes posttreatment.
 
Figure 2. Countermovement jump (CMJ) peak power normalized to baseline values for various frequencies across time at high amplitude.
 
 
 
The results of our analysis removing duration as an independent variable revealed a time 3 frequency 3 displacement interaction (p <= 0.002). Post hoc analysis demonstrated that, at low displacement, there was a decline in nPP when the 30-Hz condition was compared with 40 Hz, whereas high displacement showed the opposite pattern.
 
This analysis also showed a timexdisplacement interaction, with high displacements producing greater nPP values than low displacements (p <= 0.05).
 
DISCUSSION
 
Three significant findings resulted from this investigation. First, an interaction between time, frequency, and displacement was detected, where higher displacements elicited greater power increases at higher frequencies, whereas lower displacements elicited greater power increases at lower frequencies. Our second finding was that an acute bout of WBV led to a transient increase in power that peaked at 1 minute posttreatment, remained significantly elevated at 5 minutes posttreatment, and declined below significant levels by 10 minutes posttreatment. Finally, we found that varying the duration of exposure within the range of 30–60 seconds had no impact on subsequent power measurements.
 
To our knowledge, these are the first data that have shown a frequency-displacement relationship; however, the results from previous studies that examined Power Plate and other devices support our results to some degree. It should be recognized that these comparisons are confounded by the use of a vast array of frequencies, displacements, training protocols, and WBV devices. The Power Plate device used in this study vibrated in 3 planes and provided low (2–4 mm) and high (4–6 mm) displacement. In contrast, studies using the Galileo platform applied displacements as high as 10 mm (4,8,10,29), but this platform oscillates by pivoting laterally about a central axis. Additionally, 2 studies examined vertical vibration devices using frequencies from 24 to 40 Hz and displacements of 2–4 mm (5,38).
 
 
TABLE 3. Actual peak power values for various frequencies at low and high amplitudes.

 
In the only study evaluating the impact of a single bout of WBV on explosive power using a Power Plate, Cormie et al. (10) found that applying low-frequency (30 Hz) and low-amplitude (2.5 mm) WBV for 30 seconds significantly increased normalized CMJ height immediately after treatment. Roelants et al. (32) has reported a significant increase in muscle activity and force production after a 30-second bout at 2.5 mm and 35 Hz on the Power Plate. The results of these studies support our finding that matching low frequencies with low displacement can positively affect neuromuscular performance. However, they provide no comparisons with other frequency and displacement interactions.
 
The results from studies using horizontal displacement WBV devices can also be compared with our results; however, it should be recognized that these comparisons are affected by the differences in the patterns of displacement. In a study using a Kuntatory vibration platform at a displacement of 2 mm and frequencies of 25, 30, 35, and 40 Hz for each minute of a 4- minute protocol, Torvinen et al. (38) found no significant increase in vertical jump measured at 2 and 60 minutes after exposure. Because the final minute of the protocol employed minute protocol, Torvinen et al. (38) found no significant increase in vertical jump measured at 2 and 60 minutes after exposure. Because the final minute of the protocol employed
 
The results of studies examining the acute impact of WBV on the Galileo platform are more difficult to compare with our results. The Galileo works by tilting on a central axis to produce a vibratory stimulus. Because this is quite different from both the Power Plate and the vertical displacement devices reviewed above, the frequencies and displacements used also differ considerably. The frequencies range from 5 to 30 Hz, and the displacements range from 0 to 13 mm. In one study, Rittweger et al. (31) examined the effect of loaded squatting exercises on a Galileo WBVplatform at a frequency of 26 Hz and a displacement of 6 mm. The stimulus was applied until exhaustion as assessed by each subject’s rating of perceived exertion. Durations ranged from 150 to 500 seconds. This protocol produced no changes in serial jump heights or isometric knee extension. In a second study, Rittweger et al. (30) examined the impact of WBV at a frequency of 26 Hz and a displacement of 11 mm. Once again, the subjects remained on the platform until selfreported fatigue. The durations of these exposures ranged from 200 to 475 seconds. This protocol produced a significant decline in CMJ performance. Finally, de Ruiter et al. (11) examined the impact of WBV at 30 Hz and 8-mm displacement for five 60-second repetitions with a 120-second recovery interval. This protocol produced no change in CMJ performance. The lack of acute improvements using the Galileo may be specific to this mode of vibration, or it may reflect the exhaustive nature of these protocols. Regardless of the underlying causes, comparing frequency/displacement patterns between WBV on the Galileo and the Power Plate seems inappropriate.
 
 
Figure 3. Countermovement jump (CMJ) peak power normalized to baseline values showing different patterns of change for high and low amplitudes across 40 and 50 Hz.
 
As indicated in the summary above, our second finding was that an acute bout of WBV led to a transient increase in power that peaked at 1 minute after WBVand declined to baseline levels by 5 minutes posttreatment. This finding is in agreement with the results of a number of studies that examined acute responses to WBV (5,10,21). For example, Cormie et al. (10) found a significant increase in CMJ height immediately after WBV (30 Hz, low displacement, 30 seconds), but this increase fell below baseline by 5 minutes posttreatment (10). Bosco et al. (4) also found that a single vibration bout (26 Hz, 10 mm) showed temporary increases (lasting 10 minutes) in muscle average force, average velocity, and average power with all loads used on the treated leg. This resulted in an increase in both velocity and power post WBV for the treated leg at all external loads tested during the leg press test (4). The control legs showed no changes from pre to post WBV tests (4). These results support our findings that there is a limited window during which power performance will be maximized after an acute bout of WBV. Unfortunately, they do not allow comparison with our findings that this window seems to be maximized 1 minute after the stimulus ends.
 
 
Figure 4. Changes in countermovement jump (CMJ) peak power over time, normalized to baseline values.
 
Finally, we found that varying the duration of exposure within the range of 30–60 seconds had no impact on power. Once again, comparisons with other studies are difficult, because acute responses have not been previously compared within the 30- to 60-second range; however, results of previous studies do lend support to our findings. Bosco et al. (4) found that 10 WBV exposures lasting 60 seconds with 60 seconds of recovery between exposures on a Galileo increased average power as well as peak power in women volleyball players. This WBV treatment was applied at a frequency of 26 Hz and displacement of 10 mm. Cormie et al. (10) found significant increases in CMJ height immediately after a 30-second exposure to the triple-plane WBV device, Power Plate, at a frequency of 30 Hz and 2.5-mm displacement. The fact that both studies reported significant improvements when the duration of the WBV varied by 30 seconds supports our findings.
 
In contrast, prolonged exposures toWBVhave been shown to elicit power decrements (11,30). Rittweger et al. (30) found decreased power after exhaustive exposures (200–475 seconds) on a Galileo WBV platform at a frequency of 26 Hz and a displacement of 11 mm with additional external loads between 35 and 40% of body weight. De Ruiter et al. (11) reported a significant decrease in both maximum voluntary contraction and electrically stimulated maximum force-generating capacity of the knee extensors after five 1-minute bouts on a Galileo 200 at 30 Hz and 8-mm displacement. These declines in performance may have been the result of fatigue resulting from the application of WBV until exhaustion or an ineffective work/recovery duty cycle. We suggest that our data indicating similar responses with WBVbouts of 30, 45, and 60 seconds, and our data indicating transient power increases lasting between 1 and 5 minutes, may help when designing WBV programs to maximize power gains.

PRACTICAL APPLICATIONS
 
Short bouts ofWBVused for warm-up before explosive efforts have been shown to improve neuromuscular performance (3,22). Therefore, WBV may prove effective as a precompetition or pretraining warm-up activity for both individual and team sports where power is a dominant factor. However, our results indicate that if WBV is to be used acutely as either a precompetition or pretraining warm-up, coaches should consider combining high displacements with high frequencies or low displacements with low frequencies. Additionally, preparatory bouts should be temporally positioned so that they precede the competitive performance by 1–5 minutes. The fact that we found no differences between 30, 45, and 60 seconds of stimulation indicates that the time required to generate a positive impact on performance is relatively short, but that durations of up to 1 minute will still generate positive results.
 
The greater time efficiency and lower training volume and intensity associated with WBV compared with traditional training methodologies may make it an effective training tool for athletes (6,27), especially during tapers when reduced volume and intensity are used to prepare athletes for competition (24). Whole-body vibration may be an effective tool for providing the benefits of a taper while further stimulating neuromuscular adaptations for power. Our data also suggest that WBV may be an effective warm-up preceding plyometric training.
 
We suggest that these results be considered when designing the proper work-recovery duty cycles when WBV is used as a training tool to maximize power; however, further studies must be conducted to determine the optimal duration of recovery, the optimal number of cycles, and the frequency and duration interactions necessary to maximize power gains in both competitive athletes and other special populations, whose responses may vary because of age, gender, body composition, and training status.
 
 
Plantar vibration improves leg fluid flow in perimenopausal women
Power Plate Studies
Plantar vibration improves leg fluid flow in perimenopausal women. Am J Physiol Regul Integr Comp Physiol 288: R623–R629, 2005. First published October 7, 2004; doi:10.1152/ ajpregu.00513.2004.—Recent studies have indicated that plantarbased vibration may be an effective approach for the prevention and treatment of osteoporosis. We addressed the hypothesis of whether the plantar vibration operated by way of the skeletal muscle pump, resulting in enhanced blood and fluid flow to the lower body. We combined plantar stimulation with upright tilt table testing in 18 women aged 46–63 yr. We used strain-gauge plethysmography to measure calf blood flow, venous capacitance, and the microvascular filtration relation, as well as impedance plethysmography to examine changes in leg, splanchnic, and thoracic blood flow while supine at a 35° upright tilt. A vibrating platform was placed on the footboard of a tilt table, and measurements were made at 0, 15, and 45 Hz with an amplitude of 0.2 g point to point, presented in random order. Impedance- measured supine blood flows were significantly (P =0.05) increased in the calf (30%), pelvic (26%), and thoracic regions (20%) by plantar vibration at 45 Hz. Moreover, the 25–35% decreases in calf and pelvic blood flows associated with upright tilt were reversed by plantar vibration, and the decrease in thoracic blood flow was significantly attenuated. Strain-gauge measurements showed an attenuation of upright calf blood flow. In addition, the microvascular filtration relation was shifted with vibration, producing a pronounced increase in the threshold for edema, Pi, due to enhanced lymphatic flow. Supine values for Pi increased from 24 ±2 mmHg at 0 Hz to 27 ±3 mmHg at 15 Hz, and finally to 31 ±2 mmHg at 45 Hz (P <0.01). Upright values for Pi increased from 25 ±3 mmHg at 0 Hz, to 28 ±4 mmHg at 15 Hz, and finally to 35 ±4 mmHg at 45 Hz. The results suggest that plantar vibration serves to significantly enhance peripheral and systemic blood flow, peripheral lymphatic flow, and venous drainage, which may account for the apparent ability of such stimuli to influence bone mass.
 
plantar vibration; orthostasis; blood flow; lymphatic flow; venous return
 
THE IMPORTANCE OF BLOOD and interstitial flow in the maintenance of bone mass has long been suggested, although difficulties in studying the interstitial fluid flows in bone has slowed progress in this area. In the mid-1960s, Keck and Kelly (18) demonstrated that increased bone growth was associated with increased venous pressure. These observations led to studies of interstitial flow in bone and to the demonstration of lymphatic vessels in bone directed from the marrow to the periosteal surfaces (36). Subsequently, Kelly’s group (19) showed that high venous pressure encouraged bone formation. Later, this same group demonstrated that high venous pressure was associated with increased venous filtration (20). McDonald and Pitt Ford (23) demonstrated that an important effect of mechanical loading was the significant alteration of blood flow in bone. The influence of increased venous pressure and increased filtration on interstitial and lymphatic flow has been confirmed in a rat hindlimb suspension model of microgravity (2). Most recently, Colleran et al. (4) have shown that decreased lower limb perfusion results in decreased cancellous bone formation as well as reduced periosteal bone.
 
Results from recent studies indicate that a relatively small vibrational stimulus applied to the plantar surface of standing subjects is capable of inhibiting bone loss or even increasing bone mass (34, 43). The effects reported in these studies are similar in magnitude to those obtained in pharmacological trials of similar duration (28) and, therefore, may have clinical implications. At this time, however, the mechanism of action of this plantar stimulation remains unclear, making it difficult to rationalize its use. Because the stimulus magnitude utilized in these studies is so low, it is unlikely that the observed effects are a direct result of the mechanical strain induced into the tissue (30). An alternative mechanism is that the bone tissue response is secondary to vibrational stimulation of postural mechanisms and the skeletal muscle pump effects on blood and lymphatic flow.
 
Such a hypothesis is reasonable because loss of bone mass, whether due to aging, bed rest, or spaceflight, is consistently associated with decreased postural musculature. As early as the 1960s, Issekutz et al. (17) demonstrated that bone loss and postural muscle atrophy associated with immobilization or bed rest in young subjects could be inhibited by having study subjects stand quietly for six 30-min sessions per day. This strategy does not work with older subjects (34), for whom quiet standing appears to result in decreased bone density. Standing increases the arterial and venous hemostatic pressure in the lower limbs, causing increased venous pooling and microvascular filtration. Although vasoconstriction modifies this to a degree, the assistance of the skeletal muscle pump is required to effectively return blood and lymph to the heart. On the one hand, the skeletal muscle pump in young individuals is usually healthy, maintaining venous and lymphatic return and aiding in orthostatic tolerance (42). On the other hand, data show that with aging there is a reduction in postural skeletal muscle activity associated with reduction in type IIa fibers and reduction in the efficacy of the muscle pump (16). This reduces venous and lymphatic return during quiet standing in older individuals and reduces the benefit on bone perfusion of hydrostatic pressure. Type IIa fibers contract at rates in the range of 20–70 Hz. We proposed that plantar vibration at low levels at frequencies similar to the contraction frequency of type IIa fibers may enhance bone growth and inhibit bone loss through its influence on the skeletal muscle pump (34, 35).
 
On the basis of these observations, we developed the hypothesis that plantar vibration should have a significant effect on lower limb blood flow and lymph flow, particularly in older women at greatest risk for osteoporosis. Our aim was therefore to study perimenopausal women.
 
To test this hypothesis in perimenopausal women, we combined vibrational stimulation of the plantar surface with upright tilt table testing while examining blood flow and fluid flow parameters in the lower extremities.
 
MATERIALS AND METHODS
 
Subjects.
 
We screened consecutive female subjects aged 45–70 yr who were enrolled in a general internal medicine practice. Subjects were excluded who had a current fracture of the lower appendicular or axial skeleton, history of back pain (which could be exacerbated by the vibration protocol), known peripheral vascular disease, peripheral neuropathy, uncontrolled hypertension (systolic blood pressure >150 mmHg or diastolic blood pressure >95 mmHg despite treatment), congestive heart failure, diabetes, liver or kidney failure, hyperparathyroidism, multiple myeloma, metastatic carcinoma, Cushing syndrome, collagen vascular disease, chronic angioedema or lymphedema, uncontrolled hyperthyroidism, chronic substance abuse, or any condition precluding the subject following the protocol or providing informed consent. Subjects with excessive alcohol use (>2 drinks/ day) or who smoked were also excluded. Written informed consent was obtained, and all protocols were approved by the Committee for the Protection of Human Subjects (Institutional Review Board) of New York Medical College.
 
Laboratory evaluation.
 
All experiments started at 9 AM after a brief fast (4 h). Morning medications were withheld. The right arm and right calf blood pressure were monitored intermittently by oscillometry. A vibrating plate (see below) was placed on the footboard of an electrically driven tilt table (Cardiosystems 600, Dallas, TX). Subjects wore rubber-soled shoes to ensure electrical isolation and were asked to lie supine with their feet flush with the plate, which initially was not oscillating. We designated this situation “0 Hz.” Subjects were instrumented to measure blood flow by two forms of measurement: mercury in Silastic strain-gauge phlethysmography (SGP) with venous occlusion congestion cuffs, and impedance plethysmography (IPG). Occlusion cuffs were placed around the lower limb 10 cm above a strain gauge of appropriate size attached to a Whitney-type strain gauge plethysmograph. These are explained further below. Ag/AgCl ECG electrodes for IPG were attached to the left foot and left hand, which served as current injectors, and in pairs representing anatomic segments as follows: ankle to upper calf just below the knee (the calf segment), knee to iliac crest (the pelvic and upper leg segment), iliac crest to midline xyphoid process (the splanchnic segment), and midline xyphoid process to supraclavicular area (the thoracic segment). Output from the strain gauge and impedance leads were interfaced to a personal computer through an analogto- digital converter with a sampling rate of 200 samples/s per channel. (DataQ Ind, Milwaukee, WI). Data were multiplexed and effectively synchronized.

 
Supine measurements made during venous occlusion plethysmography. Top: a typical experiment in which blood flow is measured in triplicate by venous occlusion followed by incremental occlusions using 10- mmHg steps to determine the volume-pressure relation. Bottom left: derivation of limb blood flow by fitting a straight line to the initial portion of the occlusion curve. Bottom right: the means by which volume changes during pressure steps can be partitioned into contributions from venous filling and microvascular filtration
 
Subjects had vascular measurements made with plantar stimulation at 0, 15, and 45 Hz with the three frequencies presented in random order for a given subject. At each vibrational frequency, measurements were made supine and at 35° upright tilt.
 
Peripheral vascular evaluation by SGP.
 
We used SGP to measure calf blood flows, the calf capacitance vessel pressure (venous pressure, denoted Pv), the calf venous volume-pressure capacitance relation, calf venous capacity, and the microvascular filtration (flowpressure) relation while supine and during upright tilt to 35° in all subjects. Methods were adapted from the work of Gamble et al. (8 –10) and have been used extensively by our group (38, 40) and are summarized in Fig. 1.
 
After a 30-min resting period, flow measurements were performed in at least triplicate. After flow values returned to baseline, we increased occlusion pressure gradually until limb volume change was just detected. This represents ambient Pv (7). We used the mean arterial pressure (MAP), calculated as 0.33 x(systolic BP) +0.67 x (diastolic BP), and Pv to calculate the calf arterial resistance to blood flow [in units of mmHg.ml-1.100 ml tissue .min] from the equation (MAP - Pv)/flow. To determine overall calf capacitance, the leg was gently raised above heart level until no further decrease in volume was obtained. After recovery and with the leg flat, we used 10-mmHg steps in pressure, starting at the first multiple of 10 larger than Pv, to a maximum of 60–70 mmHg, resulting in progressive limb enlargement. Independent data indicate that the Pv distal to the congestion cuff approximates the cuff pressure (3). Pressure was maintained for 4 min to reach a steady state. At lower congestion pressures the limb size reached a plateau (Fig. 1). With higher pressures, a plateau is not reached, but (Fig. 1, bottom right) after initial curvilinear changes representing venous filling, the limb continues to increase in size linearly with time for a given pressure step. The linear increase represents microvascular filtration. At a critical pressure greater than Pv, denoted by Pi, the lymphatic system fails to compensate for filtration, and the limb interstitium enlarges at a rate proportionate to imposed pressure. This is the pressure threshold for edema formation. At occlusion pressure between Pv and Pi, the change in leg size reaches a plateau. At occlusion, pressures exceeding Pi, pressure increments result in a change in leg size, which is asymptotic to a straight line with positive slope. We used the singular value decomposition technique (29) to fit a least squares straight line to the points comprising the linear microvascular filtration portion of the filling curve at each occlusion pressure, as shown in (Fig. 1, bottom right). The linear portion is then electronically subtracted from the total curve to obtain a residual curvilinear portion that reaches a plateau. This residual portion is the change of capacitance vessel filling with each pressure step.
 
Once the volume response is partitioned, capacitance is calculated from the sum of residual portions shown as “intravascular filling” in Fig. 1 to which is added the estimate of supine venous volume obtained from raising the limb (39). The microvascular filtration relation (filtration rate vs. pressure relation) is constructed for each subject. Normalized volume is measured and expressed in units of milliliters volume change per 100 ml tissue; normalized filtration rate is expressed in units of milliliters per 100 ml tissue per minute; and the normalized filtration coefficient, Kf, the slope in the linear relation, is expressed in units of milliliters per 100 ml tissue per minute per millimeter Hg. The intercept with the pressure axis of the filtration rate-pressure graph is Pi, at which microvascular filtration exceeds lymphatic flow and approximates the net oncotic pressure gradient for microvascular filtration. The work of Pappenheimer and Soto-Rivera (27) established that net filtration does not occur at pressures less than Pi. Thus the extension of the linear fit to negative flow is a “virtual flow,” which serves to estimate the y-intercept with the filtration axis, the normalized filtered flow at zero hydraulic pressure, comprising contributions from lymphatic flow and osmotically driven filtration. We used SGP to measure Pv, Pi, the volume-pressure relation of the capacitance vessels, and thus overall capacity, and the microvascular filtration relation, including the filtration coefficient Kf.
 
Peripheral vascular evaluation by IPG.
 
IPG was used to measure segmental blood flows (26). IPG has also been used to quantify relative body fluid volumes (14). Relations between impedance and fluid compartmentalization have been established (12). Recently, changes in fluid compartment volumes and transient blood flows have been quantitated during orthostasis (5, 26). We used a tetrapolar IPG device to measure blood flows in the thoracic, splanchnic, pelvicupper leg, and calf segments during each test sequence. Measurements of baseline impedance, Z0, and pulsatile impedance changes, (delta)Z, were made. A high-frequency (50 kHz), low-amperage (0.1 mA RMS), constant-current signal between the foot and hand electrodes was introduced. Z0 values were measured in each segment continuously. Pulsatile impedance changes were used to compute the time derivative Z/ t, which we used to obtain the total (ml/min) and relative (ml100 ml body tissue-1 .min-1) blood flow responses of each body segment to each test condition. Blood flow was estimated for an entire anatomic segment from the formula (11): flow = [HR. p.L2.dTmax/Z02 2.In this formula, HR is heart rate, p is the density of blood, L is the distance between the centers of the electrodes, T is the ejection period, Z is the impedance, and Z0 is the baseline impedance.
 
To obtain IPG flows free of respiratory artifacts, we had the subjects lightly hold their breath for 10 s on exhalation, taking care that they did not perform a Valsalva maneuver. IPG flows are expressed in units of milliliters per minute for an entire anatomic segment. Normalization to tissue volume can be performed.
 
35° Upright tilt table testing. At each vibration frequency, after supine vascular measurements were complete, the subjects were tilted to 35° for 15 min to obtain circulatory measurements during orthostasis. Earlier work indicated that strain-gauge measurements were more accurately determined during 35° compared with 70° upright tilt and that the lower angle still produces an adequate orthostatic stimulus (41). Preliminary studies have shown that this angle of upright tilt can be easily tolerated by all subjects (6). Measurements were made after ˜5–7 min when blood pressure and heart rate had stabilized. Heart rate measurement, as well as arm and leg blood pressures, was repeated by oscillometry. Pv was remeasured upright. Limb blood flows were measured by SGP and IPG. Segmental blood flows were remeasured by IPG. SGP was used to reassess the volume-pressure relation and the microfiltration relation by increasing occlusion cuff pressure beginning at the new measured value of Pv and increasing in 10-mmHg steps up to a maximum pressure less than the diastolic pressure. Pi, overall capacity, and Kf were obtained by least squares analysis. The vertical height between the congestion cuff and the strain gauge was used to correct for hemostatic load differences. Thus the pressure at the calf strain gauge was adjusted by adding p.g.D.sin(35°), where p is the density of blood, g is the gravitational acceleration constant, and D is the distance from the congestion cuff to the strain gauge.
 
Plantar stimulation.
 
Plantar stimulation was applied using a custom- made apparatus devised by one of us (K. J. McLeod). The device consists of a rectangular-shaped frame constructed with an aluminum top plate against which the subjects place their feet in either a supine or upright position. The plate is circumferentially supported by an array of 12 coil springs. Centrally located on the bottom surface of the plate is an electromechanical actuator. This actuator is capable of delivering sinusoidal 15- to 120-Hz vertical displacements of about 0.004–0.24 mm to the top plate. Attached to the underside of the aluminum plate is an accelerometer, which provides acceleration feedback to the system. Digital electronic control circuitry automatically adjusted the actuator force to provide an acceleration of 2.0 m/s2 [0.2 g point to point (p-p)]. This corresponded to a surface displacement of 240 m p-p at 15 Hz and a stimulation amplitude of 25 m p-p at 45 Hz. The platform was mounted on the footplate of the tilt table throughout the protocol. This is a stable and comfortable arrangement.
 
Statistics.
 
Tabular data were compared by two-way ANOVA, with vibration frequency, position (supine and upright) on the table axes. When significant interactions were demonstrated, paired t-tests were used for compared supine and upright changes within-groups comparisons. Results are reported as means ±SD. P values <0.05 were considered statistically significant.

RESULTS
 
Over a 1-yr period, we recruited 18 subjects for the current study ranging in age from 45.5– 63.3 yr. Subject ages, heights, weights, illnesses, medications, resting blood pressure, and heart rate are shown in Table 1. All enrolled subjects were free of acute illnesses. There were no trained competitive athletes. There were no bedridden subjects.
 
Heart rate and pressure measurements. As shown in Table 2, heart rate was not affected by the plantar vibration at either frequency and tended to increase modestly with orthostasis, as expected. Arm MAP was unaffected by vibrational frequency or by orthostasis, while leg blood pressure (BP) increased 131 ±31 (P =0.005) and to 146 ±28 ml/min with 45 Hz stimulation (P =0.001)
 
Upper leg-pelvic blood flows were unaffected by orthostasis but increased during plantar stimulation while supine (Table 2). With orthostasis, plantar vibration increased pelvic flow from 707 ±90 at 0 Hz, to 952 ±123 at 15 Hz, and finally to 940 ±102 ml/min, at 45 Hz, P <0.005. Splanchnic flow was unaffected by the plantar vibration but decreased during orthostasis at all frequencies (P <0.001).
 
Thoracic blood flow decreased as expected with orthostasis (P <0.05) and was increased to a similar extent in the supine and upright positions by plantar stimulation (from 3,506 ±322 at 0 Hz, to 3,990 ±270 at 15 Hz, and finally to 4,237 ±366 ml/min at 45 Hz, P <0.02 when supine and from 2,688 ±287 at 0 Hz, to 3,391 ±688 at 15 Hz, and finally to 3,670 ±313 ml/min at 45 Hz, P <0.02 when upright).

Volume-pressure capacitance and microfiltration relations
 
The volume-pressure relation as depicted in Fig. 2 is unaffected by plantar stimulation and by orthostasis (39). As shown in Table 3, the maximum leg capacity was unaffected by tilt or stimulation.
 
However, as shown in Fig. 3, the microvascular filtration relation is shifted rightward with plantar stimulation and is during tilt as a result of the hemostatic column imposed by tilting. Leg BP was unaffected by vibrational frequency.
Table 2. Hemodynamic properties
 
Leg Pv was increased at 15 Hz and at 45 Hz compared with 0 Hz, while supine (P <0.04) but was not different from 0 Hz when upright. Tilt increased Pv similarly at all frequencies.


VIBRATION AND FLUID FLOW

Peripheral blood flow and resistance measurements.
 
Table 2 shows the expected decrease in calf blood flow measured by SGP with orthostasis, while peripheral arterial resistance increased with upright tilt. SGP recording did not identify any significant increase in calf blood flow with either plantar stimulation frequency when subjects were in the supine position. However, calf blood flow increased during plantar stimulation in the upright position (from 1.2 ±0.2 at 0 Hz, to 1.6 ± 0.4 at 15 Hz, and to 1.8 ±0.4 ml.100 ml-1.min-1 at 45 Hz, P <0.002 by paired t-testing). There was no effect of vibration on arterial resistance when supine or upright. Venous resistance was unaffected by orthostasis but decreased during 45 Hz plantar stimulation in the upright position (from 1.2 ±0.2 at 0 Hz, to 1.2 ±0.5 at 15 Hz, and to 0.7 ±0.1 mmHg.ml-1.100 ml.min, P <0.05)
IPG measurements showed that calf segmental flow was significantly affected both by orthostasis and plantar stimulation. In the supine position, calf flow increased from 137 ±18 to 150 ±21 (P =0.05) at 15 Hz, and 178 ±26 ml/min (P = 0.05) at 45 Hz. Orthostasis resulted in a significant reduction in calf flow to 99 ±15 ml/min (P =0.05 compared with supine). However, plantar stimulation at 15 Hz increased calf flow to 131 ±31 (P =0.005) and to 146 ±28 ml/min with 45 Hz stimulation (P =0.001).
 
Upper leg-pelvic blood flows were unaffected by orthostasis but increased during plantar stimulation while supine (Table 2). With orthostasis, plantar vibration increased pelvic flow from 707 ±90 at 0 Hz, to 952 ±123 at 15 Hz, and finally to 940 ±102 ml/min, at 45 Hz, P <0.005. Splanchnic flow was unaffected by the plantar vibration but decreased during orthostasis at all frequencies (P <0.001).
 
Thoracic blood flow decreased as expected with orthostasis (P <0.05) and was increased to a similar extent in the supine and upright positions by plantar stimulation (from 3,506 ±322 at 0 Hz, to 3,990 ±270 at 15 Hz, and finally to 4,237 ±366 ml/min at 45 Hz, P <0.02 when supine and from 2,688 ±287 at 0 Hz, to 3,391 ±688 at 15 Hz, and finally to 3,670 ±313 ml/min at 45 Hz, P <0.02 when upright).
Volume-pressure capacitance and microfiltration relations
 
The volume-pressure relation as depicted in Fig. 2 is unaffected by plantar stimulation and by orthostasis (39). As shown in Table 3, the maximum leg capacity was unaffected by tilt or stimulation.
 
However, as shown in Fig. 3, the microvascular filtration relation is shifted rightward with plantar stimulation and is unaffected by upright tilt. As shown in Table 3, there is no change in the slope of the relation, Kf, with vibrational frequency, but a pronounced shift in x-intercept (Pi) and yintercept. Thus while supine, Pi increases from 24 ±2 at 0 Hz to 27 ±3 at 15 Hz, and to 31 ±2 mmHg at 45 Hz (P <0.01), and while upright, Pi increases from 25 ±3 at 0 Hz to 28 ±4 at 15 Hz, and to 35 ±4 mmHg at 45 Hz (P <0.04).
Figure 4 shows individual upright subject data as a function of the frequency of plantar vibration. While scatter remains wide, there is significant increase in upright calf blood flow and Pi as a function of vibrational frequency.
 
DISCUSSION
 
In the introductory section, we proposed a mechanism for the maintenance of bone density that depends on hemostatic pressure in the lower extremities to provide for microvascular filtration and a skeletal muscle pump to ensure the adequacy of blood and interstitial flow. We provided past evidence that low-amplitude plantar vibration operating at frequencies similar to postural muscle contraction rates enhances bone growth and inhibits bone loss. We developed the hypothesis that plantar vibration stimulates or simulates skeletal muscle pump function. Here, we successfully tested the prediction that plantar vibration enhances lower body blood flow and lymph flow. Thus a method that promotes peripheral flow in one set of experiments also promotes bone growth in another.

 
Table 3. Capacity and microfiltration properties
 
This role of the skeletal muscle pump in maintaining bone mass has been lent support by a recent study on an elderly female population (25). In the latter study, the degree of skeletal muscle pump activity was determined by the surrogate measures of muscle activity (via vibromyography) and postural sway. Bone mineral density in both the femur and lumbar spine was found to be strongly associated with both increased muscle activity during quiet standing and, correspondingly, increased postural sway during standing.
 
One significant effect of plantar stimulation on circulation in this study was on calf blood flow, which, while significantly decreased by orthostasis, is improved by plantar vibration.
 
Similarly, upper leg-pelvic blood flow and thoracic flow are significantly increased by vibration and orthostatic changes blunted by plantar stimulation, particularly at 45 Hz. It is likely that the changes in leg-pelvic flow are produced as part of a generalized lower extremity effect affecting calf and thigh alike and that the increase in thoracic IPG flow relates to the increase in overall systemic flow due to improved peripheral flow and venous return. This is entirely consistent with a skeletal muscle pump mechanism in which venous return is ensured by pumping, while forward flow is enhanced by intermittent reduction of venous pressure and increased arteriovenous pressure gradient (32).
 
Another finding is that the microvascular filtration relation is right-shifted by plantar vibration as a consequence of an increase in Pi, the threshold for edema, Frequency of Vibrationwhile the microvascular filtration coefficient, Kf, remains unchanged. Our prior work indicates that Yintercept = (approx.) Kf .Pi and approximates lymphatic flow (37). Taken together, the results therefore suggest that while microvascular filtration (i.e., Kf) is unaffected by plantar vibration, there is enhanced lymphatic and venous drainage, particularly evident when upright.
 
Peripheral lymphatic transport is known to be linked to skeletal muscle pump activity. Thus while lymph is formed by the translocation of interstitial fluid into the initial lymphatics by osmotic or vesicular transport mechanisms (1), and initial lymphatics may possess some degree of actin-dependent active contractile transport from initial lymphatics to valves containing lymphatic ducts, the bulk of lymphatic flow seems to depend on tissue movement. Thus chronically immobilized tissues have almost no lymphatic flow especially in the extremities (13). Unlike veins, there is apparently little effect of “force from behind” (cardiac muscle and blood pressure) on lymphatic fluid propulsion. Instead lymph flow is enhanced by active and passive limb muscle movements, the skeletal muscle pump (31). Prior work indicates that to create unidirectional flow, these external forces must be intermittent (24). In our subjects enhanced lymphatic flow is stimulated by plantar vibration.
 
During relaxed standing, postural muscle activity, and correspondingly skeletal muscle pump activity, can be inferred through assessments of postural sway activity, as greater postural sway typically represents higher levels of lower limb muscle activity. While younger individuals are posturally more stable than their elders, we have observed that younger individuals permit themselves to sway more during relaxed standing than older individuals (21, 31). In the absence of movement (i.e., sway), tissue pressures increase in the legs (15, 31).
 
Type IIa fibers are believed to play a dominant role in skeletal muscle pump activity (33). While type I fibers typically contract at rates below 20 Hz, type IIa muscle fibers have typical contraction rates in the range of 20–60 Hz (16). The efficacy of the higher plantar vibration frequency we tested (45 Hz) in reducing edema pressure and enhancing blood flow in the lower body is consistent with the stimulus triggering type IIa muscle fiber contraction.
 
We have previously demonstrated age-related decreases in muscle contraction dynamics with a significant decrease in the magnitude of the component of vibromyography associated with type IIa muscle contractions, corresponding to approximately a 1–2% decrease in amplitude per year (16). Our ability to influence skeletal muscle pump activity in this particular population may, therefore, be a reflection of the fact that this group had lost a substantial fraction of their normal muscle pump activity due to the age-related conversion of type IIa fibers to type IIb.
 
Nonetheless, it would appear that the remaining musculature is sufficient to respond to the vibration stimulus. Stimulated skeletal muscle pump activity produced enhanced upright calf blood flow and enhanced centripetal lymphatic flow.
 
Limitations.
 
We principally studied the lower extremities. While IPG provides hints about other regional circulations, it is incomplete in this regard and varies from subject to subject, dependent on body habits and other factors. Although other regional circulations could be affected by vibration, one would guess that the most immediate effects of plantar vibration would be on the lower extremities, which along with buttocks (pelvic-upper-leg segment) are important sites for venous pooling during quiet standing (22). Thus the study addresses effects that are important to the orthostatic response.
 
IPG was used to measure regional blood flow changes. The standard for peripheral flow measurement is SGP. While SGP measures a physical size change (circumference), impedance measures electrical resistance changes. They are not equivalent. However, from a qualitative standpoint, they yield directionally similar data. Also, our main conclusions concerning the effects of plantar vibration on calf blood flow and lymphatic flow when upright were obtained equally well by straingauge measurements at 45 Hz.
 
In addition, low-angle tilts were studied. We required stable conditions during which microvascular filtration and capacitance relations could be measured. Potentially useful information could be missed. Data collection becomes more difficult and biological variability increases at higher angles.
 
Age and gender limitations to generalizability may exist. Data obtained from perimenopausal women are not representative for younger or older ages, or for men. However, we studied this group because they are at particular risk for vascular and musculoskeletal abnormalities that may affect the lower extremities and may have an impact on osteoporosis.
 
Some subjects had medical ailments and were maintained on current medications, although this was limited by exclusion criteria. Our rationale is that we wished to study average women and not necessarily perfect physical specimens. Although the results could have been affected by illnesses and medications, all illnesses (e.g., hypertension, hypothyroidism) were controlled, we withheld medications on the day of testing, the subjects represent a cross section of comparably aged women, and we could see no apparent stratification by disease or medication subtype. In regard to orthostasis and plantar vibration, the subjects served as their own controls (14). The true control group would be older women taking absolutely no medications. However, we have had great difficulty in recruiting a suitably large medication-free population. Several of our subjects had no illnesses and took no medication, while having similar response to vibration.
 

 

 
 
Effects of whole-body vibration in patients with multiple sclerosis
Power Plate Studies
Objective:
To examine whether a whole-body vibration (mechanical oscillations) in comparison to a placebo administration leads to better postural control, mobility and balance in patients with multiple sclerosis.

Design:
Double-blind, randomized controlled trial.

Setting:
Outpatient clinic of a university department of physical medicine and rehabilitation.

Subjects:
Twelve multiple sclerosis patients with moderate disability (Kurtzke’s Expanded Disability Status Scale 2.5-5) were allocated either to the intervention group or to the placebo group.

Interventions:
In the intervention group a whole-body vibration at low frequency (2.0-4.4 Hz oscillations at 3-mm amplitude) in five series of 1 min each with a 1-min break between the series was applied. In the placebo group a Burst-transcutaneous electrical nerve stimulation (TENS) application on the nondominant forearm in five series of 1 min each with a 1-min break between the series was applied as well.

Main outcome measures:
Posturographic assessment using the Sensory Organization Test, the Timed Get Up and Go Test and the Functional Reach Test immediately preceding the application, 15 min, one week and two weeks after the application. The statistical analysis was applied to the change score from preapplication values to values 15 min, one week and two weeks post intervention.

Results:
Compared with the placebo group the intervention group showed advantages in terms of the Sensory Organization Test and the Timed Get Up and Go Test at each time point of measurement after the application. The effects were strongest one week after the intervention, where significant differences for the change score (p=0.041) were found for the Timed Get Up and Go Test with the mean score reducing from 9.2 s (preapplication) to 8.2 s one week after whole-body vibration and increasing from 9.5 s (preapplication) to 10.2 s one week after placebo application. The mean values of the posturographic assessment increased from 70.5 points (preapplication) to 77.5 points one week after whole body vibration and increased only from 67.2 points (preapplication) to 67.5 points one week after the placebo application. No differences were found for the Functional Reach Test.

Conclusion:
The results of this pilot study indicated that whole-body vibration may positively influence the postural control and mobility in multiple sclerosis patients.
 
Introduction
 
Multiple sclerosis is the most common neurological illness leading to disability in the Western world.1 The variable nature of the disease results in a broad spectrum of impairments such as disorders of balance, loss of co-ordination, muscle weakness, spasticity, altered sensation, impaired vision, cognitive impairment, fatigue and loss of bladder and bowel control.1 Ataxia and balance disorders are the most incapacitating problems seen in patients with multiple sclerosis and the resulting disturbances of postural control are a common problem. The effects are walking impairment and reduction of mobility caused by worsening balance during ambulation. Patients typically describe a wide-base gait with worsening balance when initiating gait or changing directions. Physical intervention including balance training, strengthening of proximal muscles of the extremities and stabilizing muscles and compensatory techniques can be helpful. Unfortunately, balance disorders and ataxia are amongst the most resistant symptoms to therapeutic interventions and are often a major cause of disability
 
Whole-body vibration is based on the application of multidimensional whole-body vibrations (mechanical oscillations). The transmission of vibrations and oscillations to a biological system can lead to physiological changes on numerous levels. Stimulation of skin receptors, muscle spindles, vestibular system, changes in cerebral activity, such as those in the thalamus and somatosensory cortex,6,7 changes of neurotransmitter concentrations such as those in dopamine and serotonin,8 and changes of hormone concentrations 9,10 have been described. It has been demonstrated that vibration is an effective method for improving postural control in elderly subjects. 11 Whole-body vibration resulted in improvement of gait parameters and co-ordination in patients with Parkinson’s disease. In these patients an improvement of gait and of postural control as well as an improvement in manual coordination could be achieved by means of multidimensional whole-body vibration in five series of 1 min each in 1-min intervals. This effect occurred about 10 min after the application and lasted up to 48 h.
 
It appears to be reasonable to apply this method to patients with other progressive neurological diseases. Therefore the aim of this study was to test the effectiveness of whole-body vibration in improving postural control, balance and mobility in multiple sclerosis patients. This study was designed as a pilot study in order to get a first reference whether whole-body vibration could be effective in patients with multiple sclerosis.
 
Methods
 
Subjects
 
Twelve multiple sclerosis patients were included in the study. The participants were recruited from the outpatient clinic of the university department of physical medicine and rehabilitation in Vienna. Inclusion criteria were the existence of balance disorders, gait insecurities and/or ataxia and an impairment of <=5 based on Kurtzke’s Expanded Disability Status Scale (EDSS).15 The subjects needed to stand independently, without assistive devices or external support.
 
Patients were excluded from the study in case of pregnancy (female patients in their reproductive age had to ensure reliable means of contraception), electronic implants such as pacemakers, conditions following artificial heart valves, epilepsy, malignant tumours, endoprothesis, conditions following recent fracture (less than six months), osteoporosis with vertebral body fracture, conditions following thrombosis, therapy with anticoagulant medication, relapse of multiple sclerosis in the last two months and refusal to participate.
 
After it had been determined that the patients met the inclusion criteria and that no exclusion criteria were present, they were informed in detail about the study and signed a written informed consent form to participate in the trial. This study was reviewed and approved by the ethics committee of the Medical University of Vienna.
 
The patients underwent a brief clinical examination following a standardized examination protocol. The Ataxia Clinical Rating Scale was determined.16 In this scale the maximum score was equal to 78 and 0=no signs of ataxia. To assess muscle tone of the lower extremity the Modified Ashworth Scale was used.17 The Modified Ashworth Scale grades the level of resistance encountered during manual passive stretching (0-5; 0=no increase in muscle tone, 5=jjoint fixed rigid in a position). The level of disability was determined by the EDSS.15 The score ranges from 0 (no disability) to 10 (death due to multiple sclerosis). The baseline characteristics of the study population are presented in Table 1.
 
Treatment procedures
Six patients were allocated to the whole-body vibration group and six patients were allocated to the placebo group according to a randomization list. The examiner collecting the target parameters did not know which type of intervention was applied (placebo or whole-body vibration). The interventions were performed by a second staff member who was blinded to the examination results. During the examination period (two weeks) the patients’ medications were not changed and no special pysiotherapy with gait training and balance training was performed. The interventions were a one-time term of nine minutes treatment or placebo application
 
  • Group 1:  Application of a multidimensional whole-body vibration. Amplitude: 3 mm, frequency: beginning with 1 Hz slowly increasing until the patient no longer tolerated a further increase. With this frequency five series of 1 min each with breaks of 1 min each were performed. The construction of the device is designed to perform a nonharmonious generation of oscillating movements in vertical and horizontal planes in order to prevent habituation of receptors and occurrences of resonance. The Zeptor-Med system (Scisen GmbH, Germany) was used (Figure 1). While standing on the platform of the Zeptor system subjects were instructed to maintain a squat position with slight flexion at the hips, knees and ankle joints.
     
  • Group 2:  For the placebo treatment the patients stood on the Zeptor system’s platform in the same position as for the verum application. The placebo application consisted of an application of Burst-transcutaneous electrical nerve stimulation (TENS) on the nondominant forearm in order to simulate a vibration. Just as with the verum application, TENS in five series of 1 min each with breaks of 1 min each were performed. The intensity of the TENS application was increased until a muscle contraction was just visible to simulate a vibration.
     
Outcome measures
 
The following target parameters were measured before, 15 min, one and two weeks after the application. The examinations were always done at the same time of the day.
 
Posturography (Sensory Organization Test) with a SMART Equitest System (NeuroCom International, Oregon, USA)
 
Oregon, USA) Dynamic posturography uses a computer-controlled, menu-driven, moveable platform and a moveable visual surround to isolate the effects of various sensory inputs to the brain and measures their effect on balance control. Platform and visual surround movements can be ‘sway referenced’ and move in direct response to the patient’s sway. One type of posturography is the Sensory Organization Test.18,19 This test consists of six subtests: (1) eyes open, fixed platform, fixed visual surround, (2) eyes closed, fixed platform, fixed visual surround, (3) eyes open, fixed platform, moving (sway referenced) visual surround, (4) eyes open, moving platform, fixed visual surround, (5) eyes closed, moving platform, fixed visual surround, (6) eyes open, moving platform, moving visual surround. The patients were carefully positioned on the platform with the lateral malleoli as marker along

 
Table 1 Subject characteristics

Functional Reach Test
 
A yardstick was mounted at the height of the patient’s acromion. Figure 1 Multidimensional whole-body vibration device (with permission from Irschitz GmBH).The patient was asked to stretch their arm parallel to the yardstick with fist closed. Then the patient was asked to lean forward as far a possible without taking a step. The new position of the end of the metacarpal bone was marked and the difference to the starting position was calculated. The mean value of three tries was recorded. The Functional Reach Test is a simple measurement of standing balance. Additionally it yields information to what extent an everyday task of living - reaching for an object within reach - can be performed.23
 
Statistical analysis
 
The statistical analysis was applied to the change score from preapplication values to values 15 min, one week and two weeks post intervention. Due to the low sample size a nonparametric test (Mann- Whitney U-test) was performed to find significant differences between the therapeutic groups regarding the investigated parameters at 15 min, one week and two weeks post intervention. The œ-level was 0.05.
 
Since no data with this patient group and this intervention were available and this study was conceived as a pilot study no power analysis to calculate the sample size was performed.
 
Results
 
All subjects completed the study without any sideeffects except one who complained about increased fatigue. No subject dropped out (Figure 2). None of them showed clinical exacerbation at the time of the examination. During the follow-up period the patients were neurologically stable without an indication of a relapse. The average frequency (mean; SD; range) tolerated in whole-body vibration was 3; 0.7; 2-4.4 Hz.
 
There was a tendency for higher values in the posturographic assessment in the whole-body vibration group at all time points of measurement, where for the change score the statistical level of significance was just missed (Table 2). For the Timed Get Up and Go Test all measurements after the intervention tended to result in better (lower) values for the whole-body vibration group compared with the placebo group. In the examination one week after the intervention a significant difference in the change score in favour of the whole-body vibration was found (Table 3). Two weeks after the intervention the values for posturography and Timed Get Up and Go Test in the whole-body vibration group were still higher than in the placebo-group, however without reaching statistical significance for the change score (Tables 2 and 3). In the Functional Reach Test no difference between the two groups was determined (Table 4).
 
Figure 2 Flow diagram for the present study.
 
Table 2 Sequential changes of the results of the posturographic assessment (SOT) after treatment with a whole-body vibration or placebo; data are given in absolute values at each time point of assessment and in changes from preapplication to 15 min, one week and two weeks post intervention

Table 3 Sequential changes of the results of the Timed Get Up and Go Test after treatment with whole-body vibration or placebo; data are given in absolute values at each time point of assessment and in changes from preapplication to 15 min, one week and two weeks post intervention
Discussion
 
Compared with placebo, whole-body vibration showed advantages in terms of the Sensory Organization Test and the Timed Get Up and Go Test at each time point of measurement after the application. The effects of the whole-body vibration were already evident after 15 min and lasted for up to two weeks. The effects were strongest in the examination one week after the intervention, where significant differences (p=/0.041) were found for the Timed Get Up and Go Test and a tendency for improvement was found for the posturographic examination (p=/0.065). Including more patients in this study might lead to statistically significant differences in this parameter. A limitation of this study certainly is the sample size. Furthermore it must be considered that the patients had only mild to moderate disability. The follow-up period was relatively short (two weeks), and the whole-body vibration was only applied once for a short time of 9 min. A point of criticism could be that a TENS application is not the ideal placebo application, since TENS may enhance sensorimotor recovery in neurological patients.

 
Table 4 Sequential changes of the results of the Functional Reach Test after treatment with whole-body vibration or placebo; data are given in absolute values at each time point of assessment and in changes from preapplication to 15 min, one week and two weeks post intervention
 
The maintenance of postural stability depends on a continuous information flow of the visual, vestibular and proprioceptive system, which is part of the sensory system, and on a continuous information flow from motoric centres to the muscular end organs. All this information is transmitted through myelinated connections. Multiple sclerosis is a demyelinating disease of the central nervous system and these systems are therefore frequently impaired in people with multiple sclerosis.25 The effect on the visual system can lead to fuzzy vision and diplopia. If the vestibular system is affected it leads to vertigo and nystagmus. Lesions in the long ascending sensory tracts cause an impaired proprioception and reduced sensation for vibration.26 Balance disorders are caused by a limited capacity to integrate visual, proprioceptive and vestibular stimuli for the determination of the body’s position in space.
 
Balance disorders and ataxia are symptoms that are very resistant to therapy and restrict the outcome of the rehabilitation process significantly. 27 Balance disorders deteriorate the prognosis and negatively influence the transfer, the mobility and even the balance when sitting. Balance skill is one of the most important variables associated with fall risk in patients with multiple sclerosis.28 Patients with multiple sclerosis have a higher risk of falling compared with other patient groups.29 The management of the imbalance of equilibrium and ataxia is difficult and the medical and physical-/therapeutic interventions known so far are of limited value.
 
This is the first study examining the influence of whole-body vibration on postural stability, mobility and balance in patients with multiple sclerosis. It has been shown in several studies that various receptor types such as muscle spindles, skin and pressure receptors are sensitive to mechanical oscillating stimuli.30,31 The oscillating vibration stimuli can lead to the following effects: (1) stimulation of the pressure receptors on the sole of foot (Merkel’s receptor endings, Meissner’scorpuscles, Ruffini nerve endings), (2) stimulation of proprioceptors, (3) generation of reflexes. The repetitive stimulation seems to be important as well. The consequence of this might be that a rearrangement of motor control strategies (balance control) takes place. This results in an improvement of postural stability. This could be shown in our study not only by experimental tests as posturograpy. This also was evident in a functional test, such as the Timed Get Up and Go Test.
 
The therapy scheme used in this study (five series of 1 min each with a break of 1 min each between the series) was adopted from a preliminary study with patients with Parkinson’s disease. The breaks between the individual series are designed to prevent rapid fatigue. In the preliminary study with Parkinson’s disease patients, a positive effect on motor function appeared by using whole-body vibration 10 min after the intervention and for up to 48 h.12 In another study short-term beneficial effects of whole-body vibration on postural control in chronic stroke patients were shown.32 Stroke patients were subjected to one series of four consecutive repetitions of 45 s whole-body vibration with a 1-min break between the administrations. In contrast to this study where a vertical whole-body vibration was performed,32 in our study a nonharmonious multidimensional wholebody vibration was applied in order to prevent habituation of receptors.33 There seems to be great potential for the use of mechanical oscillating stimuli in the field of neuromuscular rehabilitation, especially because it circumvents the difficulties associated with the arbitrary initiation of movement. Even though the mechanisms of the effects are not fully clarified, this method could offer new approaches in the rehabilitation of neurological diseases und neuromuscular disturbances.
 
The posturographic parameters have been found to be able to detect generic failures of the postural control system. Posturography has gained wide acceptance as a method of measuring postural control and a good reproducibility has been determined.18,29,34,35 The method can therefore be used to measure the effects of therapeutic interventions and rehabilitation methods.18 The Timed Get Up and Go Test correlates with gait speed, balance and movement of the lower extremities.22 The Functional Reach Test is a simple measurement of standing balance and integrates an everyday task of living (reaching forward to grab something).23
 
This study was conceived as a pilot study. Future examinations should be carried out with a larger sample size, should test more frequent applications of whole-body vibration, and should investigate its mechanism in multiple sclerosis patients.
 
 
Could Vibration Training Be an Alternative to Resistance
Power Plate Studies
Age dependent loss of strength and muscle mass, termed sarcopenia, worsens normal functioning of the elderly, increases the risk of fall and finally leads to the loss of their independence. Sarcopenia is also closely linked to age-related losses in bone mineral density, basal metabolic rate and increased body fat content.

Resistance training may reverse sarcopenia at any age. It may also reduce associated abnormalities. However, a safe and effective resistance exercise program requires complex exercises and close supervision, and quite a lot of time before any significant effect can be seen.

A novel form of exercise based upon the application of sinusoidal vibrations to the whole body (WBV), can enhance force generating capacity in humans. Recently it has been demonstrated that vibration training can induce skeletal muscle strength increase similarly to that seen after resistance training, both in young and elderly persons. Conflicting results concerning the effectiveness of VT in improving muscle strength, as well as bone mineral density could be related to insufficient intensity of VT applied in some studies. Knowledge on various aspects of physiological response to effective VT is at present very scarce, therefor well-founded decision as to whether VT could be an alternative to RT in reversing sarcopenia seems currently premature.
 
Key words: vibration training, sarcopenia 1
 
Sarcopenia
 
Lengthening of average span of human life unveils a host of age related ailment, among them age dependent loss of strength and muscle mass, termed sarcopenia. Maximal muscle strength declines gradually, about 15% during 6th decade and further 15% in 7th decade, later the rate of loss doubles. Doherty [2003] summarizing the number of studies on age dependent loss of maximal strength, states that on average, healthy men and women in the 7th and 8th decades retain 60-80% maximal strength compared with their younger counter - parts. Very old persons retain half or less than half of their peak strength. This age dependent loss of maximal strength, reduces the strength reserve and brings elderly persons towards their "threshold for independence" [Young and Skelton 1994, Rantanen and Avela 1997, Kozakai et al. 2000], and increases the risk of fall [Morley et al. 2001]. Sarcopenia is also closely linked to age-related losses in bone mineral density, basal metabolic rate and increased body fat content [Evans 1997].

Resistance training (RT) reverses age dependent strength loss.
 
Resistance training (termed also strength training) requires the body’s musculature to move against an opposing force, hence its name. The basic unit of strength training is arepetition. The repetition is sequence of movements ending back in starting position. It normally consists of the concentric muscle action, when the muscles involved are shortening, and the eccentric muscle action, when they are lengthening. Set is usually a predefined number of repetitions of chosen exercise performed continuously without stopping. The number of repetitions per set typically range from 1 to 15.
 
The maximum number of repetitions per set (RM) is the maximal number of repetitions, which can be performed without stopping at given resistance. Thus, 1-RM is the resistance at which only one repetition can be performed. The intensity of an exercise can be expressed as a percentage of the 1-RM. The smaller intensity, the smaller percentage of 1-RM, and the more repetitions can be performed. The 85% 1-RM intensity usually allows performing 3 repetitions. Alter - natively, this intensity may be expressed as 3-RM. Using 80% 1-RM may allow performing 10 repetitions, thus this intensity equals 10-RM. The intensities allowing 1 to 6 repetitions are the most effective in increasing strength muscle. Increase of strength muscle obtained with small number of repetitions per - formed against heavy resistance can not be gained with large number of repetitions performed against light resistance. Intensity of 50% 1-RM suffices to increase strength muscle,. The intensities which allow more than 25 repetitions have small or no influence on increasing strength muscle [Atha 1981].
 
Effective resistance training increases the 1-RM, therefore maintaining the same individual percentage of 1-RM will typically require increasing resistance along with training induced increase of muscle strength.
 
Resistance training, .but not endurance training, may reverse sarcopenia. It was demonstrated that increase of muscle strength and endurance could be achieved at any age, given resistance exercise program of sufficient frequency, intensity and duration. Number of authors has demonstrated substantial increase of maximal knee extensors strength after 10 or more weeks of resistance training expressed as 1 RM gain. Frontera et al. [1988] reported over 100% of 1 RM gain (and almost 230% gain of knee flexors 1 RM) after 12 weeks of resistance training in 60-72 years old men. Also, over 100% increase of knee extensors 1 RM was reported by Fiatarone et al. [1990] in the group of 72-98 years old men and women after 10 weeks of resistance training. Lexell at al. [1995] noted even grater increase, over 150%, after 11 weeks of resistance training in 70-77 years old men and women. Harridge et al. [1999] found over 130% increase in a group of very elderly subjects (85-97 years) after 12 weeks of strength training. Other researchers reported more modest increases: from 26 to 50% of knee extensors 1 RM in men and women in their 7th and 8th decades [Charette et al. 1991, McCartney et al. 1996, Hakinnen et al. 1998, Hunter et al. 1999, Tracy et al. 1999, Yarasheski et al. 1999, Hagerman et al. 2000, Lemmer et al. 2000, Brose et al. 2003 Ferri et al. 2003]. Even lesser improvements, below 20% have been documented by Roelants et al. [2004], and Reeves et al. [2004]. Reeves et al. give following explanation for these disparate results: 1/ different average age of study participants, 2/ difference in pretraining sedentary status, 3/ effect of familiarization period (if applied) which may induce neural adaptation resulting in increased weight-lifting capacity. Also, beside these causes of disparity, details of training protocols should be taken into consideration.
 
Resistance training may also reduce associated abnormalities: in comparison to endurance training the beneficial effects of resistance training on bone density are superior, on CHO metabolism similar, and on maximal oxygen consumption, blood pressure, lipid profile, hypertension, obesity smaller [Pollock and Evans 1999].
 
Resistance training in elderly
 
A training session includes all exercises performed in a given time period. A recommended training session for older adults consists of 8 kinds of exercises, which increases strength of large muscle groups that are important in everyday activities (arms, shoulders, spine, hips, and legs). At the beginning of training 80% 1-RM (i.e. 10-15 repetitions per set) intensity is recommended. Progressive increase in 1-RM percentage is possible. At the start of training it is suggested to perform only one set of repetitions for every kind of exercise with 2-3 min of rest between sets. Progression can go from 1 to 3 sets over training time for each type of exercise. The training consisting of 2-3 session per week is recognized as sufficient for gaining health benefits, each session consisting of single sets of 8-10 exercises [Evans 1999, Fleck and Kraemer 1997]. Similar recommendations can be found also in [ACSM 1998, ACSM 2002, Feigenbaum and Pollock 1999, Mazzeo and Tanaka 2001].
 
Safe and effective resistance exercise program requires the use of complex exercises and close supervision, and quite a lot of time before any significant effect can be seen. Therefore, given the generality of sarcopenia, which will accompany everybody's aging, though to variable degree [Kallman et al. 1990], enabling safe and resistance training to all, who could benefit from it and are willing to train, seems practically impossible. In search for solution, a partially supervised training program has been proposed, in which training consists of supervised training sessions and sessions at home [Capodaglio et al. 2005].
 
Vibration training (VT)
 
Another solution could be akind of training, which eliminates aneed of employing highly qualified supervisors. Anovel form training, consisting of exercises performed on the vibrating platform, which applies the application of sinusoidal vibrations to the whole body (WBV). Such vibration has been shown to produce adaptive responses similar to the ones obtained with conventional RT [Bosco et al. 1999, Cardinale and Bosco 2003, Cardinale and Pope 2003]. Recently, group of Belgian researchers has shown, that VT and RT could bring similar increase in maximal strength in elderly women [Roelants, Delecluse, and Verschueren . 2004; Verschueren et al. 2004]. The VT consisted of unloaded static and dynamic knee-extensor exercises (high and deep squat, wide-stance squat, and lunge) performed on vibrating platform. Training progressed slowly toward higher intensity according to the overload principle. The training volume was increased by increasing duration of single session, the number of series of one exercise, and the number of exercises. The training intensity was increased by increasing the amplitude of platform vibration (from 2.5 to 5 mm), frequency of vibration (from 35 Hz to 40 Hz), and by shortening the rest periods. Additionally, training intensity was increased by changing the form of exercise from predominantly two-legged to one-legged. The total duration of vibration in one session increased from 5 min at the start of training to 30 min at its end.
 
 Resistance training was programmed in accordance with recommendation of ACSM Position Stand [1998]. Resistance training was design as slowly progressing, similarly to VT. It started from two sets at intensity of 20-RM and progressed through 15-RM, 10 -RM up to 8-RM. The trainees performed atotal body resistance training program including leg extension and leg-press. Both VT and RT lasted 24 weeks, with 3 training sessions per week.
 
Vibration training was well accepted: most subjects enjoyed vibration loading, and they did not consider it to be difficult workout. The participants reported amoderate muscle fatigue at the end of each session. There were no reports of adverse side effects. During the first sessions only there were some erythrema, edema and itching of the legs after vibration exercise, which resolved rapidly after training. Of initial 30 persons in each group, four from each group left the training because of non-training related problems. Of the remaining: one subject from VT group and 3 subjects from RT group left the training because of mild knee-joint discomfort. Further 3 subjects left RT group because of anterior knee pain (patellofemoral dysfunction, patellar tendinopathy), finally one subject left RT group because of back problems, and 2 subjects from both groups left the programs because of some knee-pain related to a history of knee-injuries. The VT training was completed by 24 persons, and RT by 20 persons. It might be said than, that among elderly women dropout from VT training was lesser than from RT. However, such conclusion is limited to this particular study, and in face of lack of other studies comparing VT and RT in elderly subjects, such conclusion need support from further studies of this kind.
 
Maximal isometric knee-extensor strength increased significantly already after 12 weeks of training in both groups: on average 12.4% in VT group, 16.8% in RT group. Next 12 weeks of training brought no significant increase, final gain being 15% and 16.1% in VT and RT groups respectively. Maximal dynamic strength, measured as peak torque during the knee extension performed at 100°/s velocity also increased significantly after 12 weeks of training: 12.1% in VT group, and 12.4% in RT group, achieving 16.1% and 13.9% increases respectively. Speed of movement of knee extension significantly increased (about 6- 7%) only at low resistance only in VT group. Counter-movement jump height rose significantly by 19.4% in VT group, and by 12.9% in RT group. These results justified the authors’ conclusion, that VT is as efficient as conventional RT in improving knee-extension maximal strength, speed of movement and counter-movement jump performance in older women. However, this study does not support notation, that VT training requires less supervision, than RT, thus being the solution to logistic problem of providing safe and efficient countermeasures against sarcopenia. Both groups, VT and RT, were supervised through all 24 weeks of training by certified American College of Sports Medicine health and fitness instructor s, with one-to-three trainer-to-subject ratio. It follows that solving accessibility problem requires specially designed study, of the kind being performed for RT [Capodaglio et al. 2005]. The positive effect of VT on nursing home residents has been found by Bruyere et al. [2005]; these researchers found that 6-week VT combined with physical therapy significantly decreased the risk of all and improved health-related quality of life, whereas physical therapy alone caused no improvement.
 
The results obtained in the group of elderly women [Roelants, Delecluse and Verschueren 2004] are similar to those obtained in young untrained women (mean age 21.4 years) [Delecluse et al. 2003]. In this study 12 weeks of either VT or RT training caused significant increse of maximal isometric (dynamic) kneeextensor strength: on average 16.6% (9%) in VT group, and 14.4% (7%) in RT group. Counter-movement jump height rose significantly by 19.4% only in VT group, maximal speed of movement remained unchanged. Of particular inter - est in this study is the proof, that static and dynamic knee-extensor exercises performed without sufficiently high amplitude of platform vibration did not influence maximal knee-extensor strength.
 
The effectiveness of VT has not been confirmed in all studies. Recently Delecluse et al. [2005] found "despite the expanding use of Whole Body Vibration training among athletes", no surplus value of adding RT to conventional training program in sprint-trained athletes. De Ruiter et al. [2003] found no improvement in functional knee-extensor muscle strength after 11 weeks of standard two-legged VT in healthy young subjects. However the training program implemented by these researchers did not obey overload principle, volume and intensity remaining constant throughout the whole training program and resembling that used in the start period by Roelants, Delecluse and Verschueren [2004]. In a study on young and adult (19-38 years) men and women Torvinen et al. [2003] found benefit in the vertical jump height and no improvement in maximal isometric strength of lower extremities after 8-month VT, training load being merely 4 min/day vibration, 3-5 times aweek. Luo, McNamara, and Moran [2005] reviewing the use of VT stated that using insufficient amplitude and duration of vibration renders VT ineffective.

Need to know physiological response to VT
 
In a recent review on VT Jordan et al. [2005] conclude essential, in face of substantial evidence regarding the negative effects of vibration on the human body [Seidel 1993], that athorough understanding of the VT implications be acquired prior to application of this type of training in athletic situations. This postulate is of particular importance if VT would be applied permanently to aged individuals. Only with sufficent body of knowledge on physiological consequences of VT in elderly will be acquired, the sound decision which kind of training: RT or VT fits better needs of aged persons could be met. At present relative abundance of knowledge about various aspects of physiological response to RT are in contrast with the scarcity of such knowledge regarding VT.
 
Cardiovascular response to RT and VT
 
Resistance training is often recommended with reservation due to belief it will be accompanied by large rise of arterial blood pressure. Indeed, during different forms of resistance training high increase in blood pressure and heart rate has been observed [Fleck and Dean 1985, MacDougall et al. 1985, Fleck and Dean 1987, Stone et al. 1991, Sale et al. 1993, Sale et al. 1994, Scharf et al. 1994]. However, relative safety of resistance training has been demonstrated [Sheldahl et al. 1983, Sheldahl et al. 1985, Vander et al. 1986, Keleman et al. 1986, Crozier et al. 1988, Sparling iCantwell 1989]. Evans [Evans 1999] states clearly: "with proper breathing technique, the cardiovascular stress of resistance exercise is minimal"). Fleck and Kraemer [1997] draw attention to significance of Valsalva maneuver; they showed that blood pressure increase could be greater during Valsalva maneuver performed without isometric exercise than during isometric exercise performed without Valsalva maneuver.
 
The mechanism of circulatory response to resistance exercises is complex. Rise of blood pressure results from rise in cardiac output (CO) and/or total peripheral resistance (TPR), with variable share of these parameters.
 
The heart rate and arterial pressure increase over repetitions, with the highest values recorded during the last two repetitions of aset and greater responses at the higher relative load [Stone et al. 1991, McCartney et al. 1993]. In dynamic resistance training, higher blood pressure but not heart rate occurs during the concentric than during the eccentric portion of repetition [Falkel et al. 1992]. Stroke volume is not elevated significantly above resting values dur - ing the concentric phase. However, during the eccentric phase stroke volume is significantly increased above resting values and is significantly greater than during the concentric phase of repetition. During the resistance training CO can increase several times when the mass of muscles engaged in exercise is large. However, during an exercise involving a smaller muscle mass rise in CO could be insignificant [Miles et al. 1987]. The blood pressure response increases with the active muscle mass, but the response is not linear. The peak blood pressure and heart rate are higher during sets at submaximal loads to voluntary failure than using 1-RM resistance [MacDougall et al. 1985, Fleck and Dean 1987, Sale et al.1994].
 
Is the claimed cardiovascular safety of RT justified? If seems more appropriate to speak about relative safety of RT, as compared to endurance training [Vinson et al. 1990, Faigenbaum et al. 1990, Green et al. 2001]. For instance Faigenbaum et al. observed much more adverse cardiac events in cardiac patients during the graded exercise stress test than during maximal contraction effort or during repetitions with submaximal force. Vinson et al. observed considerable inter-individual variability in cardiovascular response to resistance exercises as compared to response to endurance exercise: in some persons the maximal response to resistance exercise was less than to endurance exercise, however in the others reverse was true. RT did not change maximal response of arterial blood pressure in elderly men [Hagerman et al. 2000].
 
It is likely that age-dependent diminished adrenergic responsiveness of the heart would result in qualitatively different mechanism of cardiovascular response. It has been found that in elderly the cardiovascular adaptation to exercise relies to lesser degree on adrenergic stimulation and instead the mechanism of Frank – Starling law of heart is utilized [Gerstenblith et al.1987].
 
At present, we are unaware of any study on cardiovascular response to WBV.

Hormonal response to RT and WBV – disparate results
 
Whether WBV can acutely alter the hormonal profile is at present uncertain. In earlier study Bosco, Iacovelli et al. [2000] reported, that in response to WBV treatment of young males, plasma concentration of testosterone and growth hormone increased, whereas cortisol level decreased. Recent study by DiLoreto et al. [2004] reports no change after 25 min of WBV of middle-aged men in variety of hormones: insulin, glucagon, cortisol, epinephrine, growth hormone, IGF- 1, free and total testosterone. Also disparate results has been obtained concerning resistance exercises: in the study of Bosco, Colli et al. [2000] single heavy resistance training session performed by male sprinters resulted in postexercise lowering of testosterone and cortisol. Borst et al. [2002] found in young healthy adults, after acute resistance exercise, no change in testosterone but increase in cortisol level. This authors also did not find any changes in resting serum concentration of IGF-1, IGF binding proteins –1 and –3 ( IGFBP-1 and IGFBP-3), testosterone, and cortisol induced by 6 month RT. No data concerning hormonal changes induced by long-term VT are yet available.
 
Osteogenic effects of RT and VT
 
The effect of different exercise interventions on bone mineral density (BMD) has been extensively studied [Todd and Robinson 2003]. In general, these studies shown that high impact exercises like vigorous aerobic exercises, weight training, running and squash increases BMD at hip and lumbar spine, whereas low impact exercises like swimming, walking or gentle aerobic exercise offer no protection or protect against farther loss of BMD at best. Regarding RT, it has been shown that in persons, aged 60-83 years, low - and high-intensity RT increased bone turnover, as evidenced by increased serum levels of osteocalcin and bone-specific alkaline phosphatase, but high-intensity RT only increased bone mineral density of the femoral neck [Vincent and Braith 2002]. Authors concluded that exercises intensity of 80% 1-RM may be indispensable to increase bone mineral density. Experiments on animals showed that vibration treatment could also represent an effective non-pharmacological intervention for increasing osteogenic stimulus in animals [Rubin et al. 2001]. Equivocal results have been presented regarding osteogenic effects of VT. Torvinen et al. [2003] found no BMD improvement in any skeletal site, whereas Verschueren et al. [2004], applied much greater training load and noted significant increase of hip BMD. Thus, achieving BMD increase most probably requires high enough intensity of VT similarly to intensity of strength exercises required to achieve strength improvement, also.
 
Exercise-induced oxidative stress
 
Beside beneficial health effects of physical activity, many studies have reported increased generation of reactive oxygen species (ROS) [Witt et al. 1992, Sanchez-Quesada et al. 1995]. Prolonged submaximal exercise elevated whole body [Gee and Tappel 1981] and skeletal muscle [Davies et al. 1982] concentration of lipid peroxidation byproducts. However, the activities of antioxidant enzymes such as superoxide dismutase, catalase, and gluthatione peroxidase, and the concentration of glutathione can increase following aerobic exercise training [Evelo et al. 1992, Hellsten et al. 1996, Powers et al. 1998, Vincent et al. 2002]. Higher training intensities induce greater changes in the antioxidant defense [Powers et al. 1994]]. These observation led to what have been termed “ the exercise-induced oxidative stress paradox” [Leaf et al. 1999]: it was found, that exercise training increased work capacity without a concomitant increase in expired markers of lipid peroxidation, and decreased lipid oxidation product plasma malondialdehyde (MDA) level. It has been even suggested, [Radak et al. 2001] that regular exercise results in overcompensation against increased level of reactive oxygen and nitrogen species, what may lead to adecreased base level of oxidative damage and increased resistance to oxidative stress. This paradox seems of particular interest in elderly. Asenescent organism is more susceptible to oxidative stress during exercise, and muscle repair capacity is reduced, however the elderly who are physically active seem to benefit from exercise-induced adaptation in cellular antioxidant defense system [Ji 2001].

Plasma antioxidants, lipids oxidation products, and muscle damage in response to RT.
 
There are few studies on changes in plasma antioxidants and lipids oxidation products after resistance exercises. McBride et al. [1998] found increased MDA level at 6 and 24 h after high intensity resistance exercise. Alessio et al. [2000] compared effect of exhaustive aerobic and anaerobic isometric exercise on biomarkers of oxidative stress. They found 14-fold increase of oxygen consumption with aerobic and 2-fold after anaerobic exercise, and accordingly lesser and shorter lasting increase in protein carbonyls – product of protein oxidation, however lipid hydroperoxides (LH) increased more after anaerobic exercise. Both types of exercises increased total antioxidants. Vincent et al. [2002] found that 6 months of RT in elderly can prevent the elevation of serum lipid peroxidation levels following acute aerobic exercise and can enhance thiol group mediated detoxification system. Ramel, Wagner and Elmadfa [2004] examined resistance-trained and non-resistance trained subjects, and found after submaximal resistance exercise increase of fat soluble antioxidants, MDA, but not ascorbic acid. Another product of lipid oxidation – conjugated dienes –increased only in non-resistance trained group. Authors suggest that regular RT partly prevents lipid peroxidation during exercise. Recently, protective effect of RT against oxidative stress in elderly subjects has been confirmed by Parise, Brose and Tarnopolsky [2005].
 
It is believed, that beside muscle cells damage caused by physical forces, these cells could be also damaged by increased rates of lipid peroxidation resulting from oxygen radical production, especially in untrained persons, older men and women, and those with an inadequate antioxidant system [Evans 2000]. Acute resistance exercise with the eccentric overload elevated muscle damage marker plasma creatine kinase (CK) concentration 24 h and 48 h, more in untrained than in resistance trained subjects [Dolezal et al. 2000].
 
Young healthy subjects increased their oxygen uptake up to 50% of their VO2max while performing static and dynamic squats on a vibrating platform [Rittweger et al. 2000]. Oxygen uptake in young healthy subjects has been shown to increase with increase of vibration frequency and amplitude, and external load [Rittweger 2002]. Increased oxygen consumption may lead to enhanced production of free radicals, however, there is currently a lack of knowledge on the acute effects of whole body vibration and chronic effects of VT on the markers of exercise-induced oxidative stress and antioxidant defense.
 
Conclusions
 
Vibration training, if applied in accordance to the overload principle, seems, as could be judged from first studies, as effective as resistance training, in reversing sarcopenia and increasing bone mineral density. However, because VT (like RT) has to be performed permanently by elderly trainees in order to sustain increased muscle strength, potential adverse side effects have to be carefully determined and , if found, minimized by proper design of VT training. At present VT should be regarded promising alternative to RT as asarcopenia countermeasure, however scarcity of data on physiological responses to VT precludes recommending VT as standard sarcopenia treatment.
 
 
vibration training an overview training consequences and future considerations
Power Plate Studies
ABSTRACT.

The effects of vibration on the human body have been documented for many years. Recently, the use of vibration for improving the training regimes of athletes has been investigated. Vibration has been used during strengthtraining movements such as elbow flexion, and vibration has also been applied to the entire body by having subjects stand on vibration platforms. Exposure to whole-body vibration has also resulted in a significant improvement in power output in the postvibratory period and has been demonstrated to induce significant changes in the resting hormonal profiles of men. In addition to the potential training effects of vibration, the improvement in power output that is observed in the postvibratory period may also lead to better warm-up protocols for athletes competing in sporting events that require high amounts of power output. These observations provide the possibility of new and improved methods of augmenting the training and performance of athletes through the use vibration training. Despite the potential benefits of vibration training, there is substantial evidence regarding the negative effects of vibration on the human body. In conclusion, the potential of vibration treatment to enhance the training regimes of athletes appears quite promising. It is essential though that a thorough understanding of the implications of this type of treatment be acquired prior to its use in athletic situations. Future research should be done with the aim of understanding the biological effects of vibration on muscle performance and also the effects of different vibration protocols on muscle performance.
 
INTRODUCTION
 
The effects of vibration on the human body have been documented for many years. In fact, a drive along a rough road was once prescribed for individuals suffering from kidney stones due to the therapeutic effects of the bumpy ride (23). Many positive effects of vibration on the human body have also been reported in physiotherapeutic and clinical settings in which vibration has been used for pain management and to elicit muscle contractions in spastic and paretic muscles (2, 28, 30). FIGURE 1. Different vibration waveforms (23, p. 6).At present, research is being performed examining the use of whole-body vibration in the treatment and prevention of osteoporosis (40).
 
Recently, the use of vibration for improving the training regimes of athletes has been investigated (5, 6, 7, 8, 26, 27, 30, 36, 40). Vibration has been used during strength-training movements such as elbow flexion (6), and vibration has also been applied to the entire body by having subjects stand on vibration platforms (5, 8, 36). In these instances, the vibratory wave is propagated from distal to proximal links of muscle groups and the subjects are often required to perform voluntary muscle contractions throughout the vibration exposure (26, 27, 36). The application of vibration in this method has been shown to increase electromyogram (EMG) activity during the exposure to vibration (40). Vibration applied to the upper extremities during a 3-week training period has been shown to enhance gains in maximal strength in the seated row (26, 30) and in the postvibratory period, vibration applied to the elbow flexors resulted in increased power output during elbow flexion (6).
 
Exposure to whole-body vibration has also resulted in a significant improvement in power output in the postvibratory period and has been demonstrated to induce significant changes in the resting hormonal profiles of men (8). In addition to the potential training effects of vibration, the improvement in power output that is observed in the postvibratory period may also lead to better warm-up protocols for athletes competing in sporting events that require high amounts of power output. All of the above observations provide the possibility of new and improved methods of augmenting the training and performance of athletes through the use of vibration training.
 
In contrast with the literature on the positive effects of vibration training, the negative effects of vibration on the human body are also well documented and are most often observed in the work place through exposure to large vibration loads or chronic exposure to vibration (21, 22, 43, 46). In this environment, exposure to vibration has been shown to damage several biological structures, including peripheral nerves, blood vessels, joints, and perceptual function. Studies investigating the effects of vibration on animals also report changes in endocrine function, cardiovascular function, respiratory responses, central nervous system (CNS) patterns, and metabolic processes (23). Based on research from the workplace studying the physiological hazards of vibration load, standards have been developed to limit exposure to harmful vibration (23, 24). However, although exposure standards exist for the workplace, there are no standards to limit exposure to vibration in the athletic environment, yet sports such as alpine skiing, sailing, in-line skating, and horse-back riding are known to have a significant vibration load (36). While it appears that exposure to vibration may be more harmful than beneficial, it must be mentioned that the biological reaction to vibration is dependent on the frequency, magnitude, duration, and type of vibration (23).
 
While vibration may be a potent stimulus for the neuromuscular apparatus, it is crucial that coaches and physiologists have a general understanding of the effects of vibration on the human body. The purpose of this review, therefore, is to provide a general framework of understanding concerning the major factors and implications surrounding vibration training.

AN OVERVIEW OF VIBRATION
 
Vibration is defined as oscillatory motion and the study of human vibration is ‘‘a multi-disciplinary subject involving knowledge from disciplines as diverse as engineering, ergonomics, mathematics, medicine, physics, physiology, psychology, and statistics’’ (23). In the human environment, whole-body and or hand-transmitted vibration occurs in several instances, including motorized vehicles (e.g., cars, trucks, motor cycles), marine ships (e.g., boats, submarines), aircraft, buildings, and from industrial equipment (e.g., cranes, fork lifts). When an individual is exposed to vibration in these settings, the sensation that is experienced varies considerably. In the abovementioned settings, the characteristics of the vibration also varies. Vibration can vary according to the magnitude, which is determined by the size of the oscillation, and by the frequency, which is determined by the repetition rate of the vibration. The waveform may be of a deterministic or random form. Examples of different waveforms of oscillatory motion are presented in Figure 1.
 
Typically in sport situations, vibration is of a random form (e.g., the vibration experienced by an alpine skier during a downhill event). In contrast, during whole-body vibration training using commercially available vibration platforms, the resultant vibration transmitted to the subject is of a deterministic, sinusoidal waveform. That is, this form of vibration can be predicted from knowledge of prior oscillations and it is repeated in an identical time interval.
 
The frequency of vibration is measured in Hertz (Hz) and is the main factor determining the biological effect of the vibration (23). The magnitude of vibration can be represented either as the acceleration (i.e., g or m·s22) or as the displacement (i.e., mm, cm, m). Finally, the duration of the exposure must be considered when evaluating the potential effects of vibration on the human body (23).

BIOLOGICAL INTERACTION WITH VIBRATION
 
Exposure to vibration affects several physiological systems, including the neuroendocrine, cardiovascular, musculoskeletal, and sensory systems (23). For example, vibration ranging from 1 to 20 Hz can cause blurry vision, whereas vibration at frequencies between 20 and 70 Hz can result in resonance of the eye (23). Vibration between 2 and 20 Hz may elicit a cardiovascular response similar to that of mild exercise, and vibration at 120 Hz results in an increase in fetal heart rate (23).
 
While vibration may have the potential to elicit a substantial training effect, it must be clearly understood that exposure to vibration can have serious negative physiological effects. However, the different characteristics of a vibration waveform, including the amplitude, frequency, and type of vibration, make it difficult to predict the exact physiological response (23). The response also depends on the intrasubject and intersubject variability. Some of the factors that affect the intrasubject variability include the orientation of the subject (e.g., sitting in moving vehicle while facing forwards vs. facing sideways), the body position (e.g., sitting vs. standing), and the body posture (e.g., stiff posture vs. relaxed posture). The intersubject variability is affected by the size of the individual (e.g., children vs. adults), body dynamics (e.g., the amount of power that can be absorbed by the body), age, gender, and the psychological preparedness of the individual
 
A review of all the different biological reactions that occur in response to vibration is beyond the scope of this document. In order to review the consequences of vibration training, it is more suitable to focus on the effects of vibration on skeletal muscle and some of the potential hazards associated with exposure to vibration.
 
Effects of Vibration on Skeletal Muscle

Vibration of a muscle stimulates the primary endings of the muscle spindle (Ia afferents), which excites a-motoneurons, causing contraction of homonymous motor units, and this results in a tonic contraction of the muscle, referred to as the tonic vibration reflex (2, 3, 10, 11, 14, 28, 32, 36). Electromyogram data have revealed that the tonic vibration reflex has both a monosynaptic and a polysynaptic component (2). Further evidence demonstrating the complexity of the tonic vibration reflex can be observed in a decerebrate cat, as even in this condition, the tonic vibration reflex can be elicited. The response of the tonic vibration reflex is also dependent on the frequency of vibration, the level of precontraction of the muscle, and the position of the body (32, 38, 42).
 
During an exposure to vibration, the stretch reflex and the Hoffman-reflex (H-reflex) are inhibited, and this has been referred to as the vibration paradox—vibration induces the tonic vibration reflex but inhibits the stretch reflex and the H-reflex (17, 18, 34). This is due to presynaptic inhibition of the Ia afferents of the muscle spindle, which permits excitation of the pathways relating to vibration and inhibition of the pathways responding to stretch (18). The depression of the H-reflex has been shown to be greater than the depression of the stretch reflex, although the exact mechanism for this difference has been difficult to identify. In the postvibratory period, the stretch reflex displays a marked potentiation in contrast with the H-reflex that displays a gradual recovery up to normal values over a period of about 100 seconds (1, 39). The depression of the H-reflex during exposure to vibration is greater as the amplitude of the vibration increases at a constant frequency, but the depression remains unchanged if the amplitude is constant as the frequency of vibration increases (1).
 
Research also indicates that vibration compensates for the reduction in motor output in a fatigued state when the exposure is preceded by a prolonged maximal voluntary contraction (i.e., 4 minutes) (3). It has been hypothesized that, in a fatigued state, vibration compensates for the decreased ÿ-motoneuron drive by exciting the Ia afferents and this results in the reflexive excitation of the œ-motoneurons and greater force output.
 
The effects of vibration on skeletal muscle described above are well documented. However, these effects do not necessarily account for the benefits that are observed following acute and chronic administration of vibration training. While these effects provide insight into the possible mechanisms that may explain these benefits, further research must be performed to determine the physiology underlying the effects of vibration training.
 
Physiological Hazards Associated With Vibration Exposure

There is substantial evidence regarding the negative effects of vibration on the human body (23). The specific physiological effects of vibration can be considered under the following categories: (a) cardiovascular function, (b) respiratory function, (c) endocrine and metabolic function, (d) motor processes, (e) sensory processes, (f) CNS, and (g) skeletal changes. In animal studies in which high magnitudes of vibration have been used, there are reports of lung damage, gastrointestinal bleeding, and heart hemorrhage causing death in the exposed animals. Some reports on the more serious adverse affects from exposure to large vibration loads include severe chest pain and gastrointestinal bleeding in an individual exposed to vibration of 10 and 25 Hz ranging from ±3 g to ± 10 g (23). Vibration-induced pathology also includes hypertrophy of blood vessel walls, resulting in a narrowing of the arteriole lumen, damage to joints and bones, and neurological disorders (20, 24).
 
Research from the workplace provides evidence on the physiological hazards resulting from certain types of vibration exposure. In these instances, workers may be exposed to vibration for long durations or to large magnitudes of vibration for short periods of time. An example of vibration-induced pathology is hand-arm vibration syndrome (HAVS), a disorder associated with excessive exposure to vibration. HAVS is common in miners who operate jack-leg-type drills (46). In these instances, miners can be exposed to vibration for up to 3 hours per day. Patients exhibiting HAVS present with neurological dysfunction in the hands and, in later stages of this disease, vascular dysfunction can occur. The incidence of disease from exposure to vibration is a function of exposure time (37). A study examining the prevalence of vibration-induced pathology in a group of 266 chain-saw operators found that subjects with less than 2,000 hours of exposure reported symptoms of tingling, numbness, and mild pain; those with 2,000–5,000 hours of exposure displayed circulatory dysfunction, including Raynaud’s syndrome; and those workers with over 8,000 hours of exposure to vibration suffered severely from functional and organic changes (e.g., vertigo, irritability, sleeplessness, and other autonomic disturbances) (37).
 
Given the many risks associated with vibration exposure, prudence is required when integrating vibration training into the programs of athletes. While the exposure time during vibration training is quite low in comparison with workplace exposures to vibration, most vibration platforms are capable of exposing athletes to frequencies and amplitudes of vibration that could result in serious injury. In summary, the main factors that must be considered to ensure the safety of the athletes engaging in vibration training are the magnitude of vibration (frequency and amplitude), the duration of exposure, and the body position during the exposure. Careful prescreening measures for injuries that may be exacerbated by vibration training should also be included in vibrationtraining protocols.
 
VIBRATION TRAINING

Vibration Training, Equipment, and Parameters

Vibration training has been performed during upper-body movements using pulley systems and during lower-body FIGURE 2. Example of vibration training.movements using vibration platforms and vibration leg frequency. This may pose experimental problems for scientific investigations using commercially available platforms.
 
Most commercially available vibration platforms have a control mechanism for the vibration frequency and amplitude, which allows the vibration characteristics to be adjusted. Typically, the vibration characteristics during vibration training involve relatively low vibration frequencies and small amplitudes of vibration. The scientific literature reports ranges in vibration frequency from 25 to 44 Hz and ranges in amplitude from 2 to 10 mm. It must be mentioned that, at the present time, the effects of different vibration-loading parameters are not clearly understood. This is reflected in the wide range of frequencies, amplitudes, and durations of exposure used in different scientific investigations.
 
BENEFITS OF VIBRATION TRAINING IN THE GENERAL POPULATION

The interaction of the human body with vibration is complex and, despite a risk for serious injury, certain types of vibration possess healing effects and health benefits. Vibration is used to help clear the lungs in patients with respiratory problems, to improve mobility and muscle function in athletes, to help those suffering from rheumatoid arthritis, to treat the stumps of amputated limbs, and to improve muscle function in spastic and paretic individuals (2, 23).
 
An excellent example of the potential benefits of vibration on the health of the general population is the increase in bone mass that is observed after long-term exposure to whole-body vibration, an effect that may be important in preventing osteoporosis (18, 25). A study evaluating the effects of whole-body vibration on the bone mineral density of ovariectomized rats indicated that mechanical stimulation in the form of low-intensity, wholebody vibration was effective in reducing postovariectomy bone loss in rats (18). This observation may lead to benefits for humans who are at risk for bone loss. Some of these populations include postmenopausal women and astronauts who are subject to zero-gravity conditions for prolonged periods of time.
 
Effects of Vibration Training on Athletic Performance

In currently accepted training methodology, the development of the physiological qualities that relate to the expression of muscle force has been accomplished using different types of strength-training protocols (31). This involves the use of jumps, sprints, and exercises with added resistance. It is possible that vibration training may be an additional method for the development of the physiological qualities related to muscle force production (5–8, 26–28, 36, 40).
 
Vibration training can be performed by applying vibration to a limb or extremity using specially designed pulley machines, but more commonly, it is used in the form of whole-body vibration training. It has been postulated that vibration affects several factors related to intramuscular coordination (30). As previously described, vibration of a muscle activates the tonic vibration reflex, which is evidenced by an increase in EMG of the vibrated muscle during the exposure to vibration (36, 40). It is possible that the tonic vibration reflex may enhance voluntary muscle contractions and, when used in conjunction with strength-training protocols, vibration may improve neuromuscular training in athlete populations. An increase in motor-unit synchronization is one of the explanations offered in an attempt to account for the increase in muscle force observed during vibration of a muscle. Vibration training is also thought to increase the neural drive to muscles and to recruit previously inactive motor units (27). The increased recruitment of motor units is thought to occur because of several neuromuscular changes that occur during exposure to vibration, and through the recruitment of these additional motor units, vibration training may be beneficial in the development of muscle power and maximal strength.
 
Vibration training does not appear to elicit a significant cardiovascular response. A study comparing the effects of exhaustive whole-body vibration exercise in a group of 40 healthy subjects with exhaustive cycle ergometry demonstrated that there was a significantly smaller increase in heart rate during vibration training (40). Maximal oxygen uptake was also significantly lower during vibration exercise compared with maximal cycling and the mean blood lactate concentration was significantly lower after the exhaustive vibration exercise. It was concluded that exhaustive vibration exercise did not have a significant cardiovascular effect and did not result in a stimulating effect on the cardiovascular control system.
 
ACUTE EFFECTS OF VIBRATION TRAINING

Vibration training may be useful in an acute setting as a neuromuscular warm-up in preparation for explosive athletic events. Vibration training applied to the arms of 12 international-level boxers during elbow-flexion exercise resulted in increased EMG during the exposure to vibration and a significant increase in mechanical power following the exposure to vibration, despite a concomitant decrease in EMG (6).
 
In an acute setting, whole-body vibration training resulted in a significant improvement in average force, average velocity, and average power during leg press in a group of 6 female volleyball players (7). This effect was also noted in a group of 14 male subjects, who displayed a significant improvement in leg power and a concomitant decrease in EMG after a 10-minute treatment with whole-body vibration (8). In this investigation, wholebody vibration also elicited a change in hormonal profiles. Following the 10-minute vibration treatment, there was a significant increase in growth hormone and testosterone and a significant decrease in cortisol over resting values.
 
In contrast with the reports of acute improvements in performance following vibration, other investigations report a decrement in performance or a negligible improvement in performance (16, 45). De Ruiter and coauthors performed an investigation evaluating the acute effects of whole-body vibration training (16). It was found that acute exposure to vibration did not improve performance during the voluntary and involuntary muscle contractions. In fact, there was a 7% decline in force production during the maximal voluntary contraction 90 seconds following the vibration treatment. The decline in force in the maximum voluntary contraction was relatively larger than the decline in force observed during the involuntary contraction. This was reflected in a decrease in voluntary activation from 95 to 90%. This finding indicates a decline in neural activation (44).
 
Chronic Effects of Vibration Training

Vibration-training research to date suggests that this modality of training may have the potential to elicit a longterm training effect on strength and power (5, 26, 27, 36). A 21-day training block of whole-body vibration training resulted in substantial gains in strength and leg power in a well-trained alpine skier (36). Following the 21-day training block, force production measured on the isometric leg press increased by 43% over the initial value and vertical jump also increased from 38.9 to 47.8 cm. Lieberman and Issurin evaluated the effects of vibration applied to the upper extremities and found that a 3-week training period of vibration during the seated row resulted in an average increase of 49.8% in strength compared with a 16.1% increase in strength for the training group not exposed to vibrations (30). Finally, a 10-day block of whole-body vibration training resulted in an improvement in vertical jumping ability in 14 physically active subjects (5). The subjects exposed to the 10-day vibration treatment displayed a significant increase in jump height during 5 seconds of continuous jumping and a smaller improvement in jump height for the countermovement jump. The control group did not show improvement in jumping ability for either the 5 seconds of continuous jumping or the countermovement jump.
 
Evidence for the greater overload on the neuromuscular apparatus that occurs as a result of vibration training was observed in a study examining the acute effects of vibration during elbow flexion (30). Vibration applied during elbow flexion resulted in a significant increase in the 1 repetition maximum and subjects perceived their effort to be less during the exposure to vibration compared with the no-vibration condition. In addition to the increased neuromuscular activation resulting from the tonic vibration reflex, evidence also suggests that the increased load lifted for a 1 repetition maximum during exposure to vibration was obtained in part by the sensation that the load being lifted was lighter than when not subject to vibration. A possible explanation for the decreased sensation of effort during a 1 repetition maximum in elbow flexion observed in this study is that, during the exposure to vibration, the increased activity of the Ia afferents may have reduced the central feedforward element making the force produced during exposure to vibration feel like less force than during a normal contraction (12, 13). The problem in making this conclusion is that it is difficult to quantify the degree to which the feedforward element affects the perception of force during maximal voluntary force production.
 
Based on the above findings, it is clear that vibration training has the potential to elicit a training effect over a 10–21-day mesocycle. The benefits to vibration training may be a result of positive changes in neuromuscular mechanisms (e.g., improved synchronization of firing of the motor units and improved cocontraction of synergist muscles), and an increased discharge from Ia afferents and subsequent increase in EMG that occur during the period of vibration. These effects on the neuromuscular system may provide a greater overload than conventional training alone, and thus vibration training may provide a superior training effect over the long term.
 
CURRENT DEFICITS IN VIBRATION-TRAINING LITERATURE

Currently, the scientific evidence on the benefits of vibration training is mixed. Evidence does exist in support of the acute and chronic benefits of vibration training (5–8, 36), and there is also evidence demonstrating a negligible training effect following vibration training and perhaps even a decrement in performance (16, 40, 45). There are several factors that must be considered in the interpretation of the scientific evidence evaluating vibration training. Two important factors that have the potential to affect the outcome of vibration-training experiments include the vibration protocol and the specific movements that are tested following the vibration treatment.

 
TABLE 1. A comparison of vibration characteristics between different vibration training investigations.
 
VARIABILITY IN VIBRATION PROTOCOLS

The variability in the vibration-training protocols used by different investigators may be an important reason for the inconsistent results that are reported in the scientific literature. The vibration protocols can vary in the vibration characteristics (i.e., frequency and amplitude) that are used, the movement performed during the exposure to vibration, the duration of the exposure, and the length of time between the cessation of the vibration treatment and the posttreatment measurements. Table 1 provides a brief summary of some of the differences in the vibrationtraining protocols reported in the scientific literature used in investigations evaluating the acute effects of vibration on performance.
 
As with all types of exercise designed to improve strength qualities, the exact loading parameters must be carefully determined, applied, and then controlled in order to ensure that the desired training effect occurs. It is likely that different vibration protocols would elicit different physiological effects and, as a result, the exact vibration characteristics are of paramount importance when evaluating and interpreting the results of an investigation on the effects of vibration training.
 
Evidence in support of the need to account for interprotocol differences in the vibration treatment can be obtained in the basic science literature on vibration. First, the position of the body during the exposure to vibration and the degree of muscle contraction can affect the biological response to vibration (23, 29). The EMG response in the shoulder muscles during exposure to vibration has been shown to vary according to the position of the arm and the level of muscle contraction (42). Second, the biological response, and specifically the tonic vibration reflex, is highly dependent on the frequency of the vibration (23, 32). Third, the duration of the vibration exposure can greatly affect muscle function. Evidence from an investigation on the effects of prolonged exposure to vibration suggests that, during a sustained maximal voluntary contraction of the dorsiflexors muscles, vibration counteracted fatigue in the initial phase of its application but prolonged vibration accentuated fatigue as evidenced by a large decrease in EMG activity (4).
 
In summary, based on the parameters represented in Table 1, it is clear that vibration protocols used by investigators have varied according to the frequency, amplitudes, duration of exposure, the type of exercise during the exposure to vibration, and the timing of the posttreatment measurements. It is also clear that all of these parameters have the capacity to greatly affect the biological response to vibration and therefore the effects of vibration training on performance.
 
Movement-Specific Effects of Vibration

In addition to the large potential that exists for the vibration protocol to affect the outcome of vibration-training studies, it is also possible that vibration may elicit movement-specific effects. While several investigations have demonstrated a postvibratory improvement in dynamic and explosive activities (5, 7, 8), other studies have demonstrated a deterioration or negligible improvement in performance during isometric maximal voluntary contractions (16, 40, 45).
 
While accurate comparisons between the results of these different investigations may be troublesome due to the large interprotocol variability that exists in the vibration treatment, it may be possible that the posttreatment effects of vibration treatment would augment performance only in explosive activity involving a stretch-shorten cycle and not maximal isometric activity. Evidence in support of this can be obtained from an investigation that compared the effects of vibration on continuous jumping and a countermovement jump (5). In this investigation, 10 days of vibration training resulted in an improvement in jump height during 5 seconds of continuous jumping in which the subjects were asked to minimize ground contact time compared with a relatively smaller improvement in jump height during one maximal countermovement jump. In this instance, the 5 seconds of continuous jumping involved a short stretch-shorten cycle, which may have been more greatly influenced by the stretch reflex and muscle stiffness. Further evidence in support of a specific postvibratory improvement in explosive activity is obtained from the basic science literature. There is strong evidence that, in the postvibratory period, there is a marked potentiation of the stretch reflex in the human plantar flexor muscles (1).
 
The interprotocol variation, which includes the vibration loading parameters and the movement that is used to test the effects of vibration training, creates difficulties when deciding on the viability of vibration training. This further highlights the need for more research before vibration training becomes a regular component of any training regime.
 
FUTURE RESEARCH

Although several pathological conditions are associated with exposure to vibration and that there are adverse effects on workers who are exposed to high levels of occupational vibration, the severity of the symptoms that results is a function of the total exposure to and the intensity of the vibration. While the duration of the exposure to vibration is comparably less during vibration training than in an occupational setting, the risk for injury is still high if vibration training is used inappropriately. It is clear from the occupational research that further investigation should be performed in this area to minimize the risk of serious injury to athletes engaged in vibration training.
 
Further research should also be undertaken to understand the effects of different vibration protocols on performance. The current scientific investigations on vibration training use a wide range of vibration protocols and loading parameters. Some studies required the subject to perform voluntary movements with extra loads; some required voluntary movements without loads; and in some studies, the subjects stood in a static position on the vibration platform. These performance studies also differed in the frequency and amplitude of the vibration and the length of exposure for the vibration treatment. None of the studies offer a rationale explaining why one protocol is superior to another; yet with all types of training, whether it is strength training or cardiovascular training, it is imperative that the volume, intensity, and duration of exposure be controlled and manipulated, depending on the desired outcome. Based on the fact that the tonic vibration reflex is influenced by several variables, including the level of precontraction in the muscle, the position of the body and the vibration characteristics (i.e., vibration waveform, amplitude, and frequency) it is important that the effects of these variables be further investigated and controlled during performance studies on vibration training.
 
PRACTICAL APPLICATIONS

Vibration may be a potent stimulus for the neuromuscular system. However, the use of vibration as a training modality to enhance the performance of athletes is still in its early stages of development. It would appear, based on the limited scientific evidence, that vibration training may serve as a tool to develop explosive ability in athletes in both acute and chronic training conditions. The benefits of vibration training must be balanced with the potential physiological hazards associated with exposure to vibration. The following recommendations can be made to coaches and physiologists interested in using vibration training:
 
  1. Carefully consider and review the current scientific evidence evaluating the use of vibration training.
     
  2. Carefully consider the appropriate vibration loading parameters to ensure the safety of the athletes. Consideration should be given to the vibration frequency and amplitude, the duration of exposure, and the activity performed during the exposure to vibration.
     
 
Acute whole body vibration training
Power Plate Studies
Objective:

To quantify the acute effect of whole body vibration (WBV) training on arm countermovement vertical jump (ACMVJ), grip strength, and flexibility performance.

Methods:

Eighteen female elite field hockey players each completed three interventions of WBV, control, and cycling in a balanced random manner. WBV was performed on a Galileo machine (26 Hz) with six different exercises being performed. For the control, the same six exercises were performed at 0 Hz, whilst cycling was performed at 50 W. Each intervention was 5 min in duration with ACMVJ, grip strength, and flexibility measurements being conducted pre and post intervention.

Results:

There was a positive interaction effect (intervention6pre-post) of enhanced ACMVJ (p,0.001) and flexibility (p,0.05) parameters following WBV; however no changes were observed after the control and cycling interventions. There was no interaction effect for grip strength following the three interventions.

Conclusions:

Acute WBV causes neural potentiation of the stretch reflex loop as shown by the improved ACMVJ and flexibility performance. Additionally, muscle groups less proportionally exposed to vibration do not exhibit physiological changes that potentiate muscular performance.

        Whole body vibration (WBV) is a novel training intervention performed on a commercially manufactured machine known as the Galileo Sport. This instrument was originally developed to increase power and strength in athletes1–3 and its vibration production has been described elsewhere.4 However, in summary the Galileo Sport machine has a tilting platform that delivers oscillatory movements to the body of varying frequencies (0–30 Hz) around a horizontal axle. The subject stands on the machine with the feet placed on either side of the axle and maintains a steady position while the oscillations of the platform produce a vertical ground reaction force to each foot alternately. The rapidly repeating eccentric-concentric contractions presumably evoked, result in muscular work evidenced by a significant elevation in metabolic rate.4
Proven to be effective in improving strength,5 6 bone density,7 and body composition,5 this novel training intervention is becoming popular for conditioning, rehabilitation, and general health. However, the acute effects of WBV on power, strength, and flexibility are still largely untested.
Superimposed cable vibration increases maximal muscular force8 and an improved residual post-vibration effect has been noted.9–11 The latter is suggested to be the joint result of increased sensitivity of muscle stretch receptors to excitation, elevated muscle temperature, and improved blood flow.8 The improvement in muscular power following acute exposure results in performance gain that would otherwise require weeks of training.10 11
From these observations, it would be expected that a functional measure of muscular power, such as maximal vertical jump height, would be significantly enhanced by acute exposure to WBV. However, the data are equivocal, showing either no change,12 a decrease,13 or an increase14 15 following acute exposure to WBV. Nevertheless, improper control, differences in vibration duration, frequency, and amplitudes, lack of or use of a warm up, and differing exercise positions, may explain the discrepancies in the results. In addition, previous studies have not permitted arm movement in the vertical jump test in an attempt to isolate lower limb muscular performance. In sport, jumping movements generally involve arm drive, so information regarding vertical jump performance with arm countermovement (arm countermovement vertical jump, ACMVJ) after acute WBV is required.
The paucity of well controlled studies investigating the acute effect of WBV on vertical jump height is surprising considering the number of sporting activities which would be enhanced by an improvement in vertical jump height. Furthermore, the lay literature suggests that other measures, such as flexibility, are influenced by acute WBV, but there is little information in the scientific literature in this regard
Therefore, the primary aim of this study was to quantify the acute effect of WBV on ACMVJ performance in elite female field hockey players. To better understand the mechanism by which WBV may exert an ergogenic effect, a secondary aim of this study was to observe whether muscle groups exposed less proportionally to vibration are capable of enhancing muscular performance. Finally, a third aim was to test the effect of WBV on flexibility using a sit and reach test.
 
METHODS
 
Participants
 
Eighteen healthy female elite field hockey players volunteered to participate in the study. They were (mean¡ standard deviation) 21.8¡5.9 years old and 1.66¡0.06 m tall, and weighed 63.7¡7.6 kg. The protocol was approved by the Massey University Human Ethics Committee.
 
Study design
ACMVJ, isometric grip strength, and sit and reach flexibility were measured before and after the following three interventions, which each lasted 5 min:
 
  • Standing on the Galileo Sport machine with vibration (WBV)
     
  • Standing on the Galileo Sport machine without vibration (control)
     
  • Seated cycling
     
Within 15 s of completing the intervention, participants were retested on ACMVJ, grip strength, and flexibility measures in the same order as before the intervention.
 
The intervention order was allocated in a randomised, balanced design, with 24 h recovery between each testing session. During the course of the study, participants were not permitted to undertake any power or strength training and timing of the menstrual phase was accounted for. The participants were strictly instructed to refrain from undertaking any vigorous activity 24 h prior to the interventions, and, to prevent interference from variations in daily biorhythms, participants performed the tests at the same time each day. Each participant was familiarised with the equipment and tests prior to the study and performed pre and post measures of countermovement vertical jump, grip strength, and sit and reach. Every participant wore exactly the same pair of shoes to standardise vibration dampening.
 
Performance tests
 
Warm up was prohibited prior to the performance tests to reduce the possibility of influencing the outcome of the study.

Vertical jump test
 
Six countermovement jumps with arm swing (ACMVJ) were performed according to the protocol of Harman et al.16Each jump was recorded to 0.1 cm and was separated by a rest period of 10 s. The ACMVJ was measured by a system consisting of a portable hand held computer unit connected to a contact timing mat (Swift Performance, NSW, Australia). It has been previously reported that this system has a validity and reliability similar to a force plate.17
 
Hand grip strength test
 
Hand grip strength was determined by a hand grip dynamometer (Smedlay, Tokyo). Participants stood upright with arms down the side of the body. The dominant hand performed three repetitions of a 2 s grip contraction which was recorded to the nearest 0.1 kg.
 
Sit and reach test
 
The sit and reach test was conducted on a sit and reach apparatus (Figure Finder Flex Test, Novel, Rockton, IL) and followed the procedure as outlined by Church et al18 with the maximum end position being held for 2 s. Two repetitions were recorded to the nearest centimetre and separated by a rest period of 10 s.
 
Interventions
 
WBV
 
This was performed on a commercial Galileo Sport machine (Novotec, Pforzheim, Germany). The participants stood on the machine and positioned their feet around the centre of the oscillating platform, that equated to a peak to peak amplitude of 6 mm of vertical vibration. The frequency was set at 26 Hz. The positions taken by the subject were: (1) standing upright with knees semi-locked; (2) isometric squat at a knee angle of approximately 120˚; (3) kneeling on the ground with arms straight and hands placed on the platform equating to a peak to peak amplitude of 6 mm of vertical vibration; (4) squatting at a tempo of 2 s up and 2 s down at a knee angle of approximately 120˚; (5) lunge position with left leg on platform and right leg on ground; and (6) lunge position with right leg on platform and left leg on ground (fig 1). Positions 1–4 were held for 1 min and positions 5 and 6 for 30 s.
Figure 1 The six positions performed on the Galileo Sport machine. (The subject depicted in these images agreed to the photographs being published.)
 
Control
 
The control intervention was performed on the Galileo Sport machine (0 Hz, amplitude 0 mm) with the exact same six body positions and time constructs as described for the WBV intervention.
 
Cycling
 
In the seated position, the subject pedalled at a cadence of 50 rpm for 5 min at 50 W on a friction braked cycle ergometer (Monark 818 E, Varberg, Sweden).
 
Statistical analysis
 
All statistical analyses were performed using a specialised statistical software package (SPSS for Windows version 10; SPSS, Chicago, IL). Dependant variables were compared using a three way, repeated measures analysis of variance (ANOVA). Each test data measure (six for ACMVJ, three for hand grip, and two for sit and reach) was included as part of the ANOVA analysis. Pairwise comparison between means was performed using post hoc contrasts to identify intervention difference. Intraclass correlation coefficients (ICC) assessed the test-retest reliability of comparing the mean of the dependent variables between testing sessions. Significance was considered to be at or greater than the 95% level of confidence (p<=0.05).
 
RESULTS
 
ACMVJ
 
There was a significant (p<0.001) (intervention6pre-post) interaction (fig 2) such that WBV produced an ergogenic effect resulting in an 8.1±5.8% increase in ACMVJ height (fig 3) compared to both control (–2.0±3.7%) and cycling (–0.3±3.7%).

Hand grip strength
There was no significant interaction effect for WBV, control, and cycling interventions (fig 4). There was a significant effect in that each repetition of grip strength decreased (p<0.05).

Sit and reach

 
The interaction between treatment and pre-post sit and reach (p,0.05) was significantly greater after WBV (8.2¡5.4%) compared to the control and cycling interventions (5.3¡5.1% and 5.3¡4.9%, respectively; fig 5). Post hoc analysis revealed that for all interaction interventions (WBV, control, and cycling) sit and reach increased for each repetition (p,0.001). The ICC test-retest reliability analyses revealed that jump height (0.916), grip strength (0.804), and sit and reach (0.934) were consistent between sessions.
 
DISCUSSION
 
ACMVJ
 
The primary purpose of this study was to examine the acute effects of WBV (eccentric-concentric loading), control, and cycling (concentric only loading) interventions on ACMVJ performance. Our results show that ACMVJ performance is enhanced by 8.1% immediately following 5 min of WBV exposure when compared to control (no vibration) conditions. Figure 2 Mean and standard deviation pre and post ACMVJ for WBV, control, and cycling interventions. *Statistically significant (p,0.001) interaction effect (pre-post) for WBV intervention.These observations are in accordance with the findings of Bosco et al14 and Torvinen et al15 who reported increases of 2.5% and 3.8% in vertical jump height, respectively, but without arm action.
 
The ACMVJ is a common test used by conditioners and coaches in a variety of game sports to monitor the effects of training and/or rehabilitation. Additionally, it is an established measure of lower body explosive power and, as such, it should be used as part of a battery of tests for assessing the field hockey player.19 Although the ˜8% improvement in ACMVJ height seen in this study cannot be directly extrapolated to predict field hockey performance, it would have relevance in game situations such as changing direction, lunging, and acceleration, where maximal explosive power is important. In this context, only small enhancements in muscular performance are needed to provide the necessary edge in elite competition.
 
The fact that the protocol of the present study enhanced jump performance to a greater degree than in previous studies using an acute WBV intervention is difficult to explain. However, the pre-intervention measures of ACMVJ in the present study were performed without a warm up, whereas the Bosco et al14 and Torvinen et al15 studies which revealed a smaller ergogenic effect of WBV, employed a cycling warm up prior to the first measure. Although in the present study the effect of seated cycling alone on ACMVJ did not reach significance (p=0.07), it is likely that, with greater participant numbers, we would have observed improved jump performance compared to control conditions. Thus, it is possible that cycling alone may provide sufficient warm up to enhance vertical jump performance to a small degree, and this may explain the smaller ergogenic effects of the aforementioned studies in which cycling exercise was used as a warm up. It is clear, however, from the results of the present study and those of Bosco et al14 and Torvinen et al,15 that the effects of vibration are additive to any cycling based warm up.
 
In the present study different muscle contractions were utilised in the different interventions: WBV exposure elicits both concentric and eccentric contractions, whilst seated cycling requires only concentric muscle action. ACMVJ involves activation of the stretch-shortening cycle, where the stretch receptors are activated under the eccentric loading phase. Given the significant enhancement of ACMVJ by WBV compared to cycling, it could be speculated that the additional effect of WBV over cycling was due to the eccentric stimuli it provides.
 
The enhanced muscle power observed following acute vibration is suggested to occur via potentiation of the neuromuscular system whereby stimulation of muscles spindles (Ia afferents) results in reflex activation of motor neurones with increased spatial recruitment.20 21 The continued enhancement of the stretch-reflex pathway can also be attributed to the ÿ motor neurone input causing an increase in sensitivity of the primary endings. Furthermore, the tonic vibration reflex can recruit additional motor units via muscle spindle and polysynaptic activation.15
 
This current study did not include EMG recordings and therefore it is not possible to directly assess any neurogenic enhancement. However, it has been reported that the stretchshortening cycle of the ACMVJ activates the spinal reflexes to enhance jump height compared to that of a concentric squat jump.22 Therefore, it is reasonable to propose that any neurogenic changes are ably detected by ACMVJ. The accentuated jump height following acute WBV suggests that neural enhancement occurred through an increased sensitivity of the stretch reflex mechanism and is in agreement with other investigators.3 10 14 23 24 However, in the absence of measures of neural function, such an explanation remains speculative and a better understanding awaits further investigation.
 
The use of arm countermovement in the vertical jump measure in the present study is unlikely to have contributed to the greater enhancement of jump performance, unless WBV somehow improves the coordination of arm and leg movements. Additionally, arm swing in the countermovement jump has been proven to require less practice than a countermovement jump with no arm swing and has been described as a natural practiced movement.19
 
Hand grip strength
 
Maximal grip strength was not improved with either intervention compared to control conditions. The level of vibration exposure sustained by the forearm muscles during the protocol in the present study is difficult to quantify, but would have been small. The results confirm the findings of Torvinen et al,12 15 illustrating the fact that muscles not directly exposed to vibration do not show a concomitant performance enhancement as does the vibrated muscle. Therefore, the improved ACMVJ performance seen in the present study is not related to a vibration induced centrally mediated phenomenon but confirms that the effect is localised to the spinal level and/or the muscle itself.
Figure 3 Percentage change in ACMVJ, grip strength, and sit and reach for WBV, control, and cycling interventions. *Statistically significant (p,0.001) percentage change pre-post interaction.
 
Flexibility
 
The significant improvement of 8.2% in the sit and reach flexibility following WBV is comparable to the 8% increase in leg split flexibility reported by Issurin et al.2 However, their protocol of flexibility training was performed with a vibrating cable simultaneously attached to the lower limb of the participant.2 In the present study, no flexibility exercises were conducted concurrently with the three interventions.
 
As stated previously, vibration enhances the stretch reflex loop through the activation of the primary endings of the muscle spindle, Figure 4 Mean and standard deviation pre and post grip strength for WBV, control, and cycling interventions./Figure 5 Mean and standard deviation pre and post sit and reach for WBV, control, and cycling interventions. *Statistically significant (p,0.001) interaction effect (pre-post) for WBV, control, and cycling interventions.which influences agonist muscle contraction while antagonists are simultaneously inhibited.25 The enhanced flexibility measure following WBV was greater than that after the control and cycling interventions, which suggests that the vibration exposure may have activated the Ia inhibitory interneurones of the antagonist muscle. This in turn may have caused changes to intramuscular coordination to decrease the braking force around the hip and lower back joints and potentiate the sit and reach score.3
 
Increases in static and dynamic muscular contractions have been attributed to muscle stiffness, which is a function of muscle and tendon components.26 The magnitude of the stretch load and the condition of the musculotendinous complex ultimately determine which reflexes dominate.27 For pre-stretching to enhance concentric muscular contraction, excitatory responses of the muscle spindle must exceed the inhibitory effects of the Golgi tendon organ (GTO). This is normally achieved through potentiated neural input of muscle spindle sensitivity or suppression of GTO neural activity.
 
In strength and power training, performing heavy sets of squats has been shown to augment jump squat height.28 Equally in WBV, fast joint rotation and muscle stretching occur, which is likely to increase muscle stiffness following the purported neural potentiation of the stretch reflex pathway and motor neurone input.29 Moreover, vibration causes more excitatory responses to the primary endings of muscle spindles compared to secondary endings and GTOs. The joint, skin, and secondary endings also detect the vibratory stimulus whereby the neural activity of the primary endings is potentiated through the activation of the ÿ motor neurone.3 This post activation potentiation, known in the strength conditioning field as ‘‘tuning up’’, may explain the concomitant enhancement of ACMVJ and flexibility performance.
 
The vibratory stimulus of the Ia neural drive and proprioceptive loop may also replicate a warm up effect by increasing pain threshold, blood flow, and muscle elasticity.2 Kerschan-Schindl et al30 have reported an increase in mean blood flow of the popliteal artery after acute WBV. They cite a combination effect of possible vasodilation and thixotropism for reducing the viscosity of the blood and improving the mean speed of blood flow. Acute vibration exposure has also been shown to reduce pain affected areas of muscle or tendon,31 which may allow a greater tolerance of the stretch threshold.
 
As yet, there are no set guidelines for WBV exercises, hence different investigators have used very different protocols 14 15 32 with few comparisons with other controls. In this study, WBV elicited a greater increase in ACMVJ and flexibility compared to cycling, which suggests that WBV may be an effective intervention for warming up. Numerous warm up mechanisms have been described to increase performance,33 therefore it is difficult to explain how WBV may accelerate the warm up effect. However, given that WBV results in concentric-eccentric contraction but cycling in solely concentric, WBV may provide an additional eccentric stimulus that is currently overlooked in conventional warm up procedures. Incorporating a greater eccentric component in warm ups may be beneficial for enhancing performance in sporting activities that rely on the eccentric-concentric interaction, which with WBV requires little effort and time. Further investigation is required before any conclusion can be drawn that acute WBV may be used as a potential warm up intervention.
 
Conclusions
 
In conclusion, this study further substantiates the claims of other investigators14 15 34 that acute WBV causes neural potentiation of the stretch reflex loop as observed by the improved ACMVJ and flexibility performance. Additionally, muscle groups less proportionally exposed to vibration do not exhibit physiological changes that potentiate muscular performance.
 
 
Whole body vibration exercise: are vibrations good for you?
Power Plate Studies
Whole body vibration has been recently proposed as an exercise intervention because of its potential for increasing force generating capacity in the lower limbs. Its recent popularity is due to the combined effects on the neuromuscular and neuroendocrine systems. Preliminary results seem to recommend vibration exercise as a therapeutic approach for sarcopenia and possibly osteoporosis. This review analyses state of the art whole body vibration exercise techniques, suggesting reasons why vibration may be an effective stimulus for human muscles and providing the rationale for future studies.

Vibration is a mechanical stimulus characterised by an oscillatory motion. The biomechanical variables that determine its intensity are the frequency and amplitude. The extent of the oscillatory motion determines the amplitude (peak to peak displacement, in mm) of the vibration. The repetition rate of the cycles of oscillation determines the frequency of the vibration (measured in Hz).
 
Vibration has been studied extensively for its dangerous effects on humans at specific amplitudes and frequencies. On the other hand, recent work has suggested that low amplitude, low frequency mechanical stimulation of the human body is a safe and effective way to exercise musculoskeletal structures. In fact, increases in muscular strength and power in humans exercising with specially designed exercise equipment have been reported.1–7 In particular, the effects of whole body vibrations (WBVs) have been studied with subjects exercising on specially designed vibrating plates producing sinusoidal vibrations (fig 1). The exercise devices currently available on the market deliver vibration to the whole body by means of oscillating plates using two different systems: (a) reciprocating vertical displacements on the left and right side of a fulcrum; (b) the whole plate oscillating uniformly up and down.
 
WBV exercise devices deliver vibrations across a range of frequencies (15–60 Hz) and displacements from ,1 mm to 10 mm. The acceleration delivered can reach 15 g (where 1 g is the acceleration due to the Earth’s gravitational field or 9.81 m/s2). Considering the numerous combinations of amplitudes and frequencies possible with current technology, it is clear that there are a wide variety of WBV protocols that could be used on humans. Vibration exercise is quite a new topic in sport science. Many athletes and
 
fitness and rehabilitation centres are using vibration in their exercise programmes, but current knowledge on appropriate safe and effective exercise protocols is very limited, and claims made by companies and pseudo-experts can be misleading.
 
IS VIBRATION A NATURAL STIMULUS?
 
During all sporting activities our bodies interact with the external environment and experience externally applied forces. These forces induce vibrations and oscillations within the tissues of the body. Tissue vibrations can be induced from impact related events where either a part of the body or sporting equipment in contact with the body collides with an object. Examples of this are the impact shocks that are experienced through the leg when the heel strikes the ground during each running stride or the impact shock that occurs when a racquet is used to hit a ball. The initial impact causes vibrations within the soft tissues, after which the tissues continue to oscillate as a free vibration—that is, vibrating at their natural frequency, with the amplitude of these vibrations decaying because of damping within the tissues. Tissue vibrations can also be induced when the body experiences more continuous forms of vibration, such as may occur through the legs during skiing across a groomed slope or through the arms during bike riding. A continuously oscillating input force drives the soft tissue vibrations to be at the same frequency as the input force, but the amplitude of the vibrations will be greatest if the natural frequency of the tissues is close to that of the input force (resonance); however, the amplitude of these larger amplitude vibrations can be reduced by damping from the tissues. Therefore we can expect to experience soft tissue vibrations in all sporting activities, and the amplitude and frequency of these vibrations is partly determined by the natural frequency and damping characteristics of the tissues.
 
The body relies on a range of structures and mechanisms to regulate the transmission of impact shocks and vibrations through the body including: bone, cartilage, synovial fluids, soft tissues, joint kinematics, and muscular activity. Changes in joint kinematics and muscle activity can be controlled on a short time scale and are used by the body to change its vibration response to external forces. It has been proposed that the body has a strategy of ‘‘tuning’’ its muscle activity to reduce its soft tissue vibrations in an attempt to reduce such deleterious effects.8 This idea would predict that the level of muscle activity used for a particular movement task is, to some degree, dependent on the interaction between the body and the externally applied vibration forces. It has been proposed that vibrations could be used as a training aid. However, prolonged exposure to vibrations has been shown to have detrimental effects on the soft tissues, including muscle fatigue,9 reductions in motor unit firing rates and muscle contraction force,10 11 decreases in nerve conduction velocity, and attenuated perception.
 
The natural frequency of a vibrating system depends on its stiffness and mass. Within the skeletal muscles, each cross bridge between the actin and myosin myofilaments generates some stiffness,13 and so the tissue stiffness (and therefore natural frequency) can be increased with increases in muscle activity. Indeed, studies have shown that increases in the natural frequency of whole muscle groups do concur with the joint torques developed by the muscle and typically range between 10 and 50 Hz for the lower extremity muscles (zero to maximal activity14). Muscles can also damp externally applied vibrations, and, indeed, more vibration energy is absorbed by activated muscle15 than by muscles in rigor,16 suggesting that the active cross bridge cycling is an important part of the damping process. Studies have shown that the damping coefficients of whole muscle groups increase with muscle activity,15 17 supporting the idea that the cross bridge mechanisms are important. A maximally activated muscle can damp free vibrations so that the tissue oscillations are virtually eliminated after a couple of cycles.18 It is thus possible that muscles are activated to minimise the vibrations that occur within the tissues, but does this actually happen during WBV exercise?
 
VIBRATION AND MUSCLE ACTIVATION: THE MUSCLE TUNING HYPOTHESIS
 
Evidence for muscle tuning requires information on the nature of the input force, the vibration response of the tissue, and the level of muscle activity. These can be difficult to measure because vibrations induced in the tissues can cause movement artefacts, which may interfere with measurement of muscle activity. Nonetheless, in a study of hand held vibrating tools,12 it was found that activation of the triceps brachii was greatest between vibration frequencies of 8 and 16 Hz, coinciding with the resonant frequencies measured at the wrist and elbow (10–20 Hz). In a similar experiment, vibration was recorded directly from the soft tissue groups in the lower extremities while subjects stood on a vibration platform.18 The natural frequencies for the tissues for each posture were determined by measuring the vibration response to a complex vibration covering a range of frequencies and therefore accounted for changes in resonance that occurred with altered limb posture and muscle activity. Figure 1 Different designs of whole body vibrating plates.The vibration response of the soft tissues was measured for a range of input vibration frequencies (10– 65 Hz), and it was found that most vibration damping occurred at the resonant frequencies of the tissues, concurring with the highest levels of muscle activity. The responses of the lower extremities to continuous vibrations or sequences of single, impact-like input were similar. This suggests that the body has a strategy to minimise its vibrations regardless of the mode of the input force.18 These studies support the muscle tuning paradigm, but these concepts should be tested further. For instance, the effect of the amplitude of the input vibrations on the tuning response has not yet been determined. Is there a minimum amplitude below which the body is not triggered to respond? At high vibration amplitudes, the maximum damping from the tissues will not be as effective at dissipating the vibration energy. We do not yet know the most effective range of vibration amplitudes that can be applied safely while eliciting a significant tuning response.
 
Training protocols and sporting equipment that cause specific alterations in muscle activity during exercise may have important implications for athletic training, rehabilitation after injury, and competitive performance. For instance, the hardness of a shoe midsole causes changes in the time to peak impact force at heel strike.19–22 This time and the associated loading rate are a correlate of the major frequency content of the impact force; impact forces that drive the soft tissues of the lower extremity closer to resonance cause increases in muscle activity and vibration damping from those tissues.23 It is conceivable that different types of equipment may be designed in future: training equipment, which promotes increased muscle activity, and competition equipment, which reduces the muscle activity required for vibration damping and thus allows more of the muscle activity to be used for the sporting task. Vibration platforms are the most recent example. They have been developed with the idea of promoting muscle activity, hence providing an effective training stimulus. Are they effective?
 
METABOLIC EFFECTS OF VIBRATIONS
 
The possibility of using vibrations as an effective training tool can be considered a recent idea. However, it should be noted that early work by Whedon et al24 reported some positive effects of oscillating beds on plaster immobilised patients. The possibility of using vibration in an athletic setting was introduced relatively recently by Russian scientists, who developed specific devices to transmit vibratory waves from distal to proximal links of muscle groups, mainly during the performance of isometric exercises.25 Recently many studies have been conducted with the aim of understanding the acute and chronic responses to WBV training (WBVT).
 
WBVT has been shown to cause clear metabolic responses similar to other forms of exercise. In a study by Rittweger et al,26 WBVT to exhaustion with an extra weight showed an O2 uptake of less than 50% of VO2MAX. An acute reduction in vertical jump was observed, suggesting that vibration exercise to fatigue can impair neuromuscular performance. The early impairment of muscle performance was shown to be recovered 20 seconds after the end of the fatiguing vibratory exercise. Another experiment conducted by Kerschan-Schindl et al27 showed a significant increase in muscle blood volume in the calf and thigh and a significant increase in mean blood flow velocity in the popliteal artery after vibration exercise on a vibrating plate (26 Hz, 3 mm amplitude). The mean blood flow measured by Doppler ultrasound increased from 6.5 to 13 cm/s, and this acute Figure 1 Different designs of whole body vibrating plates. response was attributed mainly to the effect of vibrations in reducing the viscosity of blood and increasing its speed through the arteries. The above studies seem to indicate that WBVT may represent a mild form of exercise for the cardiovascular system.26–28 However, owing to the relatively low level of stimulation, it is unlikely that an athletic population could benefit from such a training stimulus if the aim is to improve cardiovascular performance. However, elderly people could make use of this form of exercise when other solutions are not possible. Also, because of its reported beneficial effects in reducing low back pain,29 pain sensation, and pain related limitation, it may be a viable alternative for a patient who cannot run and/or lift weights. However, the extensive literature on the dangerous effects of WBV on the spine (for a review, see Cardinale and Pope30) suggests that more, well controlled, long term intervention studies are needed before WBVT can be prescribed for patients with low back pain.
 
ACUTE EFFECTS OF VIBRATION ON NEUROMUSCULAR PERFORMANCE
 
Most of the studies so far conducted have focused on the acute and chronic effects of WBVT on neuromuscular performance. In our studies, WBV exercise has been shown to acutely enhance strength and power capabilities in well trained people.1 2 Acute application of five minutes of WBVT at 26 Hz and 10 mm peak to peak amplitude were in fact shown to shift the force-velocity and power-velocity relations to the right in the vibrated legs of well trained volleyball players.1 Finally, WBVT applied for a total of 10 minutes (26 Hz, 4 mm) was shown to improve vertical jumping ability, increase concentrations of testosterone and growth hormone, and decrease cortisol concentrations in recreationally active people.2 The results of this preliminary study have been used by many companies to advertise WBVT as a way to boost anabolic hormones, reduce stress, and accelerate muscle remodelling. For this reason, it is important to recognise that the study has many limitations, the primary one being the absence of a control condition. Also, not all studies have shown acute increases in strength/power performances and hormone concentrations. Torvinen et al,31 for example, have shown acute increases in knee extension maximal strength and vertical jumping height after four minutes of WBVT when a relatively large amplitude was applied (4 mm) with a tilting plate as compared with no significant acute effects when low amplitude whole plate oscillation (2 mm) was applied.32 Results from our laboratory have also shown that, when vibration duration is relatively long (seven minutes), an acute decrease in vertical jumping ability is observed even in well trained subjects.33 Recent work from De Ruiter et al,34 in which subjects exercised on a vibrating plate for 5 6 one minute (frequency 30 Hz, amplitude 8 mm) with two minutes rest in between, showed an acute reduction in maximal voluntary knee extension force. Also, in their well controlled study, the authors showed that vibration depressed voluntary activation of the leg extensor muscles up to 180 minutes after the exercise bout. Finally, Di Loreto et al35 have recently shown that 10 minutes of WBVT at 30 Hz with a relatively small amplitude did not produce any change in the serum concentrations of growth hormone, insulin-like growth factor 1, and free and total testosterone.
 
At this stage, owing to the differences in WBVT protocols used in the different studies, it is difficult to ascertain the acute effects of the WBVT intervention on the neuroendocrine and neuromuscular systems. However, it is important to consider that a certain degree of muscle activation is needed from lower limb muscles to damp the vibrations23 36 originated by vibrating plates. In fact, this extra muscle activity results in a greater rate of oxygen uptake during exposure to vibrations.37 38 It should be remembered that, according to the muscle tuning theory, the magnitude of the muscular response is related to the interaction between the amplitude and frequency of the vibration input and the intrinsic neuromuscular properties. It is possible that many studies have failed to show any positive effect of vibration because the applied vibrations did not stimulate the target muscles at their resonant frequencies. It should also be noted that most of the studies have focused on leg extensors, while neglecting plantar flexors which have been shown to increase their electromyographic activity up to five times the baseline values with vibration.4 It is clear that more studies are needed to ascertain the influence of the above variables on humans.
 
CHRONIC EFFECTS OF VIBRATION ON NEUROMUSCULAR PERFORMANCE
 
Chronic studies seem to provide more supportive evidence for the possibility of using WBVT effectively in different populations. A few weeks of training seem to produce conflicting results.
 
In our study in 199839 10 days of WBV (26 Hz, 10 mm, total exposure time 100 minutes) resulted in an increase in average jumping height (+11.9%) and power output during repeated hopping in active subjects. No change was observed in counter movement jump performance. Five training sessions of five minutes each (30 Hz, 8 mm amplitude, total exposure 25 minutes) did not affect maximal voluntary contraction and voluntary activation of leg extensors in untrained students.34 The same authors also analysed the effects of 11 weeks of WBVT on maximal voluntary contraction measured with an isometric leg extension task (maximal voluntary contraction), maximal force generating capacity, and stimulated maximal rate of force rise.40 The results showed no change in all variables except for an increase in stimulated maximal rate of force rise in the group undergoing WBVT detected at week 14. The subjects in this study were exposed to WBVT three times a week starting with five sets of one minute each with one minute seated rest in between. Exercise duration was progressively increased up to eight sets of one minute each. However, even if the total exposure time to WBVT was relatively high (169 minutes), it is important to note that the training period was not continuous because of two weeks of non-training allowed between week 5 and week 7 of the study. Nine days of WBVT have also been recently shown to have no effect on jumping ability, sprinting, and agility tests in sport science students.41 In the light of the above, it seems clear that, when WBVT is performed with small amplitudes for a short time by physically active people, it is unlikely to produce significant improvements in force and power generating capacity of the lower limbs. However, when resistance exercise is performed on a vibrating plate, it seems that even physically active people can improve vertical jumping ability more than by resistance exercise alone.42 When standing on a vibrating plate, young healthy people generate relatively low force levels in their lower limbs compared with their maximal voluntary capacity. Hence, even if the vibration stimulation can increase their muscle activity, it is likely that this would not be enough to produce any significant change in their ability to forcefully activate their muscles. So, if well trained populations use vibration exercise with the aim of improving neuromuscular performance, an optimal amplitude and frequency should be coupled with an optimal level of muscle activity on which the vibration stimulation can be superimposed. Of course, this should be the aim of future studies and for this reason we have recently patented an exercise device able to allow the user to perform vibration exercise while controlling the level of force and muscle mechanics (Patent Number WO2004009173).
 
On the other hand, sedentary, injured, and elderly people with impaired muscle activation capabilities may benefit from currently available WBVT applications. In this case the results seem to be more encouraging. In fact, Torvinen et al43 showed a net improvement of 8.5% in vertical jumping ability after four months of WBVT performed with static and dynamic squatting exercises with small vibration amplitudes (2 mm) and frequencies ranging from 25 to 40 Hz in sedentary subjects. A 12 week WBVT programme (frequency 35–40 Hz and amplitude 2.5–5 mm) induced a significant enhancement in isometric, dynamic, and explosive strength of knee extensor muscles in healthy, untrained, young adult women.4 Also, vertical jump improved only in the group undergoing WBVT and not in the group performing conventional resistance exercise. However, it should be noted that the resistance exercise programme in this study was of relatively low intensity (started with a load of 20 repetition maximum and reached 10 repetition maximum in the last four weeks) and the exercises (leg press and leg extensions) were performed to failure and not with explosive movements, reducing the possibility of such a programme producing significant changes in explosive measures. The same authors also showed that 24 weeks of WBVs were effective in producing a rightward shift in the force-velocity relation of knee extensor muscles and an increase in fat-free mass in untrained female subjects.5 Despite not being significantly different from the standard training groups, the results observed by both Delecluse et al4 and Roelants et al5 highlight the possibility that long term programmes of WBVT may produce significant improvements in muscle function of the leg extensors in untrained subjects. As more supportive evidence, a recent study from the same group7 showed that WBVT was superior to a low intensity resistance training programme in improving isometric and dynamic muscle strength in middle aged and older women (58–74 years). The WBVT programme was also effective in increasing bone mineral density of the hip even though the improvement was very small (+0.93%) and within the error of measurement used for establishing bone mineral density. Finally, Torvinen et al44 have shown that eight months of WBVT with small amplitude (2 mm) improved vertical jumping ability in young healthy sedentary subjects compared with a control group, but did not change dual energy x ray absorptiometry derived bone mineral content measures, markers of bone turnover, and postural sway.
 
The latest results support our idea that the current technology/methods of use of WBVT (standing on a vibrating plate with low force generation in the lower limbs) are unlikely to produce significant improvements in performance in well trained athletes and physically active young subjects, and, even if they do, conventional resistance exercise should still be superior. However, this technology may be of benefit to the elderly or in rehabilitation programmes, as little effort is required and there is no complicated technique to master. Special populations in particular seem to benefit from acute bouts of WBVT. Unilateral chronic stroke patients, for example, have been shown to improve postural stability after a few minutes of WBVT at 30 Hz and 3 mm amplitude.45 Also, heart transplant patients seem to be able to exercise on vibrating plates with no adverse events.46 Furthermore, owing to the potential of this intervention to stimulate bone remodelling,47–49 it is possible that WBVT may be a possible non-pharmacological intervention for the prevention of osteoporosis, but more evidence needs to be gathered with well controlled studies.
 
CONCLUSIONS
 
The current evidence indicates that WBVT may be an effective exercise intervention for reducing the results of the ageing process in musculoskeletal structures. It would also appear that vibration may be an effective countermeasure to microgravity and disuse. However, it is important to conduct further studies to understand the neurophysiological mechanisms involved in muscle activation with vibration in order to be able to prescribe safe and effective WBVT programmes. Not only the optimal frequency and amplitude need to be identified but also the level of muscle activation that would benefit more from vibration stimulation. Considering current WBVT technology, it is possible to confirm that the procedure seems safe when subjects stand on vibrating plates for a relatively short time with knees semiflexed to limit transmission of vibrations to the head. However, when vibration transmission frequency is too high, some can experience motion sickness-like symptoms.50 As we know from occupational medicine that prolonged exposure to WBVT can have major negative effects on health, proper care should be taken when exercise programmes are prescribed so as to guarantee safety.
 
 
Vascular adaptation to deconditioning and the effect of an exercise countermeasure
Power Plate Studies
        Vascular adaptation to deconditioning and the effect of an exercise countermeasure: results of the Berlin Bed Rest study. J Appl Physiol 99: 1293–1300, 2005. First published June 2, 2005; doi:10.1152/japplphysiol.00118.2005.—Deconditioning is a risk factor for cardiovascular disease. The physiology of vascular adaptation to deconditioning has not been elucidated. The purpose of the present study was to assess the effects of bed rest deconditioning on vascular dimension and function of leg conduit arteries. In addition, the effectiveness of resistive vibration exercise as a countermeasure for vascular deconditioning during bed rest was evaluated. Sixteen healthy men were randomly assigned to bed rest (BR-Ctrl) or to bed rest with resistive vibration exercise (BR-RVE). Before and after 25 and 52 days of strict horizontal bed rest, arterial diameter, blood flow, flow-mediated dilatation (FMD), and nitroglycerin-mediated dilatation were measured by echo Doppler ultrasound. In the BR-Ctrl group, the diameter of the common femoral artery decreased by 13 ± 3% after 25 and 17 ± 1% after 52 days of bed rest (P < 0.001). In the BR-RVE group this decrease in diameter was significantly attenuated (5 ± 2% after 25 days and 6 ± 2% after 52 days, P < 0.01 vs. BR-Ctrl). Baseline blood flow did not change after bed rest in either group. After 52 days of bed rest, FMD and nitroglycerinmediated dilatation of the superficial femoral artery were increased in both groups, possibly by increased nitric oxide sensitivity. In conclusion, bed rest deconditioning is accompanied by a reduction in the diameter of the conduit arteries and by an increased reactivity to nitric oxide. Resistive vibration exercise effectively attenuates the diameter decrease of leg conduit arteries after bed rest.
 
echo Doppler ultrasound; flow-mediated dilatation; bed rest deconditioning
 
PHYSICAL INACTIVITY OR DECONDITIONING is an independent risk factor for atherosclerosis and cardiovascular disease (3, 38). In a prospective observational study, improvement of physical fitness decreased cardiovascular mortality risk by 51% (3). Endothelial dysfunction plays an important role in the pathogenesis of cardiovascular disease and is directly related to cardiovascular mortality (29). Cross-sectional studies have demonstrated a lower vascular dimension (24) and endothelial function (19) in sedentary subjects compared with exercisetrained individuals. This may reflect either downregulation by physical inactivity or upregulation by exercise training. Although changes in blood flow after deconditioning have been observed in humans, data on vascular dimension and endothelial function are scarce. Moreover, the underlying physiological mechanism of vascular adaptation to deconditioning in humans has not been elucidated.
 
Longitudinal deconditioning intervention studies have demonstrated the detrimental effects of bed rest on muscle function (2, 5), bone density (5), and orthostatic tolerance (13, 23). Previous studies on vascular adaptation to deconditioning interventions are restricted to flow measurements and have mainly focused on the arm vascular bed (15, 47, 48). In most of these studies, the effect of physical inactivity on blood flow is confounded by the effects of head-down tilt on plasma volume (15, 47, 48). Because of their role in standing and locomotion, the legs more accurately reflect the intense deconditioning during bed rest. Vascular remodeling as a result of deconditioning will be reflected in changes in vascular dimension. Moreover, endothelial function is of paramount importance for vascular remodeling and in the pathogenesis of cardiovascular disease. Therefore, the purpose of the present study was to assess the effect of horizontal bed rest deconditioning on vascular dimension of leg and arm conduit arteries and on endothelial function of a leg conduit artery.
 
Exercise training has been shown to improve vascular dimension and endothelial function in longitudinal intervention studies (20, 33, 34), and exercise has been propagated as a countermeasure for orthostatic intolerance in both space travelers (12) and hospitalized patients (13). Therefore, a second purpose of the study was to evaluate the effectiveness of exercise as a countermeasure for vascular adaptation to bed rest. Resistive vibration exercise has recently emerged as a training modality that increases oxygen uptake (42), leg blood flow (28), muscle strength (43, 51), and bone density (51). As such, we hypothesized that resistive vibration exercise would counteract the vascular changes induced by bed rest deconditioning.
 
METHODS
 
Subjects
 
Sixteen healthy men (age 34 ± 2 yr) participated in this study and represent a subpopulation of the Berlin Bed Rest study. All subjects were screened with a medical history and physical examination and did not have any medical problems. None of the subjects suffered from diabetes or cardiovascular disease or used any medication. Cholesterol and triglyceride levels were in the normal range (Table 1).
Baseline characteristics
 
Smoking was not used as an exclusion criterion, but smoking was prohibited during the bed rest trial. Subjects were randomly assigned to bed rest (BR-Ctrl) or bed rest with resistive vibration exercise (BR-RVE). All subjects gave their written, informed consent. The Ethics Committee of the Medical School of the Free University Berlin has approved the Berlin Bed Rest study and the present experiment within it.
 
Procedures
 
The vascular characteristics of all subjects were measured three times: 2 days before, and 25 and 52 days after bed rest deconditioning (BR-2, BR25, and BR52, respectively).
 
Bed rest protocol. After the initial series of experiments, subjects were placed at complete horizontal bed rest. All personal hygiene activities were performed in supine position. Subjects were housed in a dedicated clinical ward of the University Hospital Benjamin Franklin. The subjects were monitored with video cameras to guarantee compliance with the bed rest protocol. In addition, the monitoring with force transducers of the vertical forces generated by the subjects ensured strict bed rest and avoidance of powerful movements. The diet of the subjects was controlled carefully.
 
Resistive vibration exercise. The subjects who were randomly assigned to the BR-RVE group were exposed to resistive vibration exercise (RVE) twice daily for 30 min (8 min pure exercise time), with the exception of Sundays and Wednesday afternoons. RVE was performed with a device that was specifically manufactured for the Berlin Bed Rest study with modifications for the use during supine bed rest (Galileo Space, Novotec, Pforzheim, Germany). The subjects attached themselves to the device with four supporting belts (an image of RVE is available as Fig. 5 in the online version of this article). The subjects pushed their feet against the device’s footplate and pulled on the hand and hip belts; this caused elongation of the springs and generated a force resisting the body extension. Force transducers at the end of each spring yielded the platform reaction force. The vibration of the footplate was elicited by an eccentric rotation of a mass that was phase shifted by 180° for the right and left part of the footplate. By virtue of that construction (preset vibration frequency), the acceleration of the eccentrically rotating mass changes with vibration frequency. Hence, the greater the vibration frequency, the greater the peak platform reaction force elicited on the exercising subjects. At the beginning of each training session, the length of the supporting belt was adjusted so that a certain resting platform reaction force was created in full knee extension. Then, four different exercise units were performed. 1) Squatting exercise: knees were stretched from 90° to full extension in cycles of 6 s. This unit was targeted at the knee extensors. 2) Heel raises: with the knees in almost complete extension, the heels were raised as long as the subjects could sustain this. After briefly resting back on the foot platform, the heels were raised anew. This unit was targeted at the foot plantar flexors. 3) Toe raises: with knees almost in complete extension, the toes were raised as long as possible. After briefly relieving, the toes were raised anew. This unit was targeted at the foot dorsal flexors. 4) “Kicks”: the knees were extended from 90° flexion to extension as quickly and forcefully as possible. Between each of these 10 kicks, subjects relaxed completely for 10 s.
 
For each training session, units 1–4 were done once and in ascending order. There was one morning session and one in the afternoon. In the morning sessions, exercise units 1–3 were performed for at least 60 s. If subjects managed to perform them for 100 s, the vibration frequency was increased. Initially, the vibration frequency was set to 19 Hz. In the afternoon, subjects were asked to exercise with a lower resting platform reaction force (60–80% of the value achieved in the morning) and to run through units 1–3 for 60 s each, with as many iterations as possible. Trained staff members supervised all training sessions.
 
Measurements
 
To reduce circadian and dietary effects, measurements were performed at the same time of day in each individual subject, and meals were identical for each subject on the days of the measurements. All subjects refrained from caffeine and alcohol from midnight, and from vitamin C supplements for 24 h. On the testing days, subjects did not perform exercise before testing. Because of the scheduling of the measurements it was impossible for the subjects to be tested in a fasting state. However, after midnight the diet was carefully controlled. Subjects received low-fat meals, and meals were identical for each measurement. Measurements were performed after an acclimatization period of at least 20 min after instrumentation. Blood pressure and heart rate were measured at the onset of the measurement. Blood pressure was measured manually by the standard auscultatory method. Heart rate was derived from the electrocardiogram.
 
Ultrasound measurements Resting blood cell velocity and diameter of the common femoral artery (CFA) and superficial femoral artery (SFA) were measured in the left leg, using an echo Doppler device (Megas, ESAOTE, Firenze, Italy) with a 5- to 7.5-MHz broadband linear array transducer (16, 17). The angle of inclination for the velocity measurements was consistently below 60°, and the vessel area was adjusted parallel to the transducer. In addition, resting blood cell velocity and diameter were measured in the right brachial artery and in the left common carotid artery. The brachial artery represented a conduit artery supplying a limb that is subject to less intense deconditioning during bed rest. The common carotid artery was included as a reference vessel.
 
For reactive hyperemia and flow-mediated dilatation (FMD) of the SFA, a cuff was placed around the left upper thigh 3–4 cm below the bifurcation of the CFA. The cuff was inflated to a suprasystolic pressure of 220 mmHg for 5 min. After cuff deflation, hyperemic flow velocity in the SFA was recorded on videotape for the first 25 s, followed by a continuous registration of the vessel diameter for 5 min to determine FMD. Studies in the radial and brachial conduit arteries have proven that the vasodilatation response to hyperemic response after 5 min of distal arterial occlusion is endothelium and nitric oxide dependent (21, 35). Therefore, our FMD response most likely reflects endothelium-dependent dilatation. Endothelium-independent vasodilatation of the SFA was determined in 12 subjects. After a resting period of at least 20 min to reestablish baseline conditions, a systemic dose of nitroglycerin (0.4 mg) was administered sublingually to determine the endothelium-independent vasodilatation of the SFA, which is indicative for smooth muscle function and nitric oxide responsiveness. Vessel diameter of the SFA was continuously recorded between 2 and 6 min after nitroglycerin administration. We have reported the reproducibility for the measurements in the SFA previously as 1.5% for diameter, 14% for blood flow, and 15% for relative FMD changes (17).
 
Data Analysis
 
Ultrasound. For resting diameter measurements, two consecutive longitudinal vessel images were frozen at the peak systolic and end-diastolic phase and analyzed off-line. Three measurements were performed per diameter image. Mean diameter was calculated as (1/3 . systolic diameter) + (2/3 . diastolic diameter). The average of 10–12 Doppler spectra waveforms was used to calculate peak velocity and mean velocity. Mean blood flow (ml/min) was calculated as 1/4 .3.14(pie)(mean diameter)2 .mean velocity (cm/s) . 60; peak blood flow (ml/min) was calculated as 1/4 .3.14(pie).(systolic diameter)2 peak velocity (cm/s) .60; regional peak wall shear rate (PWSR, s-1) was calculated as 4.(peak velocity/systolic diameter), and mean wall shear rate (MWSR, s -1) was calculated as 4.(mean velocity/mean diameter). Reactive hyperemic blood flow was calculated from blood velocity 15–25 s after cuff release and the baseline vessel diameter. Although maximal reactive hyperemia may occur slightly earlier, we used this time frame to obtain data from all measurements in all subjects. In addition, we made the assumption that the diameter 15–25 s after cuff release is similar to baseline diameter. Delta PWSR and delta MWSR were defined as the differences between rest and hyperemic responses. Vessel diameters after reactive hyperemia were measured off-line from videotape at 50, 60, 70, 90, 120, 180, and 240 s after cuff release and at 2, 3, 3.5, 4, and 5 min after nitroglycerin administration. FMD and endothelium-independent vasodilatation were expressed as relative (%) diameter change from baseline of the end-diastolic diameter. Because the FMD response is directly proportional to the magnitude of the stimulus (30), the FMD response was also expressed relative to the delta shear rate. Ratios were calculated for the FMD/delta PWSR and FMD/delta MWSR. The ratio between the maximal FMD and endothelium-independent vasodilatation was expressed as FMD/nitroglycerin- mediated dilatation. Ultrasound analysis has been described in more detail previously (17).

Statistical Analysis
 
Data are presented as means ± SE. Differences in the response to bed rest between the BR-RVE group and the BR-Ctrl group were tested with repeated-measures ANOVA with time as within-subject factor and group as between-subject factor (Statistical Package for Social Sciences, SPSS 12). The time factor represents the overall effect of bed rest. The time-by-group factor was used to test the effect of the RVE countermeasure. Statistically significant differences between the groups were further analyzed with unpaired t-tests at bed rest day 25 and bed rest day 52. Differences were considered to be statistically significant at P < 0.05.
 
RESULTS
 
Subjects
 
There were no significant differences between the groups for any of the baseline characteristics (Table 1). All subjects completed the study. During the bed rest period the subjects in the BR-RVE group were exposed to 89 exercise sessions of ˜30 min (8 min pure exercise time).
 
Heart Rate and Blood Pressure
 
During the bed rest period, resting heart rate increased significantly in the BR-Ctrl group (P< 0.05, Table 2). Changes in heart rate during bed rest were significantly different between the BR-Ctrl and BR-RVE groups, and heart rate was significantly lower in the BR-RVE group compared with the BR-Ctrl group at BR25 and BR52 (P < 0.05, Table 2).Blood pressure did not change significantly during bed rest and was not different between the groups.
 
Mean values of heart rate and blood pressure during bed rest
Diameter and Blood Flow of the CFA and SFA
 
The data for the CFA in the exercise group are based on seven subjects, because the diameter of the CFA could not be assessed in one subject because of vessel wall irregularities. The diameter of the CFA and SFA decreased significantly during bed rest (P < 0.001 for time). This decrease was significantly attenuated in the exercise group compared with the BR-Ctrl group in both the CFA and SFA (Fig. 1, A and C, P =0.001 and P <0.001 for grouptime, respectively). The blood flow in the CFA and SFA did not change during bed rest and did not differ between the groups (Fig. 1, B and D).
 
Diameter and Blood Flow of the Brachial and Carotid Artery
 
The diameter of the brachial artery decreased significantly during bed rest (P =0.016 for time) but did not differ between the exercise and control group (Fig. 2C). The diameter of the carotid artery, and the blood flow in the brachial artery and in the carotid artery did not change during bed rest (Fig. 2, A, B, and D).
 
Reactive Hyperemia, FMD, and Nitroglycerin-Mediated Dilatation of the SFA
 
Reactive hyperemic blood flow did not significantly decrease after bed rest in both groups (BR-Ctrl: from 989 ±98 to 716 ±58 ml/min, BR-RVE: from 1,119 ±104 to 1,076 ± 107 ml/min).
 
At bed rest day 25, one FMD measurement in the control group and one in the exercise group failed; therefore the ANOVA is based on 14 subjects. FMD increased significantly during bed rest (P =0.007, Fig. 3A, n =14). This increase tended to be less in the exercise group than in the BR-Ctrl group (P =0.07). FMD was lower in the exercise group than in the BR-Ctrl group on bed rest day 25 (P =0.008), but not on bed rest day 52 (P =0.55). Nitroglycerin-mediated dilatation increased significantly over time (P =0.002, n =12) with no difference between the groups (Fig. 3C). These findings for FMD and nitroglycerin-mediated dilatation were similar if absolute instead of relative changes in diameter were analyzed. FMD corrected for MWSR did not change significantly over time nor between groups over time (Fig. 3B, n =14). Changes in FMD corrected for nitroglycerin-mediated dilatation were significantly different between groups (Fig. 3D, n =10). At bed rest day 25, corrected FMD tended to be lower in the exercise group (P =0.05).
Changes during bed rest in diameter and blood flow of the common femoral artery (A and B) and superficial femoral artery (C and D).
 
DISCUSSION
 
This study is the first to characterize the adaptation of diameter and endothelial function of the leg conduit arteries to bed rest deconditioning. The diameter of the CFA and SFA decreased after bed rest, whereas baseline blood flow did not change. Both FMD and endothelium-independent dilatation of the superficial femoral artery increased significantly after bed rest, indicating increased reactivity to nitric oxide after bed rest, possibly by increased nitric oxide sensitivity or increased smooth muscle sensitivity to vasodilators. In addition, this is the first study to demonstrate that RVE can effectively attenuate the diameter decrease of conduit arteries of the leg.
 
Bed Rest Deconditioning and Vascular Dimension
 
After bed rest without exercise, the diameter of the CFA decreases by 13 and 17%, at BR25 and BR52, respectively. This suggests that most of the adaptation in arterial diameter occurs in the first 4 wk of bed rest deconditioning. After 4 wk of hindlimb unloading in rats, an animal model for physical inactivity and microgravity, the lumen diameter of the femoral artery also decreased significantly by ˜8% (54). Furthermore, our results are in agreement with a 12% decrease in diameter of the CFA after 4 wk of deconditioning by unilateral lower limb suspension in humans (4), suggesting the same degree of deconditioning in several models of physical inactivity. However, in the paralyzed legs of spinal cord-injured individuals the diameter of the CFA is 30% smaller than in healthy control subjects (16). This adaptation is completed within 6 wk after the occurrence of a spinal cord injury (16). Hence, the decrease in arterial diameter appears to be larger in spinal cord-injured individuals than in able-bodied, immobilized individuals. This can be attributed to the presence of some physical activity of the legs during bed rest as opposed to no activity because of paralysis. Notably, the time course of arterial diameter adaptation is very similar in bed rest and spinal cord injury. The adaptation of conduit artery diameter to bed rest deconditioning may reflect structural and/or functional changes. Nitroglycerin 0.4 mg sublingually has been shown to produce a maximal vasodilatation in both coronary and brachial arteries (10, 36). In addition, maximal dilatation of the femoral artery to nitroglycerin closely resembles maximal vasodilatation in response to another strong vasodilator stimulus, 12 min of ischemia combined with ischemic exercise (4). Therefore, the response to nitroglycerin can be used as a measure of near maximal arterial diameter in the SFA (Fig. 4). Overall, maximal diameter decreased with bed rest (P < 0.01), suggesting that structural changes occur in conduit arteries in response to bed rest.
Changes during bed rest in diameter and blood flow of the carotid artery (A and B) and brachial artery (C and D).
 
The decrease in brachial diameter of 5% was small compared with the effect of bed rest deconditioning in the legs. This may be attributed to the specific antigravity and locomotion functions of the legs. After 7 days of bed rest, baseline diameter of the brachial artery did not change (7); this is probably due to the shorter duration of bed rest in that study. The lack of effect of bed rest on carotid artery diameter can be explained by the minor effect of physical inactivity on the cerebral circulation.
 
Bed Rest Deconditioning and Blood Flow
 
Baseline leg blood flow did not decrease after bed rest. Former studies used plethysmography and reported a decrease in leg blood flow at the arteriolar level (14, 27, 31, 39). All these studies applied 6° head-down-tilt bed rest (14, 27, 31, 39). Louisy et al. (31) demonstrated that a large portion of the decrease in blood flow was already present after 1 day of head-down tilt bed rest. In the first 24–48 h, head-down tilt bed rest causes a pronounced decrease in plasma volume (11), which may be responsible for a large part of the blood flow decrease in these studies. In contrast, Bonde-Petersen et al. (6) also used plethysmography but reported no changes in leg blood flow after 20 days of horizontal bed rest. Interestingly, Takenaka et al. (49) used echo Doppler ultrasound in the same subjects and reported a decrease in leg blood flow. It is not possible to make a detailed comparison with our echo Doppler data because Takenaka et al. did not report on changes in diameter and velocity
 
The present findings are in agreement with a previous study of deconditioning due to unilateral lower limb suspension in human volunteers (4). After limb suspension, diameter of the leg conduit arteries decreased, whereas leg blood flow did not decrease. Furthermore, even in extreme deconditioning due to paralysis after spinal cord injury, with a dramatic decrease in arterial diameter, several studies have reported no differences in resting leg blood flow, as measured with echo Doppler ultrasound (17, 37). Studies using exercise training have provided clues that conduit arteries adapt primarily to peak blood flow and peak oxygen demand during exercise (20, 33). Baseline diameter seems to adapt to maximal blood flow during bouts of exercise rather than to resting blood flow (20, 33). In the present bed rest study the loss of periods of high blood flow and high shear stress in the group without exercise would explain the decrease in arterial diameter, without changes in baseline blood flow. In agreement with the results in the legs, blood flow in the arm did not change during bed rest.
 
Bed Rest Deconditioning and Endothelial Function
 
FMD, indicative for endothelial function, was significantly increased after 52 days of bed rest. This corresponds with an increase in FMD after 28 days of deconditioning by lower limb suspension (4) and with an increase in FMD in the paralyzed legs of spinal cord-injured individuals (17). However, when FMD is corrected for its eliciting stimulus MWSR (30), the increase in FMD is no longer statistically significant in the present and the cited (4, 17) studies. In the present study, the shape of the figure changes very little when this correction for shear rate is applied, with an increase in standard error (Fig. 3, A and B). This suggests that the number of subjects is too low for this type of correction. However, correction of FMD for PWSR instead of MWSR results in a trend toward increased FMD after bed rest (P =0.059) with a significant difference between groups (P =0.018). Likewise, in a previous study virtually all spinal cord-injured individuals had a higher FMD response per delta shear rate (17). Moreover, bed rest deconditioning causes a significant increase of FMD of the brachial artery (7). Combined, these data provide evidence that FMD increases after deconditioning.

Flow-mediated dilatation (FMD) and nitroglycerin-mediated dilatation (NMD) of the superficial femoral artery
 
In hindlimb-unloaded rats, endothelium-dependent vasodilatation of the lower abdominal aorta in response to acetylcholine is reduced. This decrease in vasodilatation is probably due to endothelial dysfunction, but changes in smooth muscle cell nitric oxide sensitivity may also be responsible (18). At the level of the resistance arteries and arterioles, endotheliumdependent vasodilatation and nitric oxide synthase expression are reduced in the soleus muscle after unloading (26, 46, 53), whereas endothelium-independent vasodilatation is enhanced (26). Therefore, animal data largely suggest a reduction in endothelium-dependent dilatation combined with changes in nitric oxide responsiveness at the level of the smooth muscle cells. In contrast, an upregulation of inducible nitric oxide synthase has also been demonstrated after hindlimb unloading (45, 50). The changes in endothelial function in this animal model and our human model are distinctly different. Apart from interspecies differences, hindlimb unloading causes more microgravity effects than horizontal bed rest. In addition, most changes in rats were observed in the soleus muscle with a decrease in baseline blood flow in the absence of changes in endothelial function in the gastrocnemius muscle (53), whereas in our study blood flow did not change. Nevertheless, the animal data do illustrate that deconditioning may also alter smooth muscle responsiveness.
Changes during bed rest in maximal diameter of the superficial femoral artery in response to nitroglycerin
 
In the present study, FMD corrected for endothelium-independent dilatation does not increase after 52 days of bed rest (Fig. 3D). This suggests that mainly nitric oxide responsiveness or general vasodilator responsiveness of the smooth muscle cell is enhanced after bed rest and not endothelial function and nitric oxide availability. In contrast, exercise training in animals and humans with endothelial dysfunction and vigorous exercise in healthy subjects specifically increase endotheliumdependent dilatation (34). Therefore, the physiological mechanism of the increase in vascular function as a result of exercise or deconditioning appears to be fundamentally different and seems to be located in the endothelium for exercise and mainly in the smooth muscle cell for bed rest deconditioning.
 
Exercise Countermeasure and Vascular Dimension
 
It has been suggested that increase in arterial diameter after exercise training is due to expansive remodeling in response to peak shear stress during exercise (20, 33). Parallel to this reasoning, the observed decrease in diameter after bed rest may represent inward remodeling as an adaptation to diminished exposure to periods of high shear stress. The 16% decrease in maximal diameter of the SFA in the BR-Ctrl group was attenuated to 5% in the RVE-group (Fig. 4, P < 0.01), suggesting that RVE significantly attenuated the effect of bed rest on blood vessel structure. RVE has been shown to increase heart rate (41), oxygen uptake (41), and leg blood flow (28). Therefore, periods of high shear stress are not absent in the BR-RVE group, which explains the observed attenuation of the decrease in baseline and maximal arterial diameter in the BR-RVE group. Nevertheless, the stimulus of RVE on the conduit arteries is probably too low to completely prevent vascular adaptations to bed rest. Moreover, the lack of increase in heart rate after 52 days of bed rest in the BR-RVE group as opposed to the BR-Ctrl group suggests that RVE is an effective countermeasure for some aspects of bed rest deconditioning. In accordance, resistive exercise has been shown to be an effective countermeasure against other detrimental effects of bed rest, such as loss of muscle size and function (1). Whether the effect of RVE is due to the vibration exercise component, the resistive exercise component or the combination of both cannot be determined in the present study design.
 
Exercise Countermeasure and Endothelial Function
 
The FMD and nitroglycerin-mediated response did not differ between the BR-Ctrl and BR-RVE group before and after 52 days of bed rest. Nevertheless, the BR-RVE group appears to follow a different time course of adaptation, with significant differences between groups after 25 days of bed rest. Possibly, RVE only delays the adaptation of endothelial function to bed rest, whereas the reactivity to nitric oxide increases similarly in both groups. One might argue that in the BR-RVE group exercise should have caused an increase in FMD. However, 52 days of bed rest represents an immense deconditioning stimulus, specifically in the legs. In addition, vigorous systemic exercise is needed to improve endothelium-dependent dilatation in healthy subjects without endothelial dysfunction (9). Therefore, it is well conceivable that in our bed rest study the deconditioning stimulus on the endothelium overruled the exercise stimulus.
 
Limitations
 
Endothelium dependency of FMD has been established more extensively in the conduit arteries of the arm than of the leg. However, both Rubanyi et al. (44) and Pohl et al. (40) have demonstrated that an intact endothelium is required for FMD of the femoral artery. Studies of FMD in the arm have shown that both ischemia at the measurement site and prolonged ischemia (15 min) decrease the contribution of nitric oxide to FMD (21, 35). Because we measured FMD proximal of the arterial occlusion cuff and in response to 5 min of ischemia, our results likely reflect endothelium-dependent dilatation.
 
In the setting of the study it was not possible to perform the measurements in the fasting state. Because FMD is decreased after high-fat meals (22, 52), we carefully controlled the subjects’ diets. Subjects received identical, low-fat meals before each measurement. Baseline arterial diameter and FMD are not affected by low-fat meals (22, 52). Therefore, we are confident that we minimized the confounding effects of food intake.
 
Some of the subjects smoked until the start of the study. Smokers were equally distributed among the BR-Ctrl and BR-RVE groups. Although there have been reports that smoking may not affect endothelium-dependent dilatation (25, 32), most evidence suggests that smoking decreases FMD (8). In a hallmark study by Celermajer et al. (8), former smokers with an average time since cessation of 6 yr tended to have better FMD than current smokers. To our knowledge, data on the effect of short-term cessation of smoking of maximal 8 wk on endothelium-dependent dilatation are lacking. Our FMD results were similar if smokers were excluded from the analysis. Therefore, smoking does not appear to have an important influence on our results.
 
In conclusion, the diameter of the leg conduit arteries decreases after bed rest, whereas baseline blood flow remains unchanged. Both FMD and endothelium-independent dilatation of leg arteries increase significantly after bed rest, indicating increased reactivity to nitric oxide after bed rest, possibly by increased nitric oxide sensitivity or increased smooth muscle vasodilator capacity. In addition, RVE can effectively attenuate the diameter decrease of conduit arteries of the leg but seems only to delay the effect of bed rest on endothelial function.
 
Prevention of Postmenopausal Bone Loss
Power Plate Studies
Abstract:

A 1-year prospective, randomized, double-blind, and placebo-controlled trial of 70 postmenopausal women demonstrated that brief periods (<20 minutes) of a low-level (0.2g, 30 Hz) vibration applied during quiet standing can effectively inhibit bone loss in the spine and femur, with efficacy increasing significantly with greater compliance, particularly in those subjects with lower body mass.


Introduction:

Indicative of the anabolic potential of mechanical stimuli, animal models have demonstrated that short periods (<30 minutes) of low-magnitude vibration (<0.3g), applied at a relatively high frequency (20–90 Hz), will increase the number and width of trabeculae, as well as enhance stiffness and strength of cancellous bone. Here, a 1-year prospective, randomized, double-blind, and placebo-controlled clinical trial in 70 women, 3–8 years past the menopause, examined the ability of such high-frequency, low-magnitude mechanical signals to inhibit bone loss in the human.


Materials and Methods:

Each day, one-half of the subjects were exposed to short-duration (two 10-minute treatments/ day), low-magnitude (2.0 m/s2 peak to peak), 30-Hz vertical accelerations (vibration), whereas the other half stood for the same duration on placebo devices. DXA was used to measure BMD at the spine, hip, and distal radius at baseline, and 3, 6, and 12 months. Fifty-six women completed the 1-year treatment.

Results and Conclusions:

The detection threshold of the study design failed to show any changes in bone density using an intention-to-treat analysis for either the placebo or treatment group. Regression analysis on the a priori study group demonstrated a significant effect of compliance on efficacy of the intervention, particularly at the lumbar spine (p=0.004). Posthoc testing was used to assist in identifying various subgroups that may have benefited from this treatment modality. Evaluating those in the highest quartile of compliance (86% compliant), placebo subjects lost 2.13% in the femoral neck over 1 year, whereas treatment was associated with a gain of 0.04%, reflecting a 2.17% relative benefit of treatment (p = 0.06). In the spine, the 1.6% decrease observed over 1 year in the placebo group was reduced to a 0.10% loss in the active group, indicating a 1.5% relative benefit of treatment (p = 0.09). Considering the interdependence of weight, the spine of lighter women (<65 kg), who were in the highest quartile of compliance, exhibited a relative benefit of active treatment of 3.35% greater BMD over 1 year (p = 0.009); for the mean compliance group, a 2.73% relative benefit in BMD was found (p = 0.02). These preliminary results indicate the potential for a noninvasive, mechanically mediated intervention for osteoporosis. This non-pharmacologic approach represents a physiologically based means of inhibiting the decline in BMD that follows menopause, perhaps most effectively in the spine of lighter women who are in the greatest need of intervention.


Key words: osteoporosis, anabolic, mechanical loading, vibration, low-level, frequency, osteogenic, muscle, skeleton, aging, menopause, bone, antiresorptive
 
INTRODUCTION
 
OSTEOPOROSIS, A DISEASE CHARACTERIZED by the progressive loss of bone tissue, is one of the most common complications of aging.(1) After menopause, BMD can continue to decline at a rate as high as 3%/year in some women,(2–5) resulting in 70% of women over the age of 80 having BMD measurements more than 2.5 SDs below young normal values.(6) Intervention strategies that slow the loss of bone soon after menopause may result in a significant reduction of fractures in those individuals at greatest risk.(7)
 
To date, prevention of bone loss has been approached principally through pharmacologic intervention, the longterm safety of which remains uncertain.(8) These pharmacologic approaches inherently ignore that a significant portion of the skeleton’s structural success can be attributed to bone’s sensitivity to alterations in its mechanical environment, with its “form follow function” characteristics ensuring that sufficient mass is placed to withstand the rigors of functional activity.(9) In essence, physical stimuli represent both an endogenous anabolic stimulus to bone tissue(10) and an antiresorptive factor that can actively inhibit osteoclastogenesis.( 11)
 
The skeleton’s sensitivity to its physical environment infers that such non-pharmacologic signals could provide an exogenous treatment regimen for the inhibition of bone loss. Whereas long-term exercise has been shown to increase BMD in young people,(12) this sensitivity seems to be greatly reduced in the elderly.(13) Moreover, exercise, and the predilection to falls that it may invite, could promote the very fractures that the intervention is prescribed to prevent. In contrast to the relatively well-accepted anabolic influence of high mechanical forces, recent work has led to the hypothesis that extremely small physical stimuli, at sufficiently high, but physiologically relevant, frequencies, can be critical determinants of bone morphology(14) and thus represent a unique means of mediating bone quantity and quality.
 
Using a surgically invasive model on the ulnae of aged (4 year old) turkeys, high-frequency (30 Hz), low-magnitude (200 microstrain) signals were successful in stimulating an increase in cortical bone, whereas high-amplitude (3000 microstrain), low-frequency (1 Hz) signals failed to be anabolic.(15) Delivering these signals noninvasively for 10 minutes/day, a floor plate vibrating vertically at 90 Hz, inducing strain in the bone of less than 10 microstrain, successfully inhibited disuse osteopenia caused by 23 h and 50 minutes of tail suspension in the rat, whereas 10 minutes/ day of normal weight-bearing activity failed to curb this loss.(16)
 
Using a surgically invasive model on the ulnae of aged (4 year old) turkeys, high-frequency (30 Hz), low-magnitude (200 microstrain) signals were successful in stimulating an increase in cortical bone, whereas high-amplitude (3000 microstrain), low-frequency (1 Hz) signals failed to be anabolic.(15) Delivering these signals noninvasively for 10 minutes/day, a floor plate vibrating vertically at 90 Hz, inducing strain in the bone of less than 10 microstrain, successfully inhibited disuse osteopenia caused by 23 h and 50 minutes of tail suspension in the rat, whereas 10 minutes/ day of normal weight-bearing activity failed to curb this loss.(16)
 
In longer-term animal studies, 1 year of daily, 20-minute sessions of low-level (0.3g, where g = earth’s gravitational field, or 9.8 m/s2), high-frequency (30 Hz) mechanical stimulation to the hind limbs of adult female sheep stimulated a 43% increase in bone density in the proximal femur, measured by CT.(17) This increase was achieved through a 36% increase in the thickness of individual trabeculae and a 45% increase in their number,(18) contributing to a 12% increase in stiffness and 27% increase in strength of the cancellous bone from the femur.(19)
 
The work reported here evaluates, in humans, whether such a noninvasive, low-level mechanical signal, induced noninvasively into the musculoskeletal system, is able to inhibit the bone loss that follows menopause. Considering the fiber type–specific sarcopenia that parallels aging,(20) we believe the bone wasting that occurs in older adults results not only from the diminished levels of activity, but from the attenuated 20- to 50-Hz muscle dynamics that normally arise during long-duration activities such as quiet standing. Thus, we hypothesize that “reintroducing” the lowmagnitude, high-frequency dynamics back into the musculoskeletal system will re-establish a key regulatory stimulus to the bone tissue and thus inhibit the reduction of BMD that follows menopause.
 
MATERIALS AND METHODS
 
Study subjects
 
The protocol and study design were reviewed and approved by Creighton University’s Human Use Committee, and all clinical work was completed at the Creighton University School of Medicine’s Osteoporosis Center. Women meeting the 3 to 8-year postmenopausal criteria were recruited from the greater Omaha area by newspaper, radio, and television advertising and from existing subjects within Creighton’s Osteoporosis Center. Informed consent was obtained from qualified volunteers who agreed to participate in the study. Inclusion criteria included normal nutritional status (as determined by questionnaire), stable weight maintenance (i.e., no elective weight loss or diet), estimated daily calcium intake of >=500 mg/day, and the capability of following the protocol for daily use of the device as well as understanding and providing informed consent. Because of design constraints of the oscillating device, the body mass of included subjects had to be greater than 45 kg and less than 84 kg.
 
Exclusion criteria consisted of any pharmacologic intervention for osteopenia within the previous 6 months, any use of steroids, current smoking status, consumption of excessive alcohol (>2 drinks/day), evidence of osteomalacia, Paget’s disease, osteogenesis imperfecta, gastrointestinal disease, or history of malignancy, and/or any prolonged immobilization of the axial or appendicular skeleton within the last 3 years. Subjects were also excluded if they had evidence of spondyloarthrosis, thyrotoxicosis, psychomotor disturbances, hyperparathyroidism, renal or hepatic disease, and chronic diseases known to affect the musculoskeletal system (e.g., muscular dystrophy), and/or were engaged in high-impact activity at least three times per week (including but not limited to tennis, aerobics, running, weight-bearing activity or exercise more intense than fast walking).
 
Subjects not excluded by medical history and who met the inclusion criteria of 3–8 years past menopause underwent a battery of standard laboratory tests (e.g., Health Screen 20, urinalysis, hematology, and bone-specific markers; Metra, Sausalito, CA, USA), as well as lateral X-ray views of the thoracic and lumbar spine. In this second tier examination, subjects were excluded with physical or radiographic evidence of fractures or osteophytes. No patient exclusion was based on BMD status (T or Z scores). If the inclusion/exclusion criteria were satisfied by the medical history, laboratory data, and X-ray data, the subject was enrolled in the study. Over the course of 2 years, a total of 70 women were enrolled in the study.
 
Active and placebo devices were manufactured and assigned a device number to coincide with a randomization code. Each woman successfully recruited into the study was provided with a mechanical device (see below), which was delivered to her home and set up by a technician. Throughout the course of the study, subjects and investigators were blinded as to which device was an active or placebo unit, and all information regarding the randomization scheme was kept confidential and secure.
 
Design of the vibration platform
 
To induce low-level physical stimulation in a controlled manner, an apparatus was designed that used a small, lowforce (18N), but highly linear, moving coil actuator (model LA18–18; BEI San Marcos, CA, USA) to impose peak to peak vertical accelerations of 0.2g at a frequency of 30 Hz on a body mass of up to 85 kg. The device was designed such that a very small driving force would produce vertical accelerations of the subject’s body mass and the supporting spring loaded plate (Fig. 1). With incorporation of appropriate accelerometer feedback from the plate surface, control circuitry was sufficient to reduce non-translational modes of vibration caused by motion or positional changes of the subject.(21) As demonstrated in human volunteers, foot-based, whole-body vibrations above 25 Hz (cycles per second) and below 1g can safely be transmitted into the lower appendicular and axial skeleton without producing any detrimental skeletal resonances. The measured transmissibilities in the skeleton are all significantly below 1.0 at frequencies above 25 Hz, with ˜70% of the ground-based signal reaching the trochanter of the femur and L3 in the spine.(22)

Experimental design
 
Sample size projections (discussed below) determined that 64 women would be required to address the principal hypothesis, that is, women who used an active device at least 80% of the prescribed time would show a significant inhibition of the bone loss that follows menopause. FIG. 1. (A) Noninvasive device to achieve low-magnitude mechanical stimulation consists of a spring-supported plate driven by an 18N peak force electromagnetic actuator. By incorporating the subject’s mass as part of a resonating mechanical system, perturbation of up to 0.4g (peak to peak), over the range of 5–100 Hz, can be attained for subjects up to 80 kg. (B) Accelerations measured at L4 (dotted line), while slightly out of phase with the 0.2g, 30-Hz oscillation of the plate, demonstrated a high level of transmissibility.(22)The study was also designed such that subjects who dropped out within the first months of participation would be replaced. The initial cohort of 64 women was randomly distributed into one of two groups, and individual treatment began as soon as each subject was enrolled in the study. Each subject was randomly assigned to the active or placebo group according to a confidential, randomized number sequence generated by an independent statistical consultant and without regard to baseline BMD or matching between groups.
 
In the initial recruitment group, active devices, which vibrated at 30 Hz, 0.2g peak to peak, were provided to 32 women, whereas 32 women received a placebo device. At this intensity level, with a total displacement of 55 µm, the motion of the active platform is slightly discernible because the intensity is just above the perception level for vibration.( 23) To help obscure the active/placebo status of the devices, each device emitted a low-frequency audible sound to suggest that every plate was “active.” Throughout the course of the study, neither the investigators nor the subjects were informed whether the device was active or placebo, reinforcing the blinded nature of the study.
 
Each coded device was delivered to the subject’s home, and the subject was instructed how to stand on it for two 10-minute treatments/day, separated by a minimum of 10 h, for 7 days/week. By delivering the devices to the subject’s home, each person was insulated from other participants in the study and intersubject device comparison was avoided, which also aided in the blinded study design. The subjects were advised to use the device in any location in their home that was convenient for them. Subject compliance was recorded by an electronic monitor integrated within the device, which tabulated time, date, and duration of each treatment, throughout the 1-year period. After the 10-minute treatment period, both active and placebo devices shut off automatically. If the subject interrupted any given 10- minute period (e.g., stepping off to answer the phone), this disruption was detected through a plate surface pressure switch, signaling the device to emit an acoustic warning and the treatment would pause until the subject returned. If the subject did not return within 10 minutes, the device would record the time activated and automatically shut off.
 
No incentive was given for maximizing compliance, the device emitted no visible or audible warnings if daily use was undersubscribed, and the study was designed such that the investigators did not prompt the subjects to use the device. Percent compliance was measured as the total number of treatments in which the subject stood on the device for at least 8 minutes, divided by two times the number of days the devices were in the subject’s home times x 100.
 
Clinical assessment
 
Baseline BMD was determined by DXA (QDR 2000; Hologic, Waltham, MA, USA), with measurements taken at four skeletal locations: proximal right and left femora, lumbar spine, and the distal one-third of the nondominant radius. Subjects were phoned to come in for follow-up scans at approximately 3, 6, and 12 months. Care was taken to position the patient in the same way at each scan, and the same bone density technician performed each scan. A bone phantom was used to calibrate the DXA machine each day.(24) At baseline and completion of the study, to approximate change in bone remodeling status, serum and urine samples were taken, and markers of bone formation and resorption were measured. At completion of the study, a written “exit” questionnaire was requested from each subject, which asked about ease and convenience of use and whether, in the subject’s judgment, they were on a placebo or active device.
 
Statistical analysis
 
After 12 months of treatment, the primary outcome measure was, in subjects with at least 80% compliance, a significant difference between changes in BMD of the spine and femur in the active and placebo groups. Secondary outcome measures were serum indices of bone formation and resorption. The sample size was determined by anticipating a balanced study with a difference in bone density loss between active and placebo groups of 2% over 1 year, assuming a population SD of 2.4%. A final group size of 56 was calculated to be required to attain a power of 0.80 with an œ of 0.05. With a 10% drop-out rate projected (N = 6), a recruitment goal of 64 was set (N = 32 in each group). While the active/placebo status of the devices was not revealed, any subject who withdrew within the first 3 months of treatment was replaced by a subject who received the same device status.
 
The study results were analyzed in collaboration with an independent statistical consultant (Boston Biostatistics, Wellesley, MA, USA), and no data imputation was performed. The data were initially evaluated in an “intentionto- treat” analysis using the 12-month DXA scan or the scan at the last follow-up visit, and included the results of all subjects enrolled in the study, both treatment and placebo. Analyses were performed a priori using all subjects, first by simple population t-test, and second by multiple linear regression, with body mass and compliance as covariates. Posthoc analyses were performed for all subjects with baseline and 12-month DXA data and for whom full electronically recorded compliance data were available. In posthoc testing, the interaction of compliance and treatment was assessed in a linear effects model, with least square means generated at the specified compliance levels reflecting the intercepts of the three compliance quartile boundaries (59.1%, 76.6%, 85.9%). Because of the reported relationship between osteoporotic fracture risk and body build,(25) a three-way interaction of treatment, compliance, and subject weight (bisected at 65 kg; consistent with NHANES II body weights of females in this age range(26)) was investigated both in a linear effect model and by a simple t-test dichotomizing compliance at the 80% and 60% levels. p values less than 0.05 were considered statistically significant; no posthoc corrections were undertaken.
 
RESULTS
 
In total, 70 (33 active and 37 placebo) subjects were randomized into the study and were included in the intention-to-treat analysis. Six (one active and five placebo) subjects withdrew within the first 3 months and therefore had no DXA follow-up. Each of the six people who withdrew was replaced by a new subject who entered into the same treatment type. Of the 64 subjects who had at least two DXA measurements, 8 did not have a 12-month DXA scan; therefore, the remaining 56 subjects (28 active and 28 placebo each with a 12-month DXA scan) formed the a priori analysis group. Complete electronically recorded compliance data on 10 of the remaining 56 subjects were not available, and thus the per-protocol analysis (group used for posthoc analysis purposes) considers only the subset of 46 subjects (26 active and 20 placebo) where a full electronic record of compliance was available. There was one adverse reaction of treatment reported (headache), which came from a woman in the placebo group. All active devices were reassessed at the end of the study and found to be within 5% of the 30 Hz, 0.2g criteria, as per the original dynamic parameters at the initiation of the study. Furthermore, at the end of the 12-month period, the audible acoustic signal, intended to obscure the active/placebo status of the platform, was functioning in all devices.
 
At the completion of the study, the randomization code was broken, and a comparison of the two groups, active and placebo, was determined. Although the study was not powered to detect demographic differences, age, height, femur, and spine BMD at baseline were not significantly different between the groups. However, at baseline, the placebo group’s average weight was 5 kg higher than the active group (p < 0.03), and the body mass index (BMI) of the placebo group was 2 kg/m2 higher (p < 0.04; Table 1).
 
TABLE 1. BASELINE COMPARABILITY OF THE PLACEBO AND ACTIVE GROUPS (RANGES ARE PROVIDED IN THE PARENTHESES)
 
An intention-to-treat analysis of all 70 subjects was undertaken using a bootstrap technique to permit estimation of the response in subjects with incomplete data.(27) In neither the active nor placebo group did changes in bone density exceed the detection threshold of the study design. In the femoral neck, the active group lost 0.69% of their BMD versus a 0.27% loss in the placebo group. In the trochanter, the active group lost 0.07% of their BMD versus a 0.19% loss in the placebo group . In the lumbar spine, the active group lost 0.51% versus a loss of 0.65% in the placebo population (p = 0.45).
 
Fifty-six subjects (28 active and 28 placebo) had a 12- month DXA scan, and this group constituted the a priori study group. A wide range in compliance with device use was observed in this population, ranging from 1% to 95%. When the device was used, however, 98.4% of what constituted a complete treatment (>8 minutes) was a full 10- minute treatment. Thirty-seven percent of subjects completing the study were at least 80% compliant (10 active and 7 placebo), whereas 72% of subjects were at least 60% compliant (19 active and 14 placebo). Whereas the placebo population had consistently higher losses of BMD in the lumbar spine, femoral neck, and trochanter regions of the skeleton than that measured in the active treatment groups, no significant differences were observed on population averages.

TABLE 2. MULTIPLE REGRESSIONS OF ALL SUBJECTS WITH 12- MONTH DXA WITH COVARIATES OF COMPLIANCE AND WEIGHT

 
Because of the large range of compliance, multiple regression analysis was performed on the a priori populations to identify the relationship between compliance and efficacy. Strong positive associations between device usage and changes in BMD were observed at all three sites of interest (Table 2). Using compliance and weight as covariates, BMD of the spine was found to increase 0.071% for each percent increase in compliance of device use (p = 0.0039). Projecting this correlation to an “idealized patient” who was 100% compliant, and assuming the bone remodeling response to be linear, this would correspond to a 7.1% increase in BMD over the course of the year. For the trochanter at 100% compliance, BMD would be projected to increase by 5.1% (p = 0.085), and for the femoral neck, BMD would increase at a projected rate of 1.8% over the course of the year (p = 0.54). Correspondingly, BMD changes in the placebo population demonstrated no association at the trochanter and lumbar spine and a negative association for the femoral neck (p = 0.001).
TABLE 3. PERCENT COMPLIANCE EFFECT ON TREATMENT DIFFERENCES                                                                
 
Posthoc analysis of the per protocol group, examining efficacy at each intercept of compliance quartiles, used least square means generated at the specified compliance level for those subjects in that quartile and treatment group performed without corrections. Based on the suggestion of a treatment and compliance interaction as seen in Table 2, a linear prediction model was constructed to investigate the general influence of compliance (i.e., percent of total possible treatments completed; Table 3). A significant interaction of treatment and compliance was observed for femoral neck BMD changes (p = 0.06), with the active treatment showing a relative benefit over placebo of 2.17% when the subjects were 86% compliant. Similar observations are seen at the trochanter (relative benefit of 1.23% at 86% compliance; p = 0.21) and at the lumbar spine (1.5% relative benefit; p = 0.09). Factoring in weight improves the efficacy of treatment, with the benefit of treatment ranging from 2% to 3% at all three sites, with p values ranging from 0.19 to 0.009.
Stratification based on body mass shows that the lighter women (65 kg) lost 3.32% bone from the spine over the course of the year.
 Considering weight as an interacting influence on spine BMD, the subjects were stratified into groups above and below 65 kg (Fig. 2; Table 4). In the lower-weight cohort, in the highest quartile of compliance (86%), there was a 3.17% loss of bone in the spine in the placebo group compared with a 0.18% gain in BMD in the active group, suggesting a 3.35% relative benefit of treatment (p < 0.009; Table 3). Similarly, in this lower-weight, high-compliance group for the femoral neck, there was a 2.23% loss over the course of the year in the placebo group compared with a 0.13% loss in the active group, representing a 2.1% relative benefit of treatment. For the trochanter, the relative benefit was 1.92% over the course of 1 year of treatment.
 
Figure 3 provides a plot of the quartiles derived from the linear modeling with the placebo group providing the mean of the three-quartile values for each treatment site. In the lumbar spine, a 0.1% loss in the highest quartile of compliance was relatively better than the 1.55% loss experienced by the lowest compliance group. TPERCENT CHANGE AS A FUNCTION OF COMPLIANCE AND WEIGHThis 1.55% loss in the lowest compliance group was similar to the 1.76% loss measured in subjects standing on a placebo device. In the trochanter region, a 0.76 gain was determined for the highest compliance group, whereas a 0.5% loss was experienced by the lowest compliance group, a loss that was similar to the 0.71% loss observed in the placebo group. The femoral neck, as well, demonstrated a dose-dependent response with a 0.04% gain in the highest-compliance group versus a 1.18% loss in the low-compliance group. This 1.18% loss was similar to the 1.24% loss measured in the placebo group. In the distal radius, there were no significant differences between any of the compliance groups and the placebo group.
 
Serum indices of bone formation and resorption were evaluated at baseline and at the end of the study to determine if the mechanical intervention influenced bone remodeling activity. Dietary calcium (self-reported) was the only variable that seemed significantly different at baseline. At 12 months, hydroxyproline levels fell 16% in the placebo group but only 3% in the active group, reflecting a 13% difference (p = 0.07). Phosphorus (baseline value = 3.7) was up 1.3% in the active group but fell 4% in the placebo group, reflecting a 5% difference (p = 0.08). No significant changes were seen in bone-specific alkaline phosphatase (which went up in both groups), total alkaline phosphatase (which went down in both groups), creatinine (which did not change), osteocalcin, or parathyroid hormone (PTH). Every 3 months, either by telephone or visits to the Center, patients were asked if they exercised more or changed any other aspect of their lifestyle. No trends were identified.
 
In their exit interviews, the subjects expressed concern that two 10-minute/day treatments were difficult to schedule but that they may be more encouraged to use the device if efficacy was demonstrated and if a single use per day were possible. Approximately 20% of the active subjects guessed incorrectly in terms of whether they had an active device, and 30% of the placebo subjects guesses were incorrect as to the status of their device.
 
DISCUSSION
 
This study examines the safety and potential efficacy of a very-low-magnitude physical stimulus to inhibit loss of BMD, which is based on the musculoskeletal system’s strong sensitivity to mechanical stimuli. The physical stimulus is imposed noninvasively into the weight-bearing skeleton through ground-based accelerations. The nature of the vibratory stimulus is based on providing a surrogate for the spectra of high-frequency muscle-based signals that attenuate with aging.(20) In addition to large amplitude mechanical forces (and resultant strains) associated with vigorous activity, smaller magnitude strain signals are continually evident in bone,(14,28) and it is these signals that we are trying to mimic. When the 12-month human data are considered in an a priori analysis, the results indicate a potential benefit of treatment strongly dependent on compliance, as standing on the device for close to 20 minutes/day was associated with a greater ability to prevent bone loss. Using linear regression analysis to determine the effect of full 100% compliance indicates that an “idealized” subject who used the device for the full 20 minutes/day could have up to 7% higher lumbar spine BMD and 5% higher BMD in the trochanter than those who did not use the device at all. Compliance, however, is difficult to ensure in any study,(29) and strategies to improve use must be considered.
 
The exit interviews indicated that a “twice per day” regimen made it difficult to fit into a working schedule. The ability of low-level mechanical stimulation to inhibit bone loss in weight-bearing regions was strongly dependent on compliance (femoral neckPossibly, exposure time could be reduced if the potency of the mechanical signal could be increased, perhaps by increasing the amplitude to above 0.2g, which may take advantage of the interdependence of cycle number and strain magnitude,(30) or to identify alternative frequencies or waveform combinations that may be more effective.(31) Examining subject commitment to a shorter treatment duration, a recent feasibility study has shown that, over 6 months of treatment in an elderly female population (75–90 years old), using a 10-minutes/day, 30 Hz stimulus at 0.3g, a mean compliance of 93% was maintained.(32) Considering the difficulty in fitting in two 10-minute treatment regimens, it is also possible that compliance would have been improved had a single 20-minute session been used.
 
Posthoc analysis indicates that this intervention may be more effective in lighter women than in heavier women, particularly in the spine (Fig. 2). Considering that BMD is positively correlated with body mass,(25) these data in turn also suggest that the mechanical stimulus works best in those women with lower BMD (i.e., effective in women who require it), specific to those skeletal sites that need treatment (no significant differences were observed in the radius between active and placebo subjects). The individualized “sensitivity” to the mechanical signal is consistent with findings in the mouse, where the anabolic potential of the mechanical stimulus is realized in inbred strains with low bone density (e.g., B6), whereas there is only low responsivity to altered mechanical environments in the high-density strains (e.g., C3H).(33)
 
This study indicates that low-level mechanical stimuli may have the potential to prevent bone loss in the postmenopausal population, but failed to stimulate the formation of bone. In contrast, the stimulus used in this study was shown in animal studies to be strongly anabolic,(17–19) an observation supported by recent work addressing the effects of 0.3g vibration on bone density in children with cerebral palsy(34) and adolescent females (10–13 years old) in the lowest quartile of BMD.(35) Whether the anabolic response was observed because the signal was delivered to the skeleton of children rather than adults or because the amplitude was 50% greater (0.3g rather than 0.2g) is not yet clear. Considering that the bone strain resulting from these vibrations are two orders of magnitude below those levels that initiate microdamage,(36) this indicates that anabolism can be achieved without putting the skeleton at structural risk. With this in mind, it is relevant to note that in a recent study reported by Torvinen et al.,(37) vibration 40 times greater than the signals examined here (8g as opposed to 0.2g) failed to stimulate any form of bone response. Whether this was because the study was relatively brief (8 months), used healthy young adults (and therefore there was no “signal” lacking that required replacement), or that the amplitude was so great as to be beyond any form of physiologic relevance (as in light that is too bright, sound that is too loud, or pressure that is too great), is difficult to determine at this point.
 
No adverse reactions were reported in the active group. Nevertheless, vibration of the human body is undeniably a complex issue,(38) and considering the variety of pathologies it may exacerbate, including low back pain,(39) circulation disorders,(40) and/or neurovestibular dysfunction,(41) it must be approached carefully. ISO 2631 gives “provisional guidance as to acceptable human exposure” to whole-body vibration in the 1- to 100-Hz band for a sitting or standing person,(42) defining numerical values of the “fatigue-decreased proficiency boundary” over a 24-h period. Sinusoidal frequencies in the range of 25–32 Hz allow for a 4-h exposure at 0.4g, well exceeding acceleration levels and times under investigation with this device. The safety of signals that exceed 1g, for even a short duration, may be of some concern.(43)
 
There is general perception within the skeletal disciplines that signals must be large to represent a positive influence on bone mass and morphology.(44) These data support the premise that extremely small mechanical signals may also be capable of serving as a regulatory influence on skeletal architecture, the “outcome” of which seems to be a more uniform distribution of stresses in trabecular bone under load.(45) This regulatory influence may be achieved directly, by mechanical strain, or indirectly, through amplification of the signal by intramedullary pressure(46) or fluid flow(47) in the bone tissue. Alternatively, the regulatory response may be regulated through a system such as neuromuscular feedback amplified by the low-level signals exceeding a stochastic threshold(48) or by stimulating skeletal muscle pump activity, resulting in significant effects on circulatory flows and fluid flow through the bone tissue.(49) Even considering the complicated nature of the physical mechanism, there can be little doubt that the biological means of controlling bone adaptation are even more complex.(50)
 
Bone architecture is but one of several critical risk factors associated with long bone fractures. For example, postural stability and muscle strength contribute to fracture risk on a par with BMD.(51) If the physical stimulus investigated here does represent a surrogate for the signals lost by sarcopenia, it is entirely possible that the muscle may benefit from treatment as well, enhancing muscle strength,(52) and coupled with the neurovestibular system, improve postural stability.( 48)
 
This prospective, randomized, double-blind, and placebocontrolled study has provided important preliminary results, and clinical support for the hypothesis that extremely low level physical stimuli may provide an effective means to inhibit bone loss, particularly for those who cannot or will not comply with traditional pharmacologic interventions for osteoporosis.(53)
 
 
Comparing the performance enhancing effects of squats on a vibration platform
Power Plate Studies
ABSTRACT.
 
Rønnestad, B.R. Comparing the performance-enhancing effects of squats on a vibration platform with conventional squats in recreationally resistance-trained men. J. Strength Cond. Res. 18(4):000-000. 2004.—The purpose of this investigation was to compare the performance-enhancing effects of squats on a vibration platform with conventional squats in recreationally resistance-trained men. The subjects were 14 recreationally resistance-trained men (age, 21–40 years) and the intervention period consisted of 5 weeks. After the initial testing, subjects were randomly assigned to either the ‘‘squat whole body vibration’’ (SWBV) group (n = 7), which performed squats on a vibration platform on a Smith Machine, or the ‘‘squat’’(S) group (n = 7), which performed conventional squats with no vibrations on a Smith Machine. Testing was performed at the beginning and the end of the study and consisted of 1 repetition maximum (1RM) in squat and maximum jump height in countermovement jump (CMJ). A modified daily undulating periodization program was used during the intervention period in both groups. Both groups trained at the same percentage of 1RM in squats (6–10RM). After the intervention, CMJ performance increased significantly only in the SWBV (p < 0.01), but there was no significant difference between groups in relative jump height increase (p =0.088). Both groups showed significant increases in 1RM performance in squats (p < 0.01). Although there was a trend toward a greater relative strength increase in the SWBV group, it did not reach a significant level. In conclusion, the preliminary results of this study point toward a tendency of superiority of squats performed on a vibration platform compared with squats without vibrations regarding maximal strength and explosive power as long as the external load is similar in recreationally resistance-trained men.

KEY WORDS. whole body vibration, resistance training, strength adaptations, squat, CMJ
 
INTRODUCTION
 
Lately, it has been hypothesized that mechanical vibration at a low amplitude and high frequency of the whole body can positively influence muscle performance (8–10, 15, 49, 55, 57–59). Nazarov and Spivak (38) were among the first to highlight the association between strength and power development and whole-body or segment-focused vibration training. They assumed that repetitive, eccentric vibration loads with small amplitudes would effectively enhance strength, because of a better synchronization of motor units. In the last decade, remarkable enhancements in strength and power after vibration training have been presented. A single vibration bout has been shown to result in acute and temporary effects when it comes to muscle power and/or strength of the lower extremities and arm flexors
 
The mechanisms mediating this acute effect of vibration on neuromuscular performance are not entirely understood. The mechanical action of vibration mediates fast and short changes in the length of the muscle-tendon complex. This may induce a nonvoluntary muscular contraction termed the ‘‘tonic vibration reflex’’ (TVR). TVR is believed to depend upon the excitation of the primary muscle spindle (Ia) fibers (11, 18, 35, 47). Thus, potential extra excitatory inflow during vibration stimulation is partly related to the reflex activation of the a-motoneuron. Accordingly, researchers have reported an increase of the root mean square EMG (EMGrms) of the biceps brachii muscle in boxers exercising with a vibrating dumbbell that was twice as high as a voluntary arm flexion with a load equal to 5% of the subject’s body mass (7). Also, Torvinen et al. (57) found an increase in EMGrms in the calf muscles during whole body vibration (WBV). In accordance with the latter, studies have demonstrated a facilitation of the excitability of the patellar tendon reflex by vibration applied to the quadriceps muscle (12), vibration induced drive of a-motoneurons via the Ia neuron loop (48), and activation of the muscle spindle receptors after applying vibrations (30). However, if muscle spindles are stimulated for a long period of time by vibration, they will finally fatigue (6). This, in turn, is seen as reduction in EMG activity, motor-unit firing rates, and muscle contraction force. It is possible that the ideal vibration period to achieve acute strength/power gains is individual, thus fatigue may explain why some of the studies find no positive effect after one acute bout of vibration (16, 45, 58). However, a confounding explanation exists. Vibrations also seem to depress some monosynaptic spinal reflexes (e.g., H-reflex) (17, 34). The decrease in the reflex is primarily related to a presynaptic inhibitory mechanism, involving a depolarization of Ia afferents (21). The practical effects of these reflexes regarding resistance training are unclear.
 
Some studies have examined the effect of WBV training on muscle performance over a longer period. Bosco et al. (8) studied the effect of a 10-day training program with daily series (5 x 90 seconds) of vertical sinusoidal vibrations at a frequency of 26 Hz on subjects who had no previous experience with resistance training. They found significant improvement in the height and mechanical power during a 5-second continuous jumping test. However, a period of 10 days is too short to determine the long-term effects of WBV. Runge et al. (49) presented gains of 18% in chair-raising time in fit elderly persons after 8 weeks of WBV training (3 times a week at 27 Hz). Recently, Torvinen et al. (59) presented a study of 8- month WBV (4 minutes per day, 3–5 times per week, with 25–45 Hz). The subjects were young and healthy nonathletic adults. They found a significant 7.8% improvement in vertical jump height in the vibration group. On the isometric extension strength of the lower extremities, grip strength, shuttle run, and postural sway the vibration intervention had no effect. Similar results have been presented after 4 months of WBV training with an identical training protocol (58).
 
Neither of the studies mentioned above compared the performance-enhancing effects of WBV with those of conventional resistance training, so we cannot tell if there is a difference in strength improvement between the two training methods. FIGURE 1. Squat performed on a vibration platform.However, other studies have compared these 2 training methods for a longer period (6–12 weeks), and have concluded with similar and significant improvement in strength regarding WBV and conventional resistance training with moderate intensity (15, 55). Both these studies included only untrained subjects, and untrained people improve their strength dramatically in the beginning of a strength-training period (40). Thus, if there are any differences in strength gain between the WBV training and conventional resistance training, it is difficult to detect it in previously untrained subjects. Regarding conventional resistance training, studies indicate that training at an intensity similar to 80–90% of 1 repetition maximum (1RM) is best for improving strength (4, 64). The studies of Delecluse et al. (15) and Schlumberger et al. (55) did not carry out conventional resistance training in this intensity zone, so it can be claimed that it was not an optimal strength training regime. Issurin et al. (26) took the latter into consideration when they studied the effects of ‘‘vibratory stimulus training’’ on strength, using a ‘‘sitting bench-pull apparatus’’ with 44 Hz vibration frequency 3 times per week for 3 weeks with men who had not previously trained on resistance exercises. A control group performed exactly the same training protocol except from the vibration stimulus (6 sets of sitting bench-pulls with the load gradually increasing from 80 to 100% of 1RM). The group using vibration showed an increase in maximum strength of 49.8%, whereas the group using conventional resistance training without vibration showed an improvement of 16.1%.
 
In the latter study, the vibration training induced significant greater strength improvement compared with conventional resistance training. Because WBV training is used by professional athletes (9, 27, 33, 36), it is of great interest to repeat the study of Issurin et al. (26) on resistance-trained subjects. Thus, the purpose of this study was to compare the effects of squats performed on a vibration platform (VP; NEMES-LC, Ergotest, Rome, Italy) with conventional squats without vibrations on 1RM and countermovement jump ([CMJ]; a measurement of explosive strength after stretch shortening of the muscles), in resistance-trained men during a 5-week overreaching period of peaking. Both groups trained with a load equal to 6–10RM. With the results of Issurin et al. (26) in mind, it was hypothesized that squats performed on a VP are superior to conventional squats when the subjects are training with the same external load on the Olympic bar.
 
METHODS
 
Experimental Approach to the Problem
 
To address the question of whether squats performed on a VP are superior to conventional squats without vibrations in resistance-trained men, the effects of 5 weeks with squat training on 1RM and CMJ were compared. Both groups trained at the same intensity (number of RM); the only difference was that 1 group performed the squats on a vibration platform (Figure 1). The subjects carried out all squats (both testing and training), in both groups, on a Smith Machine (Gym Bo, Gelsenkirchen, Germany) to avoid a balance problem on the VP during the squats.
 
Subjects
 
Sixteen men (age, 21–40 years; height, 177.8 ± 6.5 cm; weight, 76.2 ± 8.8 kg) served as subjects. Two subjects withdrew before completion of the study, due to causes unrelated to the study. All subjects had participated regularly in resistance training (minimum 3 times a week during the last year) and completed at least 1 bout of squats each week. TABLE 1. Training regime for both the SWBV and S groups*To be included in the study, the lifters had to lift at least 2.2 times their body weight in a 1RM squat. To make sure there were no differences in training periodization, the subjects provided written information about their training regimen during the last year. Full advice was given to the subjects regarding the possible risk and discomfort that might be involved, and the subjects gave their written informed consent. The study was approved by the Regional Ethics Committee of the Norwegian Research Council for Science and Humanities.
 
Subjects were randomly divided into 2 different training groups. The ‘‘squat whole body vibration’’ (SWBV) group (n = 7) trained squats on the VP on a Smith Machine. The ‘‘squat’’ (S) group (n = 8) trained conventional squats (without a VP) on a Smith Machine.
 
Testing was administered at the beginning and at the end of the 5-week training intervention. Because all the subjects had completed at least 1 bout of squats per week during the last year, we did not spend time on familiarization with the squat exercise. The order of tests was similar before and after the training intervention. The posttests were accomplished at approximately the same time of the day as the pretests, 3 days after the last workout to avoid acute effects of WBV and to reassure proper recovery after the last workout. All subjects completed at least 91% of the workouts.
 
Training
 
The 5-week training period consisted of 3 workouts during the first, third, and fifth weeks, and 2 workouts during the second and fourth weeks. The subjects completed 13 workouts on nonconsecutive days (Table 1). Each subject performed a standardized 10-minute aerobic warmup before each workout; 2–3 warm-up sets of squat were also performed with gradually increased weight. All subjects were supervised by the investigator at every workout during the first 2 training weeks, and thereafter at least once a week.
 
Training volume (total reps performed) and intensity (RM) were altered similarly for the 2 groups. During the first week, both groups performed 3 sets of 10RM in each bout of exercise, during the second and third training week they completed 4 sets of 8RM, and during the last 2 weeks they trained with 4 sets with 6RM (Table 1). Subjects were encouraged to continuously increase their RM loads during the intervention. Subjects were allowed assistance on the last rep. However, to achieve a modified daily undulating periodization, the subjects were told to reduce their load on the Olympic bar by 10% approximately every third workout (this was coordinated between the 2 training groups). Daily undulating periodization is characterized by frequent alterations in the intensity and volume (43, 44). This program seems to place considerably stress on the neuromuscular system, because of the rapid and continuous change in program variables (44), and thus elicits greater strength gains than a linear periodized program. The subjects in SWBV group performed their squats on a VP with a frequency of 40 Hz. Subjects were prohibited from performing any other strength-building exercises on the legs during the 5-week training intervention.
 
Testing
 
We used 1RM as a measure of pretraining strength in squats. Squat testing and training was performed on a Smith Machine. The pre- and posttesting was done on the same equipment with identical subject-equipment positioning overseen by the same trained investigator.
 
Jumping Measurements
 
The subjects performed a 10-minute warm-up, consisting of cycling at a workload of 60–70 W. Thereafter they performed 4 trials of CMJ. The flight time of each single jump was recorded using an infrared light mat (Muscle Lab, Ergotest Technology A.S, Langesund, Norway), interfaced to a personal computer. To avoid immeasurable work, horizontal and lateral displacements were minimized, and the hands were kept on the hips throughout the jumps. During CMJ, the angular displacement of the knees was standardized so that the subjects were required to bend their knees to approximately 908. The obtained flight time (t) was used to estimate the height of the rise of body center of gravity (h) during CMJ (i.e., h = gt2/8, where g = 9.81 m·s-2). The coefficient of variation regarding test-retest reliability for a similar test has been found to be 4.3 % (63). The best performance was used for statistical analysis.
 
1RM Measurement
 
Before the 1RM squat test, subjects performed a standardized warm-up consisting of 3 sets with a gradually increasing load (40, 75, and 85% of expected 1RM) and decreasing number of reps (12, 7, and 3). The knee-angle during the 1RM squat had to be 908 to be accepted. To assure similar knee angle in the pre- and posttest for all the subjects, the subjects’ squat depth was individually marked at the pretest depth of the buttock on a list. Thus, the subject had to reach his individual depth (touch his list with the buttock) in the posttest to get his lift accepted. The first attempt in the test was performed with a load approximately 5% below the expected 1RM load. After each successful attempt, the load was increased by 2–5% until failure in lifting the same load in 2–3 consecutive attempts. The rest period between each attempt was 3 minutes. The coefficient of variation for test-retest reliability for this test has been found to be <2% (41).

Statistical Analyses
 
All values given in the text, figure, and tables are mean ± SD. Paired t-tests were used for within-groups comparisons, and unpaired t-tests were used to compare the relative changes in strength and jump height between groups. Bonferroni adjustments were made to account for tests of 2 variables. Thus, p values of 0.025 were used for each of the 2 variables (1RM and CMJ).
 
RESULTS

1RM test
 
There was no significant difference between the groups at the pretest in 1RM. In both groups, 1RM squat increased during the training intervention (p < 0.01, Table 2). Although there was a trend toward a greater relative strength increase in the SWBV group compared with the S group (32.4 ± 9.0% vs. 24.2 ± 3.9%, respectively; p = 0.046), it did not reach a significant level when Bonferroni adjustments were made (Table 2).
 
One repetition maximum loads in squat and counter-movement jump performances recorded before (pretraining) and after (posttraining) the 5-week training intervention.
 
CMJ test
 
There was no significant difference between the groups at the pretest. Only the SWBV group significantly improved their jump height (p < 0.01, Table 2), but there was no significant difference between groups in relative jump height increase (p =0.088).
 
DISCUSSION
 
This is the first study on resistance-trained subjects that compares the effects of WBV training and conventional resistance training on 1RM in squats and maximal CMJ, where the external load is similar between 2 groups. The preliminary results of this study point toward a trend in which squats performed on a VP is superior to conventional squats regarding maximal strength and explosive power. It seems that this advantage depends on heavy external loading in addition to WBV. Both groups increased their 1RM in squats during the training intervention, and the relative strength increase was greater in the SWBV group than the S group (32.4 ± 9.0% vs. 24.2 ± 3.9%, respectively; p = 0.046). The jumping performance, CMJ, was significantly improved in the SWBV groups, but there was no significant difference between the groups (p = 0.088). It may be speculated that the lack of significant differences between the groups is related to the fact that this study contains only 7 subjects in each group and the intervention lasted only 5 weeks.
 
Several other studies have found positive effects of WBV on CMJ (8, 15, 56, 58, 59). In contrast to the present study, Delecluse et al. (15) found in untrained subjects that WBV training is superior to conventional resistance training when it comes to improvement in CMJ. However, in the latter study, there was a significant higher CMJ performance recorded in the conventional resistance group compared with the other groups in the pretest condition. Thus, it may be argued that the potential for progression in CMJ was smaller for this group.
 
The first adaptation mechanism of a skeletal muscle to resistance training is believed to be neural change, due to an almost immediate increase in strength at the onset of training and the absence of (measurable) hypertrophy (3, 13, 51). The exact mechanism by which resistance training can improve neuromuscular activation is not known, but there are several possible explanations which could cause this enhancement (e.g., increase in motor unit synchronization, co-contraction of the synergistic muscles or increased inhibition of the antagonist muscles [52]). These explanations have also been used to explain the effects of WBV on jumping performance (7, 15, 58, 59). All these studies were accomplished with subjects who had no previous resistance training, and neural adaptation seems to dominate in the early adaptation phase of resistance training (51, 53). The present study was carried out with resistance-trained men, with whom the neural adaptation phase should have reached a plateau. However, neural adaptation can not be ruled out, because of the specificity principle: a change in the training program, such as different exercises and/or intensity, could trigger a transient burst of neural and muscular adaptations (52). The modified daily undulating periodization training regime and the introduction of vibration training could potentially result in neural adaptations. This is supported by the relatively great improvement in 1RM strength in both the S and SWBV groups (24.2 ± 3.5 and 32.4 ± 8.9%, respectively). In line with this, Ha¨kkinen et al. (24, 25) found increased integrated EMG activity in elite weightlifters, indicating the importance of neural adaptations in experienced strength and power athletes.
 
The trend toward superiority of the SWBV group regarding 1RM strength in this study is in accordance with earlier results with untrained subjects. Issurin et al. (26) found that, with previously untrained subjects, applying vibrations (44 Hz) while training with a load 80–100% of 1RM, is superior to training with the same external load without vibrations. However, Delecluse et al. (15) and Schlumberger et al. (55) compared conventional resistance training with WBV training and found no differences regarding strength improvement. This result may have been caused by the lack of external load in the WBV group. Other studies have not found improvement in maximum strength after WBV interventions (16, 54, 58– 60). The reason is unclear, but the lack of external load in all these studies may indicate that this is important to achieve strength gains after WBV training.
 
The mechanisms mediating the apparently superior effect of performing squats on a VP vs. conventional squats, regarding 1RM strength, are not fully understood. An increase in isometric contraction strength induced by TVR has been well documented after local vibratory stimulation applied to the tendon or muscle (1, 18, 29). Armstrong et al. (2) found similar results when subjects were holding a cylindrical handle vibrating at 40 Hz, resulting in 52% increase in grip strength. The TVR may have contributed to the results of Bosco et al. (7), who found an increase of the EMGrms of biceps brachii muscle in boxers who were exercising with a vibrating dumbbell twice as high as a voluntary arm flexion, with a load equal to 5% of the subject’s body mass. Also, Torvinen et al. (57) found an increase in EMGrms in the calf muscles during vibration. In accordance with the latter, studies have demon strated a facilitation of the excitability of the patellar tendon reflex by vibration applied to quadriceps muscle (12), vibration induced drive of a-motoneurons via the Ia loop (47), and vibration activation of the muscle spindle receptors (30). Rittweger et al. (46) also found significantly greater EMG mean frequency over the vastus lateralis after exercise with vibrations than without vibrations. These studies indicate that exercising with vibrations achieves superior excitation of the motoneurons to exercising without vibrations. Sale (50) suggested that full activation of the muscle may lead to motor unit fatigue, and due to this training effect, may increase the strength. The motor units in the SWBV group did perhaps get more fatiguing stimulus because of increased TVR, and thus superior gains in 1RM compared with the S group.
 
The alpha-motoneuron is the final point of summation for all the descending and reflex inputs, and the net membrane current of this motoneuron determines the discharge pattern of the motor unit and thus the muscle activity (37). De Gail (14) states that TVR is able to cause an increase in recruitment of the motor units through activation of muscle spindles and polysynaptic pathways. In addition, the WBV waves propagate from the distal links to muscles located proximally and activate a greater number of muscle spindles. Their discharge activates a larger fraction of the motor pool and recruits many previously inactive motor units into contraction (27). This increased activity of motor units may enable the SWBV group to train with heavier loads than the S group, and thereby optimize the stimulation of higher recruitment threshold motor units and muscle tissue mass with each workout (42).
 
Another possible explanation concerns the difficulty in achieving full muscle activation by voluntary effort during dynamic exercise, when large muscle groups are involved (28). It is likely that the vibrations may cause partial activation of the muscles, and their mobilization at the beginning of the effort will be faster. Thus it is possible that the group which trained with vibrations could train with heavier loads and get a better stimulus for strength increase. Evidence also indicates that voluntary activation is a limiting factor in force production, and that improvements in force generated per unit cross-sectional area are responsible for the initial gain in strength (20). The possibility of enhanced capacity of the muscle to perform work when vibrations are applied simultaneously with external load was demonstrated by Liebermann and Issurin (33). The 1RM in isotonic elbow flexions for Olympic athletes increased significantly (8.3%) while applying vibrations (44 Hz) to the maximum lift, compared with conventional maximum lift without vibrations. Similar results were presented by Issurin and Tenenbaum (27). They found significant increase in mean and maximal power in elite athletes when vibration was applied (44 Hz). This is in accordance with the result of this study, where the SWBV group tended to train with a higher percent of their 1RM, compared with the S group, although this difference was not significant (data not shown). Thus, it seems as the vibrations increases the intensity of the lift rather than reduce it.
 
Although not measured in this study, a certain degree of hypertrophy may be expected after 5 weeks of intensive resistance training (56). In rats, a vibration-induced enlargement of slow- and fast-twitch fibers has been demonstrated (39). Thus it is possible that the vibrations gave an extra hypertrophy stimulus. Another potential explanation is that the vibrations resulted in greater stretch/ tension on the contractile elements (either directly through the TVR itself, or by increased capacity to lift heavier loads via the TVR). Stretch/tension seems to be an essential stimulus for muscle growth
 
Another stimulus for muscle growth is the androgen hormone testosterone. Testosterone is able to affect muscle growth via increased amino acid uptake and protein synthesis in the muscle cells (5, 19, 23, 61). Bosco et al. (10) found that acute exposure to WBV causes increased plasma concentrations of testosterone. The same acute testosterone response is also seen after a single bout of resistance exercise when the workout involves large muscle groups, relative heavy resistance (85–95% of 1RM), moderate to high volume of exercises, and short rest intervals between the sets (31). Whether the addition of vibrations in the SWBV group induced a larger testosterone response than the S group is not known.
 
It may be argued that differential psychological factors due to training on the VP might affect the motivation, and because of that promote greater effort in each single session in the SWBV group compared with the S group. This study did not control for psychological factors, but the results of Delecluse et al. (15) indicate no placebo effect of vibration training.
 
The study design makes it impossible to answer the reasons behind the tendency of difference in 1RM gain between the 2 groups, because no neurogenic enhancement or changes in the morphological structure of the muscles could be demonstrated (neither EMG recordings nor muscle biopsies were performed).
 
In conclusion, this preliminary study on recreationally resistance-trained men indicates that CMJ height was significantly increased only by the squats performed on the VP. Both training interventions led to a significant improvement regarding 1RM in squats. There was a tendency toward superior 1RM improvement in the SWBV group, compared with the S group, but this did not reach a statistically significant level (p = 0.046). Possible explanations for this tendency toward differences in training adaptations may be related to neural adaptation, TVR, or a more favorable hormone milieu regarding muscle growth during the SWBV strength-exercise protocol.
 
The above-noted findings suggest that vibration is a potentially efficient training stimulus. Future studies should include a sufficient number of subjects and focus on comparing the long-term effects of WBV with external loads to conventional resistance training to explore the mechanisms behind these apparent differences.
 
PRACTICAL APPLICATIONS
 
This study indicates that when recreationally resistancetrained men perform squats with the same external load, there is a tendency toward superiority of squats performed on a VP compared with conventional squats without vibrations regarding 1RM in squat and maximal CMJ height. Consequently, it seems as though optimal strength gains in resistance-trained subjects are achieved by adding vibration to the conventional resistance training. This superior effect of vibrations on strength seems to depend on relatively heavy external resistance (6– 10RM). Therefore, instructions from a qualified instructor are advised before adding the relatively heavy external load needed to optimize strength gains.
 
 
Short-Term Effects of Whole-Body Vibration in Unilateral Chronic Stroke Patients
Power Plate Studies
ABSTRACT
 
The short-term effects of whole-body vibration as a novel method of somatosensory stimulation on postural control were investigated in 23 chronic stroke patients. While standing on a commercial platform, patients received 30-Hz oscillations at 3 mm of amplitude in the frontal plane. Balance was assessed four times at 45-min intervals with a dual-plate force platform, while quietly standing with the eyes opened and closed and while performing a voluntary weight-shifting task with visual feedback of center-of-pressure movements. Between the second and third assessments, four repetitions of 45-sec whole-body vibrations were given. The results indicated a stable baseline performance from the first to the second assessment for all tasks. After the whole-body vibration, the third assessment demonstrated a reduction in the root mean square (RMS) center-of-pressure velocity in the anteroposterior direction when standing with the eyes closed (P < 0.01), which persisted during the fourth assessment. Furthermore, patients showed an increase in their weight-shifting speed at the third balance assessment (P < 0.05) while their precision remained constant. No adverse effects of whole-body vibration were observed. It is concluded that whole-body vibration may be a promising candidate to improve proprioceptive control of posture in stroke patients.
 
Whole-Body-Vibration Training Effective in Older Women CME
Power Plate Studies
        June 18, 2004 — Whole-body-vibration (WBV) training is as efficient as standard resistance (RES) training for improving strength and speed in older women, according to the results of a randomized trial published in the June issue of the Journal of the American Geriatric Society.

       "Recently, WBV training has been promoted as an efficient alternative for resistance training," write Machteld Roelants, MS, from Katholieke Universiteit in Leuven, Belgium, and colleagues. "Even if performed to exhaustion, the increases in heart rate, blood pressure, and oxygen uptake during WBV are mild, so the cardiovascular risks of WBV in older adults are negligible."
 
       In this controlled trial at the Exercise Physiology and Biomechanics Laboratory in Leuven, 89 postmenopausal women were randomized to WBV training, RES training, or to a control group that did not participate in any training. All women were not receiving hormone replacement therapy (HRT), and age range was 58 to 74 years.
 
       Both active intervention groups trained three times a week for 24 weeks. The WBV group performed unloaded static and dynamic knee-extensor exercises on a vibration platform, which provokes reflexive muscle activity. The RES group performed dynamic leg-press and leg-extension exercises, increasing from low (20 repetitions maximum) to high (8 repetitions maximum) resistance, to train knee extensors.
 
       A motor-driven dynamometer measured isometric strength and dynamic strength, speed of movement was measured using an external resistance equivalent to 1%, 20%, 40%, and 60% of isometric maximum, and countermovement jump performance was determined using a contact mat.
 
       After 24 weeks of training, isometric and dynamic knee-extensor strength increased significantly in both groups (P < .001 vs. baseline), and the training effects were similar in both groups (P = .56). Isometric and dynamic knee-extensor strength were 15.0% ± 2.1% and 16.1% ± 3.1%, respectively, in the WBV group, and 18.4% ± 2.8% and 13.9% ± 2.7%, respectively, in the RES group.
 
       After 24 weeks of training, speed of movement of knee extension significantly increased at low resistance (1% or 20% of isometric maximum) to 7.4% ± 1.8% and 6.3% ± 2.0%, respectively, in the WBV group only. There were no significant differences in training effect between the WBV and the RES groups (P = .39 and P = .14, respectively).
 
       Countermovement jump height improved from baseline (P < .001) after 24 weeks of training in the WBV group (19.4% ± 2.8%) and the RES group (12.9% ± 2.9%). Most of the improvement in knee-extension strength, speed of movement, and in countermovement jump performance occurred after 12 weeks of training.
 
       "WBV is a suitable training method and is as efficient as conventional resistance training to improve knee-extension strength and speed of movement and countermovement jump performance in older women," the authors write. "As previously shown in young women, it is suggested that the strength gain in older women is mainly due to the vibration stimulus and not only to the unloaded exercises performed on the WBV platform."
 
J Am Geriatr Soc. 2004;52:901-908
 
Learning Objectives for This Educational Activity
 
Upon completion of this activity, participants will be able to:
 
• Describe features of WBV training.
• Compare effects of WBV training with RES training on knee-extension strength and movement in older women.
 
Clinical Context
 
        Age-related decrease in physical activity and reduction in sex hormones are linked to strength loss and muscle atrophy. The latter predispose elderly women to falls and fractures with subsequent morbidity and mortality. WBV consists of standing unloaded on a platform generating vertical sinusoidal vibration at a frequency of 2.5 to 40 Hz with amplitudes of 2.0 to 10.5 mm transmitted to the body to stimulate sensory receptors such as muscle spindles. Unloaded exercise is performed on the platform. Tonic vibration is believed to facilitate activation of high-threshold motor units affecting fast-twitch fibers, which play a role in muscle strength and power. The major part of the gain in strength is believed to be due to muscle activity provoked by vibration.
 
        One study by Delecluse and colleagues published in the June 2003 issue of Medicine and Science in Sports and Exercise showed increased isometric and dynamic knee-extensor strength of 16.6% and 9.0%, respectively, in previously untrained young women. WBV has the potential to enhance muscular performance in older adults who are unwilling or unable to perform standard RES exercises.
 
This is the first randomized study to investigate long-term (24 weeks) effects of WBV training on muscle strength, measured as knee-extension isometric, dynamic strength and speed in postmenopausal women not receiving HRT and to compare these effects with similar standard RES training.
 
Study Highlights
 
• Inclusion criteria were postmenopausal women not receiving HRT and not engaged in regular organized physical activities
.
• Exclusion criteria were metabolic or neuromuscular disease, osteoporosis, osteoarthritis, orthopedic injuries, or two or more cardiovascular risk factors.

• 89 women were randomized to a WBV group (n = 30), a RES group (n = 30), and a control group (n = 29) with no exercise program. 69 women completed the program (n = 24, 20, and 25 for the 3 groups, respectively) and were included in the analysis.

• The study was powered at 80% with an α of .05 to detect a difference between the training and control groups assuming a dropout rate of around 30%.

• Baseline assessment included complete medical examination, ergometer test for maximal exercise capacity, and modified Baecke Questionnaire for Elderly to document baseline activity level. • The 2 training programs consisted of seventy-two 30-minute sessions for 24 weeks. Training frequency was 3 times a week with at least one rest day between sessions.

• The WBV group performed a supervised total-body training program with high, deep, and wide-stance squats and lunges with warm-up and cool down. Training volume was increased by extending the duration of the vibrations. Training intensity was progressively increased by reducing rest periods or increasing the amplitude or frequency of vibrations.

• The RES group performed cardiovascular exercises with increasing resistance for leg extensions and leg presses under supervision from instructors. Repetitions were increased using the American College of Sports Medicine guidelines for individuals older than 60 years.

• Outcomes (comparing baseline to 12 and 24 weeks) were percentage changes in strength of knee extensors, dynamometry for isometric and dynamic knee strength and speed, maximal strength, speed of movement, and countermovement jump performance.

• Participants in the 3 groups were similar for age (64 years), weight (66-70 kg.), and body mass index (26 kg/m2). The 69 who completed the program attended all 72 sessions.

• Isometric and dynamic knee strength increased significantly (P < .001) in the WBV group (mean, 15.0%) and the RES group (18.4%) compared with no change in the control group, after 24 weeks of training. There was no significant difference between the two groups (P = .56).

• Speed of movement of knee extension significantly increased at low resistance in the WBV and RES groups after 24 weeks of training compared with the control group which showed no improvement from baseline. Improvements in speed of movement were greater in the WBV than the RES groupat low resistances of 1% and 20%. • Countermovement jump height enhanced significantly (P < .001) in the WBV group (19.4%) and the RES group (12.9%) after 24 weeks.

• Most of the gain in knee-extension strength and speed of movement and in countermovement jump performance were realized after 12 weeks of training.

• Reasons for dropout before 24 weeks included incompatibility of training program with lifestyle, mild knee discomfort, and health problems unrelated to the programs. 3 participants dropped out because of anterior knee pain (patellofemoral dysfunction and patellar tendinopathy) associated with the training program.
 
Pearls for Practice
 
• WBV is a safe, appropriate, and efficient strength-training method that can be applied in geriatric settings as a low-impact exercise program.
• Regular WBV training three times weekly for 24 weeks in postmenopausal women not receiving HRT significantly improved knee-extension isometric strength, dynamic strength, and speed of movement at levels comparable to similar RES training.
 
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Medscape Medical News 2004. © 2004 Medscape
 
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        The material presented here does not reflect the views of Medscape or the companies providing unrestricted educational grants. These materials may discuss uses and dosages for therapeutic products that have not been approved by the United States Food and Drug Administration. A qualified health care professional should be consulted before using any therapeutic product discussed. All readers or continuing education participants should verify all information and data before treating patients or employing any therapies described in this educational activity.
 
 
Transmissibility of 15-Hertz to 35-Hertz Vibrations to the Human Hip and Lumbar Spine:
Power Plate Studies
Study Design.
 
Experiments were undertaken to determine the degree to which high-frequency (15–35 Hz) ground-based, whole-body vibration are transmitted to the proximal femur and lumbar vertebrae of the standing human.
 
Objectives.
 
To establish if extremely low-level (<1 g, where 1 g = earth’s gravitational field, or 9.8 ms-2) mechanical stimuli can be efficiently delivered to the axial skeleton of a human.
 
Summary of Background Data.
 
Vibration is most often considered an etiologic factor in low back pain as well as several other musculoskeletal and neurovestibular complications, but recent in vivo experiments in animals indicates that extremely low-level mechanical signals delivered to bone in the frequency range of 15 to 60 Hz can be strongly anabolic. If these mechanical signals can be effectively and noninvasively transmitted in the standing human to reach those sites of the skeleton at greatest risk of osteoporosis, such as the hip and lumbar spine, then vibration could be used as a unique, nonpharmacologic intervention to prevent or reverse bone loss.
 
Materials and Methods.
 
Under sterile conditions and local anesthesia, transcutaneous pins were placed in the spinous process of L4 and the greater trochanter of the femur of six volunteers. Each subject stood on an oscillating platform and data were collected from accelerometers fixed to the pins while a vibration platform provided sinusoidal loading at discrete frequencies from 15 to 35 Hz, with accelerations ranging up to 1 gpeak-peak.
Results.
 
With the subjects standing erect, transmissibility at the hip exceeded 100% for loading frequencies less than 20 Hz, indicating a resonance. However, at frequencies more than 25 Hz, transmissibility decreased to approximately 80% at the hip and spine. In relaxed stance, transmissibility decreased to 60%. With 20-degree knee flexion, transmissibility was reduced even further to approximately 30%. A phase-lag reached as high as 70 degrees in the hip and spine signals.
 
Conclusions.
 
These data indicate that extremely lowlevel, high-frequency mechanical accelerations are readily transmitted into the lower appendicular and axial skeleton of the standing individual. Considering the anabolic potential of exceedingly low-level mechanical signals in this frequency range, this study represents a key step in the development of a biomechanically based treatment for osteoporosis. [Key words: spine, hip, osteoporosis, transmissibility, vibration, biomechanics, anabolic] Spine 2003;28:2621–2627
 
Adaptive responses of human skeletal muscle to vibration exposure.
Power Plate Studies
ABSTRACT
 
The study was performed in order to test the possibility wheather a single whole body vibration (WBV) session will produce human skeletal muscle response. In 6 female volleyball players movement velocity, muscle force and power were recorded when they performed maximal leg press exercises with extra load of 70,90,110 and 130 kg. The testing took place before and after a 10-min WBV exposure. During WBV subjects were in the standing position with the toes of one leg on the vibration platform (E leg) while the other leg (C leg) was risen from the ground. WBV induced statistically (P
 
INTRODUCTION
 
Skeletal muscle is a specialised tissue which modifies its overall function capacity in response to chronic exercise with high loads (e.g. McDonagh and Davies 1984). Intensive prolonged strength training is known to induce a specific neuromuscular (e.g. Sale, 1988) and hormonal (e.g. Guezennec et al., 1986) adaptive responses in the human body in few months, while the changes in the morphological structure occur later (e.g. Sale,1988). However, the exact mechanism which regulate how the body adapts to the specific demands upon it, is still unknown. Even less knowledge are available in respect to fatigue, relative strength loss and hormonal changes during one acute session of exercises (e.g. Hakkinen & Pakarinen 1995, Bosco et al. 1998, inpress). It should be remind, that specific programs for strength and explosive power training are based on exercises performed with rapid and violent variation of the gravitational acceleration (Bosco,1992). In this connection it should be remind that changes of the gravitational conditions can be produced also by mechanical vibrations applied to the whole body. Whole body vibration applied for ten minutes during 10 days treatment period have induced an enhancement of explosive power performances in physical active subjects (Bosco et al. 1998, submitted for publication). A question arises from these results: how human skeletal muscle response to a single session of 10 minutes application of whole body vibration in well trained athletes? The present study was performed in order to answer of the question.
 
METHODS
 
Six female volleyball players of national level (age : 19.5 ± 2.1 years ; weight : 65.1 ± 3.7 kg ; height : 174.9 ± 3.2) voluntarily participated to the study. They were physically active and were engaged in team sport training program 5 times a week _ Each subject was instructed on the protocol and signed an informed consent to participate in the experiment. Subjects with previous history of fractures or bone injuries were excluded from the study, The study design was approved by the ethical committee of the Italian Society of Sport Science.
 
Procedures:
 
Ten minutes warm up was performed:5 minutes of bicycling at 25 km-h-’ on a cycle ergometer (Newform s.p.a., Ascoli Piceno, Italy) and five minutes of static stretching for the quadriceps and triceps surae muscles. After the warm up, all the subjects, well accustomed with the exercises, performed maximal dynamic leg press exercises on a slide machine (Newform s.p. a., Ascoli Piceno, Italy) with extra loads of 70,90,110 and 130 kg . One leg per time was used for each load. The best trail of three measurements for each load was used for statistical analysis. During the test, the vertical displacements of the loads were monitored with simple mechanics and sensor arrangement (Ergopower ®,Ergotest Technology A. S _, Langensund, Norway). The loads were mechanically linked to an encoder interfaced to an electronic microprocessor (Muscle Lab, Pat. No. 124 1671). When the loads were moved by the subjects a signal was transmitted by the sensor every 3mm of displacement. Thus it was possible to calculate average velocity (AV), acceleration, average force (AF), and average power (AP), corresponding to the load displacements (for details see Bosco et al., 1995).
 
Reproducibility of measurements
 
The dynamic exercises reproducibility testing gave a test-retest correlation r = 0.45 for the average power (P) (Bosco et al., 1995).
 
Treatment Procedures
 
Subjects were exposed to vertical sinusoidal whole body vibration (WBV) using the device called GALILEO 2000 ( Novotec, Pforzheim, Germany). The frequency of the vibrations used in this study was set at 26 Hz (displacement = 1Omm ; acceleration = 27 m l s-2). The subjects were exposed ten times for a duration of 60s with 60s of rest between the treatment each.
 
Type of treatment employed
 
The application was performed in the standing position with the toes of one leg on the vibration platform, the knee angle was pre-set at 100” flexion, while the other was risen from the ground. During all the treatments the subjects were asked to wear gymnastic-type shoes to avoid bruises. The leg which was exposed to vibration was assigned to E group , while the other not exposed was assigned to C group. Thus , in each subject one leg was exposed to vibration (E) and the other was considered as control (C). The leg randomly assigned to each E or c groups demonstrated similar mechanical behaviour exposure (Table 1). Testing procedures were administered at the beginning (Pre) and immediately after (Post) the VT period.
 
Statistical Methods
 
Conventional statistical methods used included mean, standard before the vibration (VT) deviation , paired and unpaired Student’s t-test. The, level of significance was set at P <.05
 
RESULTS
 
Before the VT period, no significant differences was found in the mechanical behaviour between E and C legs in parameters studied (AF,AV, and AP) for all loads used (70, 90,110 and 130 kg) (Table 1) . After the VT period the legs affected by vibration (E) showed statistically significant improvement (Pre vs Post) of the AF, AV and AP developed with all loads used (P < 0.05 - 0.005) (Table 1). In result, the velocity-force (V-F) and the power-force (P-F) curves (Fig. 1), established by the variables shown in Table 1, were shifted to the right after the VT period. Only the AF developed with 70 kg remained unchanged after the VT period . In contrast , the mechanical behaviour of the C legs, demonstrated no changes in mechanical variables studied by the Pre - Post test analysis (Table 1). Only the AV developed with 130 kg showed statistically significant improvement (near 3 % ) in the Post evaluation test (P< 0.05).
 
DISCUSSION
 
As expected the Pre vs Post test analysis performed for the C legs did not show any modification in the mechanical properties studied. This is not a surprising finding, since, in half -squat exercises performed with extra load (100 % of subject’s body mass) no change has been observed in twelve female and male throwers during same day (Bosco et al., 1995). However , the AV developed with 130 kg showed statistically significant improvement in the Post evaluation test of C leg (P< 0.05). Reasonable explanation for this improvement cannot be easily found, considering that the athletes of the present experiments were well accustomed with this type of exercises and therefore any learning effect of the movement executed could be excluded. The mechanical behaviour of the E legs demonstrated a dramatic alterations in the V-F and P-F relationships after VT lasting only ten minutes. Changes and shifting to the right of force-velocity (F-V) relationship have been observed after several weeks of heavy resistance training (e.g. Coyle et al., 1981: Hakkinen & Komi, 1985). The improvement of the of the F-V relationship has been attributed to the enhancement of the neuromuscular behaviour caused by the increasing activity of the higher motor center (Milner-Brown et al., 1975). Thus , it is likely that also the VT have caused a dramatic enhancement of the neural traffic regulating the neuromuscular behaviour (Bosco et al., 1998, submitted for publication) .
 
During vibration of the body skeletal muscles undergo small changes in muscle length. Facilitation of the excitability of spinal reflex has been elicited through vibration to quadriceps muscle (Burke et al., 1996). Lebedev and Peliakov (1991) pointed on the possibility that vibration may elicit excitatory flow through short spindle - motoneurons connections. Burke et al. (1976), suggested that vibration reflex operates predominantly or exclusively on alpha motoneurons and does not utilise the same cortically originating efferent pathways as are in the performance of voluntary contractions, However, a facilitation of voluntary movement cannot be excluded. In the present study, any neurogenic potentiation has not been demonstrated since no EMG recordings were performed. Nevertheless, enhancement of the mechanical behaviour strongly suggests that a neurogenic adaptation have occurred in response to the vibration treatments. Therefore, even if the intrinsic mechanism contributed, the adaptive response of neuromuscular functions to VT could not be explained by it. The duration of the stimulus seems to have relevant importance_ Adaptive response of human skeletal muscle to simulated hypergravity conditions (1. 1 g), applied for three weeks, caused a drastic enhancement of the neuromuscular functions of the leg extensor muscles shifting the F-V relationship to the right (Bosco, 1985). In the present experiment, even if the total length of the VT application period was only 10 minutes, the perturbation of the gravitational field was rather consistent (2.7 g). An equivalent length and intensity of training stimulus can be reached only by performing 150 times leg press or half squat exercises with extra loads of 3 body mass twice a week for 5 weeks (Bosco,l992).
 
 
The influence of whole body vibration on the mechanical behaviour of skeletal muscle
Power Plate Studies
ABSTRACT
 
The aim of this study was to investigate the effects of whole body vibrations on the mechanical behaviour of human skeletal muscles. For this purpose, fourteen physically active subjects were recruited and randomly assigned to an experimental (EG) and a control group (CG). The EG was treated fur ten days with 5 sets of vertical sinusoidal vibrations lasting up to two minutes each, for a total volume of ten minutes per day. The subjects of CG were asked to maintain their normal activity and avoid strength or jumping training. Subjects were tested at the beginning and at the end of the treatment with specific jumping tests performed on a resistive platform. Results showed remarkable and statistically significant enhancement in the EG of the height of the best jump (1.6 %, P
 
INTRODUCTION
 
myogenic factors [22]. The first phase of adaptation is characterised by an improvement of neural factors, while the myogenic factors becomes more important as the adaptations continues over several months (e.g. [20]. Enhancement of explosive power performance (e.g. jumping abilities) and the corresponding biological adaptations to a specific training stimulus are still not understood. Gravity normally provides the major portion of the mechanical stimulus responsible for the development of the muscle structure during everyday life and during training. It should be remind, that strength and explosive power training specific programs are based on exercises performed with rapid and violent variation of the gravitational acceleration [8 ] In this connection, simulation of hypergravity (wearing vests with extra loads) conditions has been utilised for enhancement of human explosive muscle power [5,6], On the other hand, changes of the gravitational conditions can be produced also by mechanical vibrations applied to the whole body. Thus, in light of the above observations, it was assumed that application of whole body vibration to physical active subjects could influence the mechanical behaviour of the leg extensor muscles
 
METHODS
 
Fourteen subjects voluntarily participated to the study, they were physically active and were engaged in team sport training program 3 times a week. The subjects were not engaged in strength and explosive power training but participated regularly for tactical and technical training program according to the discipline practised (handball and water polo). They were equally divided into two groups: an experimental group (EG) and a control group (CG). Each subject was instructed on the protocol and signed an informed consent, approved by the ethical committee of the Italian Society of Sport Science, to participate to the experiment. Subjects with previous history of fractures or bone injuries were excluded from the study together with the ones under the adult age. Table 1 presents physical characteristics of the subjects.
 
Procedures:
 
Anthropometric measures (height and weight) were recorded together with the age of the subjects. Following this phase a ten minutes warm up was performed consisting of 5 minutes of bicycling at 25 kmh-1 on a cycle ergometer (Newform s.p.a., Ascoli Piceno, Italy) and five minutes of static stretching for the quadriceps and triceps surae muscles. After the warm up, the subjects peformed the followings jumping exercises: counter movement jump (CMJ) and 5s of continuous jumping (5s CJ).The flight time (tf) and contact time (tJ of each single jump were recorded on a resistive (capacitative) platform [4] connected to a digital timer (accuracy ± 0.001s) (Ergojump, Psion XP, MA.GI.CA.Rome, Italy). To avoid unmeasurable work, horizontal and lateral displacements were minimised, and the hands were kept on the hips through the gravity above the ground (h in meters) in were measured from flight time (tf in seconds) applying ballistic laws:
 
h=tf2.g.8-1(m)
 
where g is the acceleration of gravity (9.81 m . s-2) During CJ exercises the subject were required to perform the maximal jumping effort minimising knee angular displacement during contact. From the recordings of tf and tc the average mechanical power (AP), average rise of center of gravity (AH) were calculated for the total 5s continues jumping. From 5s CJ the best jumping performance was selected and maximal mechanical power (PBJ) as well as the highest rise of center of gravity (HBJ) were obtained using the equation introduced by Bosco et al [4] :
 
AP = Tf . T . 24.06 e ( Tc )-1 (W *.kg brn-‘) 
 
where P is the mechanical power per kilogram of body mass, Tf the sum of the total flight time, Tt the total working time (5s), and Tc the sum of the total contact time. The average height during 5s CJ and the HBJ were computed using formula 1. 
 
Reproducibility of measurements  
 
The reproducibility of the mechanical power test (5s CJ) and CMJ performances were high with respectively r =.95 and r =,90 [4,27] 
 
Statistical Methods: 
 
Conventional statistical methods used included mean, standard deviation and paired Student’s t-test. The level of significance was set at p<.O5. 
 
Treatment Procedures
 
Subjects were exposed to vertical sinusoidal whole body vibration (WBV) using the device called GALiLEO 2000 (Novotec, Pforzheim, tiermany) . The frequency of the vibrations used in this study was set at 26 Hz (displacement = 1 Omm; acceleration = 27 m.s-2). The subjects were exposed five times for a duration of 90s with 40s of rest between the treatment each. This procedure was repeated for ten days, each day five seconds were added for each treatment up to a total of 2 minutes per position. Following the ten days the subjects of both groups were again tested and data were statistically analysed. Type of treatment employed: The first applicaticm was performed in the standing position with the toes on the vibrations platform. The second bout was performed with the subject in the half squat position. The third application was realised with the feet rotated externally on the vibration platform. The knee angle was pre-set at 900 flexion. The fourth treatment was performed with the subjects standing on the leg on the right side of the vibration platform with the knee at 90” flexion. Finally the fifth application was given while the subjects standing on another leg on the left side of the vibration platform with the knee at 90” flexion. During the 4th and 5th treatment subjects were allowed to keep themselves in balance with the aid of a bar mounted on the platform. During all the treatments the subjects wear gymnastic-type shoes to avoid bruises. The E group was treated with WBV for ten days, the C group was not treated during the project and was asked to maintain their typical activities. Testing procedures were administered at the beginning and at the end of the experiments for both E and C groups. 
 
RESULTS 
 
After almost two weeks of regular technical and tactical training program, the subjects of the C group, as expected, failed to showed changes in any of the mechanical or anthropometric parameters studied (P>0.05). The jumping height in 
 
CMJ remained the same in E group after 10 days of WBV (Table 2). This treatment, in contrast, produced remarkable and statistical significant (P< 0.05) enhancement of the HBJ ( Fig. 1) and the PBJ ( Fig. 2). In addition, the average height during 5s CJ was also improved in E group, demonstrating a statistical significant difference of P< 0.01 (Table 2). On the other hand, the average power developed during 5s CJ failed to demonstrate statistically significant change after the treatment (Table 2). 
 
DISCUSSION 
 
Less than two weeks of regular tactical and technical training programme, as expected, did not induce any modification in the mechanical properties and anthropometric profile of the control subjects_ This is not a surprising findings, since no changes, in jumping performances, was noted after four weeks either in physical active subjects [ 14], or in volleyball players [2]. In contrast, a remarkable improvement of the neuromuscular characteristics studied was observed after the WBV period in the E subjects. Significant enhancement was noted for the HBJ (Fig. 1), PBJ (Fig. 2) and the average jumping height during 5s CJ (Table 2). On the other hand, no changes were noted for the AP during 5s CJ. It should be remind that, during the continues jumping test [4], the average jumping height possessed higher significance and sensitivity than AP in differentiating athletes [28] or in revealing the effect of creatine supplementation [9]. In addition no changes in CMJ were noted after the vibration treatment in E group. Apparently these are contradictory results. However, a reasonable explanation can be found analysing the mechanical behaviour of the leg muscles during CMJ and 5s CJ. In fact, both exercises are characterised by the so called stretch- shortening cycle (SSC). This means that, before the concentric work (pushing phase), leg extensor muscles are actively stretched (eccentric phase) in both exercises. Nevertheless, the neuromuscular activation in CMJ is different than that found in 5s CJ. The CMJ is characterised by large angular displacement and slow stretching speed (3- 6 rad . s-l) [3], while 5s CJ are performed with fast stretching speed (1 O-12 rad . s-l) and small angular variation [7]. This means that, only in 5s CJ the leg extensor muscles experience fast stretching which may elicit a concurrent gamma dynamic fusimotor input that would enhance primary afferent discharge. This notion is supported by the studies of Bosco, et al. [3], who showed that during eccentric phase of drop jumping exercises (similar to 5s CJ), EMG activity was high and comparable to maximal concentric ballistic movements. Thus there is a possibility of enhanced neural potentiation either via spinal or cortical reflex. On the other hand, it is likely that CMJ is not a suitable activity to elicit stretch reflex, since high EMG activity has not been recorded during the stretching phase (e.g. [3]) 
 
On the background of these considerations it is likely that the effect of WBV treatment elicits a biological adaptation connected with neural potentiation. Thus, it can be argued that, the biological mechanism produced by vibration treatment is similar to the effect produced by explosive power training (jumping and bouncing exercises). In fact, this suggestions is consistent with knowledge that mainly the specific neuronal components and its proprioceptive feedback mechanism are the first structure to be influenced by specific training [2,14]. 
 
Training with high stretching loads may improve stretch-reflex potentiation and increase the threshold of firing for the Golgi tendon organs (GTO). The latter one, would then improve the possibility to recruit greater amount of motor units during eccentric phase [2]. Furthermore, there are several ways in which the explosive power training can infIuence neural activation, for example by increasing the synchronisation activity of the motor units [21]. I t cannot be excluded also an improvement of co-contraction of synergist and increased inhibition of antagonist muscles. In any case, what ever it is the intrinsic mechanism which enhance neuromuscular activation after specific explosive power training, it is likely that, the vibration treatment have to improve the proprioceptors’ feedback mechanism, since it is filly operating and elicited during 5s GJ performance, which was enhanced after WBW. On the other hand, the lack of modifications observed in GMJ test after the VBV treatment suggests that the proprioceptors’ feedback mechanism is not strongly operating in CMJ . In fact, this exercise is strongly influenced by the voluntary recruitment capacity and by the fiber type composition of leg extensor muscles [ 1]. However, there is no doubt that stretch reflex play an important role in stiffness regulation [IS], and that muscle spindles and GTO operate actively in the control of muscle length and tension [ 16]. Consequently, it can be suggested that WBV treatment may affect dramatically the neuromuscular functions and properties which are regulating muscle stiffiess through the control of length and tension. 
 
During vibration the body and the skeletal muscle undergo to small changes in muscle length. Facilitation of the excitability of spinal reflex has been elicited through vibration to quadriceps muscle [ 11). The idea that vibration may elicit excitatory flow through short spindle - motoneurons connections in the overall motoneuron inflow has been suggested also by Lebedev and Peliaksv [IX] pointed on the possibility. It has been shown also that vibration drives alphamotoneurons via la loop, producing force without descending motor drive [25]. Burke et al. [ 101, suggested that vibration reflex operates predominantly or exclusively on alpha motoneurons and that it does not utilise the same cortically originating efferent pathways as are in the performance of voluntary contractions. In addition, the results of Kasai et al. [ 17] are consistent with vibration induced activation of muscle spindle receptors not only in the muscle where vibration is applied, but also to the nearest muscles. Mechanical vibration (10 - 200 Hz) applied to the muscle belly or the tendon can elicit a reflex muscle contraction (e.g. [ 13]). This response has been named tonic vibration reflex (TVR). It is not known wheather it can be elicited by low WBV frequency (l-30 Hz), even if it has been suggested to occur [26]. 
 
Finally, it should be remind that not only nervous tissue, but also muscle tissue can be affected by vibration [ 23]. In fact, 5 hours daily for 2 days of vibration exposure at two different frequencies were sufficient to induce enlargement of slow and fast fibers in rats [24]. 
 
In the present study, no neurogenic potentiatian or modification in the morphological structure of the muscles was demonstrated since neither EMG recordings nor muscle biopsy sampling were performed. However enhanced mechanical behaviour during 5 s CJ, strongly suggests that a neurogenic adaptation have occurred in response to the vibration treatments. Even if the intrinsic mechanism of the adaptive response of neuromuscular functions to WBV could not be explained, importance. Adaptive hypergravity conditions the effectiveness of the stimulus seems to have relevant response of human skeletal muscle, to simulated (1 . 1g), applied for only three weeks, caused a drastic enhancement of the neuromuscular functions of the leg extensor muscles [6]. Chronic centrifugal force (2 g) for 3 months [ 19] has initiated conversion of fiber type. In the present experiment, the total length of the WBV application period was not very long (only 100 minutes), the perturbation of the gravitational filed was rather consistent (2.7 g )_ An equivalent length and intensity of training stimulus can be reached only by performing 200 drop jumps from 60 cm, twice a week for 12 months. In fact, the time spent for each drop jump is less than 200 ms, and the acceleration developed can hardly reach 2.7 g [8]. This means to stimulate the muscles for 2 min / week for the total amount in one year of 108 minutes, which is almost the total time of vibration applied to the E subjects. 
 
 
JBMR WebFirst
Power Plate Studies
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Shake, rattle and roll have now entered the osteoporosis quietness with two papers on the clinical effect of whole body vibration in this issue of JBMR. Besides, we can already see these products at scientific meetings. To what extent does the present knowledge address the potential for such approaches in the prevention of fractures

Good vibrations
 
Clinical studies have been based on animal experiments that have shown a positive effect on bone strength and mass of various forms of loading. The basis of these experiments is the concept that trabecular bone adapts to its mechanical environment – Wolff’s law. Support for these experiments has also come from epidemiological findings that greater physical activity – mechanical stimulation is associated with greater bone mass and, in some studies, fewer fractures.
 
The question of the ideal form of stimulation has been addressed in animal studies. High frequency (30 Hz), low magnitude (200 μstrain) signals stimulated large increases in cortical bone in turkeys. However higher amplitude and lower frequency was not anabolic in that model. In a longer term study in sheep over one year, daily 20 minutes sessions of high frequency mechanical stimulation of sheep produced a 35% increase in bone density. This kind of vibration may also affect the sarcopenia that occurs at the same time as bone loss with aging. Other animal studies have shown similar results. Low magnitude mechanical loading became osteogenic when rest is inserted between each load cycle. Effects of loading frequency on mechanically induced bone formation and periosteal osteogenesis suggested a complex interaction between extracellular fluid forces and cellular mechanics in mechanotransduction, best predicted by a mathematical model that assumed: 1) bone cells are activated by fluid shear stresses and 2) that stiffness of the bone cells and the extracellular matrix near the cells increases at higher loading frequencies because of viscoelasticity. These animal experiments have formed the scientific basis for studies in humans.
 
Shakin´ all over
 
In humans extremely low level, high frequency mechanical accelerations have been shown to be readily transmitted into the lower appendicular and axial skeleton of the standing individual. In a recent study, 21 male and 35 female volunteers (age d 19-38 years) were randomly assigned to a vibration or control group. Individuals stood on a vibration platform that was either stationery or oscillated in an ascending order from 25 to 45 Hz, corresponding to maximum vertical accelerations from 2g to 8g intervention for 4 minutes/day, 3-5 times per week over an 8-month period. Although there was no effect on bone mass or serum markers or other performance and balance tests, there was an increase in vertical jump height in the vibration group.
 
In this issue, Verschueren et al report on a 6-month study of whole body vibration in older women with respect to hip density, muscle strength and postural control. The 70 volunteer women, aged 60-70 years, who were healthy and had a BMD T score >-2, were randomized to a control group with no organized training; resistance training knee extensor by dynamic leg press and leg extension exercises or whole body vibration, where the subjects performed the same exercises for 20 minutes per day on a vibration platform that had a vibration frequency of 35-40 Hz and peak acceleration of 2.3-5.1 g. The vibration training improved the isometric and dynamic muscle strength by 15 and 16%, respectively and increased BMD by 0.93%. No hip BMD changed was observed in the resistance training or the age-matched controls. Serum markers of bone turnover did not change in any groups. The authors concluded that whole body vibration training might be a feasible and effective way to modify well recognized risk factors for falls and fractures in elderly women.
 
Also in this issue, Rubin et al, report another trial for 1 year in 70 healthy women, 3-8 years post menopause (mean age 57 years). Those randomized to the vibration platform were exposed to a peak vertebral acceleration of 0.2 g at a frequency of 30 Hz. Compliance was not good as in many other exercise (and pharmacological) interventions and the intention-to-treat analysis did not show an effect. In an analysis limited to those in the highest quartile of compliance (86% compliant), vibration subjects gained 0.04% in femoral neck BMD while placebo subjects lost 2.13% over the year in the femoral neck. The corresponding figures for the lumbar spine were +0.1 and –1.6%. Interestingly, the lower body weight (<65 Kg) women experienced the greatest benefit, 3.4% increase in the highest compliance group and 2.7% in the mean compliance group. The authors conclude that these preliminary results indicate a potential for a non-invasive mechanical mediated intervention for osteoporosis that is perhaps more effective in lighter women who are at greatest need of intervention.
How shall interpret these trials? First of all there are differences in age. The study by Torvinen al had younger and perhaps healthier participants. The greater benefit in lighter individuals in the Rubin study could explain some of these differences. Although there were differences in study duration these overlapped and do not seem to explain any of the differences reported. The Torvinen study used a short exposure period (4 minutes) for each treatment and somewhat greater loads although at similar frequency.(7) None of the studies showed any differences in bone turnover markers but there were observable differences in muscle strength e.g. jump height. These possibilities require examination in further studies with respect to study sample age and weight as well as vibration exposure and amplitude.
 
Any side effects?
 
Vibration of the human body has been proposed from epidemiological studies to cause back pain. However no such major side effects were reported from these studies and whole body vibration exercise has been proposed for treatment of chronic low back pain.(10) Another possible safety aspect is that the displacement could be large enough for the patient to fall but this was not reported in these studies.

What could be the biological mechanisms of this whole body vibration?
 
The vibration is sufficiently low to be unappreciable by the participants so it seems unlikely to be a direct effect of the mechanical strain. It could be an indirect effect through amplifying of signals by intramedullary pressure or through fluid flow in the bone tissue. For the neuromuscular or muscular effects, stimulation of the skeletal muscular pump has also been proposed to affect circulatory flows and flow through the bone tissue. However these potential mechanisms are still to be fully studied.
 
How do these effects compare with published studies on pharmacological interventions?
 
Leaving aside any potential muscle or balance effects, the net benefit versus placebo ranged from 1.55% to 2.2% and up to 3.4% in those of lower weight and best compliance. These effects over 1 year are difficult to compare with pharmacological studies over 2-3 years but in a bisphosphonate study with a 1-year endpoint, the difference from placebo was 2.4%. This comparison might suggest somewhat similar benefit provided good compliance could be achieved. However it is recognised that change in BMD cannot be easily translated to fracture reduction. Thus the burning question is what fracture reduction could be achieved with whole body vibration.
 
Future
 
A tantalizing possibility is that there could an interaction between whole body vibration and pharmacological treatment. Could whole body vibration enhance the effect of an anabolic agent or an antiresorptive? In one study in rat tail, there was a synergistic effect of PTH and mechanical stimulation on trabecular bone formation. It remains to be seen whether similar interactions could be seen in humans, where no major effect on bone turnover from whole body vibration has been observed. A further harder development in the future might be shock wave treatment, in animals shown to be positive to bone mass in fractured limbs.
 
What's Shakin'?
 
What are the requirements to bring this equipment to the market place? Vibration platforms are regarded as ‘devices’ and not a pharmaceutical intervention and hence are subject to different regulatory criteria for safety and efficacy. Hence for considerations of clinical application, it is important to determine what kinds of data are needed to support vibration as a valid and rational treatment option. Should BMD change be sufficient or should we require fracture reduction data ? Is analysis by compliance reasonable in light of our judgements about other randomized placebo controlled trials where ITT is and should remain the gold standard? Although vibration platforms seem to be relatively safe, it will be important to establish their anti-fracture and BMD efficacy as well as their safety in larger and more adequately powered randomized double blinded controlled trials.
 
 
Efficacy of training program for ambulatory competence in elderly women
Power Plate Studies
Abstract.
 
The optimal prevention of osteoporotic fractures in the elderly consists of increasing the bone density and preventing falls. We report on the efficacy of training program to promote ambulatory competence in elderly women. Twenty-five elderly women were enrolled in our training program, which is a three-month program consisting of dynamic balance training with Galileo 900 (Novotec, Pforzheim, Germany) once a week, combined with daily static balance (standing on one leg like a flamingo) and resistance (half-squat) trainings. The mean age of the participants was 72.8 years (range, 61–86 years). After 3 months of training, the step length, knee extensor muscle strength, and maximum standing time on one leg were significantly increased, while the walking speed and hip flexor muscle strength were not significantly altered. During the study period, no serious adverse events such as new vertebral fractures or adverse cardiovascular symptoms were observed in any participant. The present preliminary study shows that our training program may have the potential to promote ambulatory competence in elderly women.(Keio J Med 53 (2): 85–89, June 2004)
 
Key words: elderly women, fall, balance training, strength training, walking ability
 
Introduction
 
Fall-related injuries, e.g., hip fractures are serious problems in the elderly with osteoporosis. Osteoporosis is known to be especially common among elderly women. The incidence of spinal fractures increases with age after menopause in women, and the incidence of hip fractures increases exponentially after 80 years of age in elderly women. More than 90% of hip fractures result from a fall. Falls are also becoming increasingly common among the elderly, so that up to 80% of all 80-year-olds sustain at least one fall per year. While the life expectancy of women is higher than that of men, the proportion of those who are functionally independent is higher in elderly women than in men, which is known as the ‘‘gerontologic paradox.’’ Thus, prevention of trauma, i.e., injuries by falls would reduce disability, improve the quality of life, and reduce the cost of treatment for fall-related injuries, especially in elderly women with osteoporosis.
 
Exercise is generally thought to be effective for the prevention of falls in the elderly. However, randomized controlled trials have not consistently shown beneficial effects of exercise on fall prevention, and the efficacy of exercise for promoting ambulatory competence and preventing falls in the elderly has not yet been unequivocally established. Strategies for the prevention of falls, as well as for the management of osteoporosis in the elderly still need to be established.
 
Recently, vibration training has been developed as a new modality in physiotherapy. It has been suggested that vibration training possibly increases muscle power in athletes, and muscle blood volume in healthy adults, and improves muscular performance and body balance in young healthy subjects. Although highintensity vibration training may increase both the muscle strength and volume in young subjects, we surmise that low-intensity vibration training may have a greater potential to promote ambulatory competence in elderly women with an increased risk of falls, by stimulating the neuromuscular system. safe, easy to practice, and easy to continue in the elderly, and may promote ambulatory competence, including by improving balance and muscle strength, more effectively when combined with daily static balance and resistance training. We report on the efficacy of our three-month training program consisting of onceweekly dynamic balance training with the Galileo system (Novotec, Pfozheim, Germany), combined with daily static balance and resistance training, aimed to promote ambulatory competence in elderly women.
 
Subjects and Methods
 
Twenty-five elderly women, who visited our hospital during the 3 months between October and December in 2002, were recruited to our training program. The program consisted of dynamic balance training with the Galileo 900 (frequency, 20 Hz; duration, 4 minutes) once a week, combined with daily static balance training (standing on one leg like a flamingo for one minute: ‘‘Flamingo exercise’’) and resistance training (10 sets of half-squats) daily. Tables 1 and 2 show the characteristics and physical fitness of the participants, respectively. The training program was administered for 3 months, and all the participants completed the program. None of the participants experienced any fall during the study period.
 
All data are presented as means plus/minus standard deviation (SD). The correlation of age with ambulatory competence and grip strength was examined by single regression analysis. The longitudinal changes in the parameters were evaluated by one-way analysis of variance (ANOVA) with repeated measurements. All statistical analyses were performed using the Stat View J-5.0 program (SAS Institute, Cary, NC, USA) on a Macintosh computer. The significance level of P < 0:05 was used for all comparisons.
 
Results

Characteristics of Participants/Physical Fitness of Participants
Figure 1 shows the correlation of age with the ambulatory competence and grip strength. Age was significantly positively correlated with the 10-meter walking time (r = 0:622, p < 0:001) and significantly negatively correlated with the step length (r = -0:661, p < 0:001). However, no significant correlation was found between age and the maximum torque of the knee extensor and hip flexor muscles, maximum standing time on one leg, or grip strength.
 
 
 
Figure 2 shows the changes in the ambulatory competence and the grip strength. The step length, maximum torque of the knee extensor muscle, and maximum standing time on one leg were found to have significantly increased by 4.5%, 6.8%, and 72.5%, respectively, while the 10-meter walking time and maximum torque of the hip flexor muscle were not significantly altered. During the study period, no serious adverse events such as new vertebral fractures or adverse cardiovascular symptoms were observed in any participant.
 
Discussion
 
The results of the present study showed that the walking speed and the step length were correlated with the age of the subject, and that our training program increased the step length, knee extensor muscle power, and maximum standing time on one leg in the subjects. Age-related decrease in the muscle strength is known to be much more marked in the lower extremities than in the upper extremities, and gait in the elderly is characterized by a decreased step length and step width.12 Impairment of muscle strength of the lower extremities, balance/postural control, and gait has been found to be important risk factors for falls.1 While we found age-related impairment of gait in elderly women with osteoporosis, we could not detect any age-related changes in the strength of the knee extensor and hip flexor muscles in these subjects, probably because the sample size in the study was small and younger women without disability were not included.
 
The optimal prevention of osteoporotic fractures in the elderly consists of increasing bone density and preventing falls. With regard to fall prevention, objective measures of balance and muscle strength that meet the criteria of reliability and validity are required as the bases for exercise regimens. However, randomized controlled trials have not consistently shown beneficial effects of exercise on fall prevention.3–5 The reason for this may be that the type, duration, frequency, and intensity of the exercise components varied across the studies. In particular, one or more of endurance, resistance, and static and dynamic balance training was administered. Of the balance exercises, Tai Chi has proved to be the most successful for decreasing the likelihood of falls.
 
Whole-body vibration training with Galileo was developed as a new modality in physiotherapy. Galileo is a unique device for applying whole body vibration/ oscillatory muscle stimulation. The subject stands with bent knees and hips on a rocking platform with a sagittal axle, which alternately thrusts the right and left leg 0.7–4.2 mm upwards and downwards at a frequency of 20 Hz, thereby stretching of the extensor muscles of the lower extremities. The reaction of the neuromuscular system is a chain of rapid muscle contractions. This type of training provides reflex muscle stimulation to improve balance and possibly the muscle strength, with no serious side effects.
 
Correlation of age and physical fitness. The correlation of age with the ambulatory competence and grip strength was examined by single regression analysis. Age was significantly positively correlated with the ten-meter walking time and significantly negatively correlated with the step length. However, no significant correlation was found between age and the maximum torque of the knee extensor and hip flexor muscles, maximum standing time on one leg, or grip strength.
 
 
The type of exercise that is considered to be safe, well-tolerated, and sustainable in the elderly, is mild static and dynamic balance, and mild resistance trainings. Based on this view, our training program consisted of once-weekly dynamic balance training using Galileo, combined with daily static balance training (standing on one leg) and resistance training (half-squat training). These training sessions were administered to produce a stable gait, and during the study period, no serious adverse events such as new vertebral fractures or adverse cardiovascular symptoms were observed in any participant.
 
Our training program increased the step length, knee extensor muscle strength, and maximum standing time on one leg, but no improvements were found in the walking speed or hip flexor muscle strength. We tried to measure the hip abductor muscle strength, but could not obtain any valuable data in most of the participants, because of severe age-related impairment of the hip abductor muscle. With regard to the effect of exercise on the ambulatory competence in the elderly, a few studies have been reported.1,15,16 Buchner et al.15 reported that the combination of aerobic and anaerobic training in the elderly with mild gait disturbance prevented falls despite no significant improvement of muscle strength and body balance. On the other hand, Runge et al.1 reported that whole-body vibration training with Galileo decreased chair rising time as a result of increased muscle power caused by reflex muscle stimulation in geriatric patients without serious adverse events. Muto et al.16 reported that their original training program consisting of various types of gymnastic training protocols, improved the walking speed and maximum standing time on one leg. Thus, the reported effects of exercise on the ambulatory competence vary among different studies. The reason for this could be the different type of exercise training protocols and different parameters assessed among different studies. Muscle weakness and poor balance underlie most falls, and strength training against resistance and dynamic balance training have been suggested to improve both strength and balance.3 Thus, combination training with strength and balance training appears to be required to prevent falls.
 
Changes in physical fitness. All the data are presented as meansGSD. The longitudinal changes in the ambulatory competence and grip strength were examined by one-way analysis (ANOVA) with repeated measurements. The step length, maximum torque of the knee extensor muscle, and maximum standing time on one leg were found to have significantly increased, while the ten-meter walking time and maximum torque of the hip flexor muscle were not significantly altered.
 
Because our training program consisted of the combination of three types of exercise training, it remains uncertain which one most significantly improved balance and muscle strength. Half-squat training (resistance training) was aimed at increasing knee extensor muscle strength, while Flamingo exercise (static balance training) was aimed at increasing maximum standing time on one leg. Whole-body vibration training (dynamic balance training) was aimed at stimulating neuromuscular system, promoting balance, and possibly increasing the muscle strength in the lower extremities. Thus, the increases in the knee extension muscle strength and standing time on one leg might have been a result of the combined effects of all three types of training.
 
On the other hand, the reason for the increase in the step length remains uncertain. Basically, each stride during walking consists of the stance and swing phases. Thus, increased maximum standing time on one leg and knee extensor muscle strength can produce a more stable gait. That is, the more the stance phase of each leg was stabilized by training, the greater the swing of the other leg became, resulting in an increase in step length. Because impaired gait in the elderly is possibly associated with decreased step length, the increased step length may also indicate improved ambulatory competence. In order to significantly increase the walking speed, endurance training using bicycle or walking training might be needed.
 
The limitations of the present study should be discussed. First, there were no control groups. Thus, only longitudinal changes of ambulatory competence from the baseline were available only in the trained subjects. Second, the sample size might have been too small for detecting the significance of the longitudinal increases in the walking speed. Third, the duration of the training program might have been too short for the efficacy of our program for fall prevention to be unequivocally established. Further studies may be needed to confirm the efficacy of our program for musculoskeletal health in elderly women.
 
In conclusion, the present preliminary study shows that our training program may have the potential to promote ambulatory competence in elderly women.
 
 
Effects of whole-body vibration exercise on the endocrine system
Power Plate Studies
ABSTRACT.
 
Whole-body vibration is reported to increase muscle performance, bone mineral density and stimulate the secretion of lipolytic and protein anabolic hormones, such as GH and testosterone, that might be used for the treatment of obesity. To date, as no controlled trial has examined the effects of vibration exercise on the human endocrine system, we performed a randomized controlled study, to establish whether the circulating concentrations of glucose and hormones (insulin, glucagon, cortisol, epinephrine, norepinephrine, GH, IGF-1, free and total testosterone) are affected by vibration in 10 healthy men [age 39±3, body mass index (BMI) of 23.5±0.5 kg/m2, mean±SEM]. Volunteers were studied on two occasions before and after standing for 25 min on a ground plate in the absence (control) or in the presence (vibration) of 30 Hz whole body vibration. Vibration slightly reduced plasma glucose (30 min: vibration 4.59±0.21, control 4.74±0.22 mM, p=0.049) and increased plasma norepinephrine concentrations (60 min: vibration 1.29±0.18, control 1.01±0.07 nM, p=0.038), but did not change the circulating concentrations of other hormones. These results demonstrate that vibration exercise transiently reduces plasma glucose, possibly by increasing glucose utilization by contracting muscles. Since hormonal responses, with the exception of norepinephrine, are not affected by acute vibration exposure, this type of exercise is not expected to reduce fat mass in obese subjects. (J. Endocrinol. Invest. 27: ??-??, 2004)
 
High-Frequency Vibration Training Increases Muscle Power in Postmenopausal Women
Power Plate Studies
Objective:
 
To test whether training on a high-frequency (28Hz) vibrating platform improves muscle power and bone characteristics in postmenopausal women.
 
Design:
 
Randomized controlled trial with 6-month followup.
 
Setting:
 
Outpatient clinic in a general hospital in Italy.
 
Participants:
 
Twenty-nine postmenopausal women (intervention group, n=14; matched controls, n=15).
 
Intervention:
 
Participants stood on a ground-based oscillating platform for three 2-minute sessions for a total of 6 minutes per training session, twice weekly for 6 months. The controls did not receive any training. Both groups were evaluated at baseline and after 6 months.
 
Main Outcome Measure:
 
Muscle power, calculated from ground reaction forces produced by landing after jumping as high as possible on a forceplate, cortical bone density, and biomarkers of bone turnover.
 
Results:
 
Over 6 months, muscle power improved by about 5% in women who received the intervention, and it remained unchanged in controls (P.004). Muscle force remained stable in both the intervention and control groups. No significant changes were observed in bone characteristics.
 
Conclusion:
 
Reflex muscular contractions induced by vibration training improve muscle power in postmenopausal women.
 
Key Words: Bone density; Exercise; Muscles; Postmenopause; Rehabilitation; Vibration; Women.
 
MUSCLE POWER, the capacity of muscles to produce work in the environment, declines significantly over the life span. In women, the rate of decline accelerates after menopause and leads to reduction in physical functioning.1 It has been hypothesized that this process may be responsible for the development of physical frailty and mobility disability1,2 in old age. Although evidence is overwhelming that physical exercise positively affects muscle strength at all ages, compliance of older persons with traditional exercise programs has generally been low, and only a small percentage of older persons exercise regularly.
 
Vibration exercise on ground-based platforms that oscillate at high frequency has recently been proposed as an intervention for the prevention and the treatment of osteoporosis.4-6 Highfrequency (28Hz), very-low-magnitude (0.3g) vibration exercise has recently been reported to increase bone mass in experimental animals and in humans.6-10 However, the mechanism by which vibrations influence the bone tissue remains unclear.10
 
The high-frequency postural displacements induced by the alternating movements of the platform produce reflex muscle contractions aimed at stabilizing posture.11 Thus, vibration can be viewed as a special form of muscle training that may particularly affect muscle power.12 It has been proposed that the force applied to bone during muscle contraction has a pivotal role in the homeostatic and adaptive regulation of bone strength.13,14 This hypothesis may explain, in part, the mechanism by which vibration improves bone strength. To test this hypothesis, we conducted a small, randomized controlled trial (RCT) to discover whether training on a high-frequency vibrating board for 6 months would improve muscle power in postmenopausal women and, in turn, positively influence bone characteristics.
 
METHODS
 
Design
 
All the study procedures, including recruitment, measurements, and intervention, were performed in the Nuovo San Giovanni di Dio Hospital in Florence, Italy. The recruitment phase began in spring 1999 and was completed in fall 1999. The intervention began in the winter 1999–2000 and was completed by summer 2000. Among the 67 women belonging to a hospital volunteers association (Associazione Volontari Ospedalieri), 39 women who were at least 1 year postmenopausal and not affected by conditions that contraindicated the vibration training were enrolled in the study population (fig 1). Women on hormone replacement therapy were considered eligible. Women with metabolic bone disorders were excluded from the trial.

Flow diagram of the RCT.
 
The screened women entered a 3-month run-in phase during which they received daily 1g of calcium carbonate and .25µg of activated vitamin D (calcitriol). This supplementation was administered to all the participants for the entire study period to avoid any influence of insufficient calcium or vitamin D intake on the effects of vibration exercise on bone apposition and mineralization. Because of the nature of the intervention, no blinding or placebo was considered. Of the 67 screened women, 33 agreed to participate in the study, signed an informed consent, and were randomized to either vibration or control group. A simple randomization procedure was applied using a series of random numbers. Six of the 39 eligible women refused to participate in the trial owing to family problems (n=2) and little interest (n=4).
 
Measurements
 
Blood and urine tests were performed to exclude from the trial subjects affected by metabolic bone disorders like primary hyperparathyroidism or hyperparathyroidism secondary to renal failure. All blood samples were drawn in the morning between 8:00 and 9:00 AM, in the fasting state. Routine biochemical parameters, which included total serum calcium, serum phosphorus, and creatinine, were measured using standard laboratory methods. Serum parathyroid hormone (PTH) was measured by a double-antibody chemoluminescence methoda (interassay cell volume [CV]=2%), and serum bone-specific alkaline phosphatase was measured using an immunoenzymatic methodb(interassay CV=5%). Deoxipiridinoline and N-terminal telopeptide were measured using a 1-step chemoluminescence methoda (interassay CV=3%) and immunoenzyimatic methodc (interassay CV=10%), respectively. To collect the 2-hour morning urine, participants were instructed to get up early in the morning and void. After 2 hours of fasting, during which only ingestion of water was allowed, participants voided again, and all urine samples were collected and used for measurements. To assess muscle power, participants, starting from a standstill, jumped as high as possible and landed on a forceplated that measured ground reaction forces.15 The best of 4 attempts was used in the analysis. The acceleration of the center of gravity (COG) was calculated as the ratio of force (N) and body mass (kg). The integration of acceleration by time gives the instantaneous velocity of the COG (m/s). The power (W) is obtained as the product of force and velocity. Tibial bone density, mass, and geometry were assessed by a recent generation, high-resolution, peripheral quantitative computed tomography device (XCT 2000d). Volumetric total bone density (mg/cm3) was measured as the average density of the whole cross-section of the tibial metaphysis (4% of the tibial length from its distal end); that is, the section mainly composed of trabecular bone surrounded by a thin cortical shell. At the same site we assessed trabecular bone density (mg/cm3) by excluding cortical bone. Measures of cortical bone density (mg/cm3) and cross-sectional area (mm2) were obtained from a cross-sectional image of the tibial diaphysis at 38% of the tibial length from its distal end. In these images, all of the voxels with a density above 710mg/cm3 were considered to belong to cortical bone.
 
Intervention
 
The active intervention consisted of brief training sessions conducted twice weekly for 6 months. In each session, vibration was provided by a commercially available device (Galileo 2000d). By means of an oscillating board, this device delivers high-frequency vibration through the legs to the whole body. Participants stood with feet side by side on the board, which produced lateral oscillations of the whole body with accelerations in the range of 0.1 to 10g. At the beginning of the training, participants stood on the board with the knees slightly flexed and received three 1-minute bouts of vibration separated by 1-minute resting periods. During the first month of treatment, the frequency of vibration was progressively increased from 12 to 28Hz to allow for gentle adaptation. During the following 5 months of treatment, the frequency was always set at 28Hz, and the bouts of vibration were prolonged to 2 minutes. Participants were invited to separate the feet as far as tolerated to increase the amplitude and speed of the vertical displacement. Previous studies11 have demonstrated that the oscillating movement of the board produces muscle stretching, which elicits alternating reflex contraction of the flexor and extensor leg muscle groups. Participants in the active group attended on average 34 sessions, corresponding to about 200 minutes of treatment, out of 44 sessions potentially available.

 
Statistical Analysis

 All analyses were performed using the SAS, version 8.2, statistical software.e Data are reported as mean±standard error (SE). Baseline characteristics of the intervention and control group were compared by 1-way analysis of variance (ANOVA). The magnitude of change over time in muscle and bone parameters in the intervention versus control group was compared using a repeated-measures ANOVA.

RESULTS

 Women who received the active intervention were similar to controls in age, baseline muscle power, years since menopause, anthropometric measures, routine biochemical measurements, and biomarkers of bone turnover (table 1). Final measurement of the primary outcome (muscle power) was obtained in 29 of the 33 women who had been originally randomized (14 active treatment, 15 controls). Dropouts in the intervention group were caused by family problems (n=2) and knee pain (n=1). In 1 control, a measure of muscle power at the final follow-up could not be obtained because of posttraumatic muscle pain.
 
After 6 months, muscle power improved by about 5% (from 178.9±9.6W to 187.3±9.5W) in women who received the active treatment (table 2), whereas it declined slightly in controls. In a repeated-measure ANOVA, change over time in muscle power differed statistically between the 2 groups (P<.02). Overall, muscle power improved in 80% of the women in the treatment group and in 46% in the controls (P=.06). The velocity increased in the intervention group to a similar extent as the power (from 163.7±6.2m/s to 171.7± 5.3m/s, P<.005), whereas muscle force did not change significantly in either group.
Characteristics of the Participants at Baseline
 
Cortical bone density remained stable in the intervention group, whereas it declined significantly in the control group (P<.05). However, in a repeated-measure ANOVA, the decline in cortical bone density over time did not differ statisti- cally between the 2 groups (P=.09). All other bone parameters, including biochemical indices of bone turnover, did not change significantly during the study period in either group.

Transient, slight lower leg itching and erythema, a known side effect of the vibration exercise,16 was also observed in 6 of 17 treated participants in this study. In no case, however, did this problem persist after the first 3 training sessions or cause interruption of the intervention. Knee pain of moderate intensity, without objective clinical signs, was observed in 2 overweight participants with preexisting knee osteoarthritis. The pain subsided in both participants after a few days of rest. One of them, however, refused to continue and was dropped from the study population.

 
DISCUSSION
 
In the present study, 200 minutes of high-frequency wholebody vibration, distributed in biweekly sessions over 6 months, improved muscle power and the velocity of movement in postmenopausal women without significant changes in muscle force. These results suggest that vibration training improves muscle power mainly by enhancing the pattern of recruitment of muscle fibers.
 
This study is the first to show an improvement of muscle power in postmenopausal women using vibration exercise. The decline in muscle power is an early and apparently inexorable occurrence in the life of a woman, perhaps contributing to physical frailty and mobility disability in late life. Studies17 have demonstrated that such a decline may be slowed by strength training exercise. However, the compliance of older persons in traditional exercise programs is poor.
 
High-frequency vibration on a ground-based platform stimulates continuously alternating reflex contractions of flexor and extensor muscle groups of the lower extremities.11 We hypothesized that vibration is a special type of exercise that may be particularly suitable for older persons. It does not require much time or effort, does not cause potentially traumatic vertical displacements of the involved joints, and specifically trains type II muscle fibers, which are selectively lost during the aging process.16,18 The availability of a simple, safe, and wellaccepted training method that can improve muscle power in postmenopausal women opens a new perspective for the prevention of age-associated loss of muscle function in this group of women.
Effect of 6 Months of High-Frequency Vibration Training on Muscle and Bone Parameters
 
Previous studies have demonstrated that vibration exercise improves bone mineral density in animal and human models. Our findings provide a possible explanation for this effect of vibration exercise. Mechanical stress produced by muscle contraction plays a critical role in the maintenance of bone strength.19,20 Thus, improvement in muscle force and power may be a strategy for improving bone characteristics and preventing osteoporosis in postmenopausal women. In accordance with this hypothesis, our study showed that the decline in cortical bone density tended to be greater among control women than among women who received the active treatment. Our findings on cortical bone volumetric density are consistent with earlier reports21 and support the hypothesis that vibration exercise may positively affect bone characteristics.10 However, clinical trials that address these issues would require longer follow-up and, probably, a more intensive intervention. Based on earlier reports and on the present findings, our conclusion is that vibration exercise may be a more useful tool for the prevention and treatment of osteoporosis than pharmacologic treatment of osteoporosis,22,23 a disease that is generally underdiagnosed and undertreated.24,25
 
The vibration training was safe overall. The only clinically significant side effect was knee pain, which was observed in 2 participants with preexisting osteoarthritis of the knee. This pain caused cessation of treatment in 1 subject. The frequent occurrence of transient lower leg erythema reported16 previously was often observed in the present study, but it was always transient, mild, and not disturbing.
 
The present study has several limitations. First, the small number of participants and the relatively short duration of the intervention might have prevented us from identifying treatment effects on secondary outcomes such as muscle force or bone parameters. However, the effect on the primary outcome, muscle power, was small but clear-cut and therefore unlikely to be due to chance. Likewise, the treatment’s safety clearly needs to be tested in larger studies. Second, the compliance with the treatment sessions was suboptimal; in fact, only 34 of 44 sessions were attended on average. However, an important reason for the low attendance was the restricted choice of days and time offered to the participants for the training sessions (because of our lack of financial resources). The training was perceived as very useful by the participants, who uniformly reported an improved well-being as a consequence of the training. Moreover, it can be considered a striking finding of this study that a substantial improvement in muscle power was obtained with only 200 minutes of training.
 
CONCLUSION
 
The results of this small RCT suggest that high-frequency vibration exercise is a feasible, safe, convenient, and efficacious intervention, which could prevent the decline in muscle and bone strength in postmenopausal women. Such intervention can easily be added as a component of an exercise-based prevention program or even prescribed as the sole intervention when traditional exercise is not feasible.


 
 
Acute changes in neuromuscular excitability after exhaustive body vibration exercise
Power Plate Studies
Summary
 
The effects of hard squatting exercise with (VbX+) and without (VbX)) vibration on neuromuscular function were tested in 19 healthy young volunteers. Before and after the exercise, three different tests were performed: maximum serial jumping for 30 s, electromyography during isometric knee extension at 70% of the maximum voluntary torque, and the quantitative analysis of the patellar tendon reflex. Between VbX+ and VbX) values, there was no difference found under baseline conditions. Time to exhaustion was significantly shorter in VbX+ than in VbX) (349 ± 338 s versus 515 ± 338 s), but blood lactate (5.49 ± 2.73 mmol l)1 versus 5.00 ± 2.26 mmol l-1) and subjectively perceived exertion (rate of perceived exertion values 18.1 ± 1.2 versus 18.6 ± 1.6) at the termination of exercise indicate comparable levels of fatigue. After the exercise, comparable effects were observed on jump height, ground contact time, and isometric torque. The vastus lateralis mean frequency during isometric torque, however, was higher after VbX+ than after VbX). Likewise, the tendon reflex amplitude was significantly greater after VbX+ than after VbX) (4.34 ± 3.63 Nm versus 1.68 ± 1.32 Nm). It is followed that in exercise unto comparable degrees of exhaustion and muscular fatigue, superimposed 26 Hz vibration appears to elicit an alteration in neuromuscular recruitment patterns, which apparently enhance neuromuscular excitability. Possibly, this effect may be exploited for the design of future training regimes.
 
Introduction
 
Although present in many classical sports, vibration loads have been neglected until very recently (Issurin et al., 1994; Mester et al., 1999). Vibration exercise (VbX) is a new type of exercise, that has been designed with the idea of stimulating muscles via spinal reflexes. It is currently being tested in different fields, ranging from the training of elite athletes (Bosco et al., 1999) to the therapy of osteoporosis (Spitzenpfeil & Mester 1997; Rubin et al., 1998, 2001) and chronic low back pain (Rittweger et al., 2002b).
 
Some unexpected observations have been made in the application of vibration exercise. For example, the blood volume has been reported to increase (Kerschan-Schindl et al., 2001), and an erythema may occur over the activated limbs (Rittweger et al., 2000). Another finding has been that of an increased electromyographic (EMG) median frequency in isometric contraction immediately after exhaustive vibration exercise, which is in contrast to the general observation of a decreased EMG frequency in muscle fatigue (Hakkinen & Komi 1983).
 
It is evident that a better understanding of the physiolgical mechanisms involved in vibration exercise will help to identify the potential benefits of this technique. In this respect, an alteration in the neuromuscular functioning is of crucial interest. It was thus decided to investigate the acute neuromusclular effects of exhaustive whole body vibration in an integrative approach.
 
Muscle function is characterized by the production of force and power, but also by the capacity to maintain force and power over a given period of time (=endurance). Both central nervous and peripheral mechanisms contribute to these functions.
 
The maximum muscular power output can be observed after single jerks or jumps (Bosco et al., 1983b). In young healthy subjects, the power output during continuous maximum jumping is usually maintained over 10–20 s and then declines as a function of fibre type composition and hence fatigability (Bosco et al., 1983a).
 
One mechanism of fatigue is due to the relative insufficiency in oxidative energy supply. As a result, lactate accumulates in the blood. Within the usual range, lactate efflux from the muscle is linearly related with the intracellular concentration and is inversely related to the slow twitch muscle fibre proportion (Bangsbo et al., 1993). During intense, dynamic exercise blood lactate may thus serve as an indicator of fast twitch muscle fibre fatigue.
 
During isometric maximum voluntary contraction (MVC) in healthy young subjects, the greatest part of the recruitable force is reliably elicited (Allen et al., 1995). In sustained contractions, the EMG frequency and the EMG power picked up over the working muscle typically decrease, both in concentric contractions and in isometric contractions (Tesch et al., 1983; Hakkinen & Komi 1983). This is due to changes in the recruitment patterns, as smaller motor units have a smaller conduction velocity (and hence EMG frequency) and amplitude (and hence EMG power) than larger units (Kupa et al., 1995). Thus, the analysis of EMG patterns serves as an indicator of central nervous recruitment patterns.
 
The stretch reflex is a pathway with one central nervous synapse, relaying information about length changes to the a-motoneurone. Within this pathway, the Ia-afferent nerve fibres have a predominant effect on larger motor units, containing predominantly fast twitch muscle fibres. After demanding exercise the reflex amplitude is typically decreased (Avela et al., 2001; Zhang & Rymer 2001).
 
We have thus decided to investigate the influence of vibration exercise on neuromuscular force and power production and maintenance, as assessed by isometric contraction and the serial jumping test, and on the central nervous neuromuscular recruitment, as assessed by EMG analysis during isometric contraction and by quantitative stretch reflex analysis. We chose to test the frequency (26 Hz) that has been used by many other investigators including our own laboratory. From the experience in our laboratory and by reports from other colleagues, a vibration frequency below 20 Hz induces muscular relaxation (we have successfully applied 18 Hz vibration exercise in patients with chronic lower back pain), whereas there are reports that at frequencies above 50 Hz severe muscle soreness and even haematoma may emerge in untrained subjects.
 
Methods
 
Participants
 
Ten female and nine male subjects were recruited from the University campus. Before inclusion, written informed consent was obtained from all subjects (approval of the local ethical committee under signature Galileo/Physio/Elektrik). The female subjects had a mean age of 21.8 ± 2.7 years, height of 172.6 ± 6.1 cm, body mass of 63.2 ± 3.4 kg and a body mass index (BMI) of 21.3 ± 1.8 kg m-2. The male subjects had a mean age of 24.4 ± 2.8 years, height of 181.8 ± 5.5 cm tall, body mass of 75.3 ± 6.4 kg and a BMI of 22.7 ± 1.2 kg. m-2.
 
Study design
 
A randomized cross-over study was designed, that compared neuromuscluar effects of exhaustive exercise in squatting with (VbX+) and without (VbX-) whole body vibration. Three different neuromuscular measurements (i.e. serial jumping, isometric torque, and patellar stretch reflex) were performed separately before the exercise (Pre), and immediately after termination of the exercise (Post), and again after 10 min of recovery after termination (Rec). Hence, each subject performed six trials (3 x 2), with at least 3 days interval between visits. Each subject performed the different tests in random order.
 
Measures
 
Neuromuscular testing comprised EMG during isometric knee extension, quantitative measurement of the patellar tendon reflex and serial vertical jumping. All tests were performed for 30 s.
 
Vertical serial jumping was performed on a resistive contact plate. The subjects were instructed to jump as high as they could, with the ground contact time as short as possible. During the whole time, the hands were placed on the hips.
 
Isometric torque typically declines after exhaustive vibration exercise (Rittweger et al., 2000). Experiments carried out for preparation of this study have shown that a level of 70% of the maximum isometric knee extension can be performed for 30 s even after exhaustive exercise. Hence, the isometric knee extension in this study was performed at 70% of MVC. Contractions were performed on a specially manufactured chair, with the knee angle set at 1000(00 = full extension). The maximum isometric extension force was assessed in three trials with eyes closed and verbal encouragement by the experimenter. A 100% MVC was defined as the greatest torque value in theses three trials. Two electrodes for EMG recordings were placed with 3 cm distance over the vastus lateralis muscle at one-third of the femur length from its distal part. For the testing conditions, the subjects were asked to elicit a torque of 70% of their maximum voluntary effort. They controlled the torque exerted on a computer screen on which the range between 65 and 75% was displayed.
 
The patellar tendon reflex was tested in the same chair as the isometric torque. The ankle was attached to a leg supporter with a velcro tape. The hanging foot exerted a negative torque under resting conditions, which was helpful in asserting the relaxation of the leg muscles. Stretch reflexes were elicited manually every second with a hammer bearing a strain gauge on its front. The hammer impact was controlled on-line.
 
Both the isometric knee extension and the patellar tendon reflex were tested on the non-dominant leg.
 
Procedures
 
Before starting, the subjects warmed up (10 min bicycling at 50 W and stretching). Whole body vibration exercise was performed with a prototype of a commercially available product (Galileo 2000; NovoTec, Pforzheim, Germany). In brief, the device evokes platform oscillations around a central axis, that is located between both feet of the subject. Hence, the left and right leg are stretched and shortened alternatingly. The vibration amplitude was set to 6 mm (12 mm from top to bottom), and the frequency was set to 26 Hz.
 
During the exercise, squatting was performed from almost complete extension of the knee to an angle of 90. An additional load of 40% of the body mass was applied via a string that was attached to a hip belt. The length of the string was adjusted so that the weight touched the ground at flexions greater than 90, thus controlling the squatting range. For the temporal control of the squatting exercise, a metronome was set at 1 Hz, and the subjects were instructed to move 3 s down and 3 s up as evenly as they could. The precision of the movements was controlled by the experimenter confirming that the hanging weight almost touched the platform during each squat cycle.
 
Before and 2 min after all exercise bouts, the blood lactate was measured with the Accusport device (Roche Diagnostics, Mannheim, Germany), using capillary blood from the finger. During exercise, the rate of perceived exertion (RPE) was assessed every minute (Borg 1976). At the end of the exercise the subjects reported their level of fatigue. The load was then discharged by a quick release button and the subject was placed on the chair or on the jumping plate, respectively. Testing began exactly 10 s after termination of the exercise.
 
Data analysis
 
Signals from the contact plate, the hammer strain gauge, knee extension torque, and the electromyogram of the m. vastus lateralis were amplified and digitized with a 12-bit resolution and a sampling rate of 1000 Hz. The neuromuscular test variables were averaged over epochs of 10 s, yielding three consecutive epochs for each test condition (10, 20, and 30 s).
 
For each jump, the time-in-air (tAir) and the ground contact time (tGround) between jumps was assessed. The jump height was computed as hJump = 1.23tAir2(m/s2).
 
During 70% MVC knee extension, the average torque was assessed. The EMG recordings of the m. vastus lateralis were analysed for the median frequency (Kupa et al., 1995; Rittweger et al., 2000). Power spectra were computed every 200 ms with 100 ms overlap and applying a Hanning window as described before (Rittweger et al., 2000). From these spectra, the median frequency was assessed. 
 
The hammer beats were detected from the strain gauge recordings by a threshold algorithm. They were used to compute triggered averages of the torque signal. The reflex amplitude was assessed as the difference between the peak torque after the hammer beat and the torque at onset of the beat. The reflex latency was computed as the time delay between the beat onset and the torque value of 25% of the reflex amplitude. 
 
Statistical analysis 
 
Statistics were performed with the SPSS software in its PC version 10.0. The paired t-test was used to examine for differences in exercise time and RPE. A within subject repeated measures ANOVA with post-hoc simple contrasts was performed to examine for differences in blood lactate and reflex latency before and after VbX) or VbX+. Within subject repeated measures ANOVA was also performed to analyse effects of treatment (VbX) or VbX+) and epoch (10, 20 and 30 s) in torque, EMG frequency, EMG power, reflex amplitude, jump height and jumping ground contact time. 
 
If not indicated otherwise, data are given as mean ± standard deviation (SD). Significance was assumed if P<0.05. Significance levels were adjusted for multiple comparisons applying Bonferroni’s rule. 

Results 
 
The exercise time was significantly shorter in VbX+ than in VbX) sessions. There were no significant differences in RPE between the two treatments (VbX+ and VbX)). Blood lactate increased after the exercise, but again there was no treatment related effect (see Table 1). A significant correlation was found between VbX) and VbX+ in values of exercise time (r =0.67, P<0.001), but not in RPE or blood lactate. 

Serial jumping 
 
Under Pre conditions, jump height was decreased in the second and third 10 s epoch (P<0.001), but there was no treatment effect observed (P>0.2). Likewise, ground contact time was prolonged in the second and third epoch. Post exercise, the epoch effect on jumping height and on ground contact time was not present (P = 0.15, and P = 0.13, respectively). As before, there were no treatment effects on jump height (P = 0.60) or ground contact time (P = 0.30). 
General descriptive data for exercise.
 
Isometric knee extension 
 
Pre exercise, there were no effects of epoch or treatment on the average torque, indicating that knee extension was equally maintained at 70% of the maximum voluntary torque. The m. vastus lateralis EMG median frequency decreased from the first to the third epoch (VbX) 52.7 ± 7.3 Hz versus 47.3 ± 5.3 Hz, and VbX) 48.6 ± 11.6 Hz versus 41.9 ± 12.0 Hz), but it was comparable between treatments. 
 
Post exercise, the torque declined significantly from the first epoch (VbX): 121 ± 18 Nm, VbX+: 111 ± 29 Nm) to the second epoch (VbX): 116 ± 21 Nm, VbX+: 105 ± 28 Nm). No significant treatment effect was observed. There were 12 subjects who discontinued the isometric contraction in the third 10-s epoch after VBX) exercise, and 10 after VbX+ exercise, reporting severe fatigue. The EMG frequency was greater in VbX+ than in VbX) (P<0.001, for example 55.2 ± 5.8 Hz versus 42.4 ± 9.4 Hz in the first epoch). No significant epoch effect was found Post exercise (Fig. 1). 
Under Rec conditions, there was no effect of epoch or treatment on torque, indicating that knee extension was again equally maintained. Although the EMG median frequency still appeared to be higher after VbX+ than after VbX), the difference was no longer significant (P = 0.15). No differences were observed in EMG power before or after exercise. 
 
Patellar tendon reflex 
EMG mean frequency picked up over the vastus lateralis muscle after exercise with (VbX+) and without vibration (VbX)). The mean frequency was significantly higher after VbX+ than after VbX). This effect persisted after 10 min of recovery.
 
Reflex latency was comparable under Pre, Post and Rec conditions, and there was no treatment effect observed, indicating that evoking the reflexes was quite stable. No significant effect of treatment or epoch was found in reflex amplitude under Pre conditions. There was, however, a significant treatment effect on reflex amplitude in Post exercise (P = 0.005), with an increase after VbX+, but a decrease after VbX- exercise (see Fig. 2). A weekly significant interaction effect was observed between treatment and epoch (P = 0.043), indicating that the greater reflex amplitude after VbX+ was most pronounced in the first and second 10-s epochs. During Rec, these effects were no longer observed. 
 
Discussion 
 
The observed total exercise time, the changes in blood lactate and the RPE values suggest that a comparable degree of exhaustion and muscular fatigue was reached more rapidly with vibration than without. This becomes plausible when considering that whole-body vibration increases the oxygen consumption when applied in addition to the squatting exercise (Rittweger et al., 2001). A substantial correlation was observed between the individual exercise times with or without vibration, indicating a contribution of the individual resistance to fatigue for both types of exercise. 
 
Comparable levels of fatigue were also observed in the serial jumping test, as demonstrated by the jumping height and ground contact time that were found to be reduced after the exercises and which no longer depicted a decline of power output over the subsequent epochs. Moreover, the neuromuscular capacity to maintain force over time was comparably disrupted after both types of exercise, as shown by the decline in torque in the second and third epoch after exercise and the inability of about 50% of the subjects to maintain 70% of the maximum voluntary torque over 30 s. 
 
Both types of exercise produced comparable levels of exhaustion and neuromuscular fatigue and there were also differences observed in the neuromuscular function. First, and consistent with our former reports (Rittweger et al., 2000), the EMG median frequency increased over the vastus lateralis muscle during isometric contraction and was greater after exercise with than without vibration. Although no effect of vibration was observed on EMG power, the difference in EMG frequency suggests a central nervous recruitment of predominantly large motor units 

Amplitude of the pateller tendon reflex after exercise with (VbX+) and without vibration (VbX)).
 
Secondly, we found differences in the stretch reflex amplitude, which after exercise with vibration was comparable with baseline conditions or even increased, while it was clearly decreased after exercise without vibration. This is remarkable because the declines in muscular force and power output were comparable in both exercise types. Interestingly, the effect faded away after 10–20 s. 
 
Considering that an attenuation of stretch reflex amplitude is a common finding after demanding exercise, the maintained (or even increased) stretch reflex amplitude observed after vibration exercise is most likely due to an enhanced central motor excitability, particularly with respect to the phasic (fast twitch) fibres and motor units. This view might find support in the reports by Torvinen et al., (2002) who found an increase in jumping height and in isometric torque after non-exhaustive vibration exercise, possibly supporting the view of an increased neuromotor excitability. 
 
The tonic vibration response, which is thought to be elicited via the spindle loop, causes a mitigation of reflex levels, probably due to presynaptic inhibition. This seems to be in contrast with the above interpretation. It should be remembered, however, that the tonic vibration response can be only elicited if the subject relaxes the limb to which the vibrator is attached. Voluntary movements disrupt the response. Vibration exercise, on the other hand, is in combination with slow voluntary movements (squatting), thus presumably counteracting the response in passive vibration. 
 
Studies of oxygen uptake during non-exhaustive vibration exercise have shown that the energy turnover elicited by the vibration can be parametrically controlled by vibration amplitude, vibration frequency and by additional loads applied (Rittweger et al., 2002a). Together with the known influence of passive vibration on muscle spindle activity (Ribot-Ciscar et al., 1998), and with the neuromuscular findings presented here, we assume a substantial evidence for vibration exercise interacting with spinal reflex loops and possibly influencing these pathways. 
 
Thus, the view may be emerging that vibration exercise is a means to alter central motor control patterns. It should be considered that the present observations have been made in healthy young adults of moderate levels of physical fitness. Moreover, they have been obtained after exercise until exhaustion with an additional load at one vibration frequency and amplitude. As indicated above, the vibration frequency plays an important role. Based on the observation that oxygen consumption increases fairly linearly between 18 and 34 Hz, the same might be expected for the neuromuscular effects. Yet the effects of exercise duration, vibration frequency, amplitude and load that are optimum to evoke the observed neuromuscular excitability remain to be clarified in young adults, but also in athletes or elderly subjects and patients, after acute and after chronic application. Once explored, the mechanisms involved may be exploited for the application of vibration as a training method and for the design of training schedules. 
 
 
The acute effects of two different whole body vibration frequencies
Power Plate Studies
Aim.Vibration exercise is a novel exercise intervention, which is applied in athletes and general populations with the aim of improving strength and power performance. The present study was aimed to analyse the adaptive responses to different whole body vibration frequencies.
 
Methods.Fifteen untrained subjects were randomly assigned to a 5 min whole body vibration (WBV) training session on a vibrating plate producing sinusoidal oscillations at 20 Hz (low frequency) and 40 Hz (high frequency) with constant amplitude. Squat jump, countermovement jump and sit and reach test were administered before and after the WBV treatment.
 
Results. Low frequency WBV stimulation was shown to significantly increase hamstrings’ flexibility by 10.1% (p<0.001) and squat jump by 4% (p<0.05). High frequency (40 Hz) of WBV stimulation determined a significant decrease in squat jump (-3.8%; p<0.05) and in counter movement jump (-3.6; p<0.001).

 
Conclusion. The results showed the influence of WBV frequency on acute adaptive responses. In particular, the untrained subjects in the presented study, showed acute enhancement in neuromuscular performance with low-frequency WBV stimulation.
 
Key words: Vibration exercise - Neuromuscular performance - Vertical jump - Vibration frequency.

Introduction:
 
Mechanical stimulation in the form of vibration has been recently shown to produce specific adaptive responses in humans.1-9 It has been hypothesized that a low-frequency, low-amplitude vibratory stimulation it is a safe and effective exercise intervention. A single session of 5 min of whole body vibration (WBV) has been shown to induce a shift of the force-velocity and power- velocity curve to the right in well trained female volleyball players.3 A single session of 10 min divided in 2 sets of 5 bouts of 60 s WBV with 60 s rest in between sets has been shown to improve vertical jumping performance and determine an increase in testosterone and growth hormone production in well-trained individuals.10 Recent evidence from Torvinen et al.5 suggests that short-time exposure to WBV cab lead to an improvement in vertical jump performance and force generating capacity in lower limbs. On the other side, prolonged administration has been shown to induce fatigue and inhibit neuromuscular performance.1, 8, 11 Although vibration is being employed from athletes in their training regimes, it is still unclear how it is possible to effectively use this novel exercise intervention. Few studies looking at the acute effects of WBV have shown contradictory results. In particular, the effect of different vibration frequencies on neuromuscular performance is still unclear. Therefore, the purpose of this study was to analyse the acute effects of 2 different WBV frequencies on vertical jump and flexibility.
Data are expressed as mean±SD.
 
Materials and methods
 
Subjects
 
Fifteen subjects (2 women and 13 men) voluntarily participated to the study. They were all involved in recreational sport activities. They were randomly divided into 2 groups: a high frequency group (HFG) and a low frequency group (LFG). Subjects with previous history of fractures or bone injuries were excluded from the study. The HFG was constituted from 7 subjects (age: 20.4±0.5 years, height: 1.79±0.05 m; weight: 78±9.4 kg). Eight subjects were assigned to the LFG (age: 21±2.2 years, height: 1.76±0.1 m; weight: 75.2±18.2 kg). The protocol was approved by the local ethics committee.
 
Procedures
 
The subjects were familiarized with the protocol and the WBV treatment the day before the experimental trial. At the beginning of the experimental session anthropometric measures (height and weight) were recorded together with the age of the subjects. Following this phase a 10 min standard warm up consisting of running, jumping and stretching exercises was performed. After the warm up, the subjects performed the followings tests: sit and reach test (S&R), squat jump (SJ), and counter movement jump (CMJ). Between groups comparison did not reveal statistically significant differences at baseline for all the measured variables.
 
Sit and reach test was performed on a sit and reach box in which subjects were seated with their heels firmly planted against the heel board, and feet approximately 1 foot apart. Testing procedures have been described elsewhere. 12 Three trials were performed and the best one was used for statistical analysis.
 
Vertical jumping tests were conducted on a resistive platform 13 connected to a digital timer (accuracy±0.001s) (Ergojump, Psion XP, MA.GI.CA. Rome, Italy) which was recording the flight time (tf) and contact time (tc) of each single jump. In order to avoid unmeasurable work, horizontal and lateral displacements were minimised, and the hands were kept on the hips through the tests. The rise of the center of gravity above the ground (h in meters) in was measured from flight time (tf in seconds) applying ballistic laws:

h=tf 2·g·8-1 (m) [1]
where g is the acceleration of gravity (9.81 m·s-2).

 
Two different jumping tests were performed: squat jump (SJ), in which subjects were jumping from a semi-squatting position without counter movement and counter movement jump (CMJ) in which subjects were allowed to perform a counter movement with lower limbs before jumping. Three trials for each test were performed, the best result was considered for statistical analysis. 

Treatment procedures 
 
Subjects were exposed to vertical sinusoidal WBV using the device called NEMES LC (Ergotest, Greece). The frequencies used in this study were 20 Hz for the LFG and 40 Hz for the HFG (Peak-to-peak displacement= 4 mm; theoretical acceleration=6.4 g (20 Hz) and 25.7 (40 Hz) g, where g is equal to 9.81 m·s-2). The subjects were exposed to 5 bouts lasting 60 s each of WBV while standing on the vibrating plate in semi-squatting position. Sixty s rest in between each bout was allowed. Sixty s following the last bout of WBV testing took place again. 
Percentage changes in squat jump graphThe percentage changes in counter movement jump after the vibration intervention. *p<0.005 for between treatments comparisons.

 
Statistical methods 
 
Conventional statistical methods used included mean, standard deviation and paired and unpaired Student’s t-test. The level of significance was set at p<0.05. 
 
Results 
 
Whole body vibration with low frequency (20 Hz) determined a statistically significant increase in hamstrings flexibility (+13.5%; p<0.001) and squat jump (+3.9%; p<0.05). Counter movement jump also improved but did not reach statistical significance (+2.3%; p=0.07). Whole body vibration stimulation with high frequency determined a non statistically significant reduction in squat jump (-4%; p<0.07) and counter movement jump (3.8%; p<0.001). A non statistically significant decrease in flexibility was also observed (-3.3 %; p=0.268) (Table I). Between treatments analysis revealed a statistically significant difference in all variables analyzed (Figures 1, 2, 3). 
 
Discussion and conclusions 
 
The results of this study have shown that different acute effects can be observed with different vibration frequencies in sedentary subjects. The reduction in vertical jumping ability observed following 10 min of vibration exercise with a frequency of 40 Hz is in line with previous investigations which have identified an acute impairment in neuromuscular performance following WBV exercise.6, 8, 11, 14 They reported a significant marked reduction in vertical jump 6, 14 and maximal knee extensors force 11 following WBV exercise with similar protocols. 
 
Five minutes of WBV with a low frequency (20 Hz) were shown to acutely enhance neuromuscular performance as measured by vertical jumping ability. This observation is also consistent with previous findings 1-3, 5 which found acute improvements in vertical jumping ability and force-generating capacity in humans following 4 to 5 min WBV at frequencies ranging from 15 to 30 Hz. 
 
Any acute effect of WBV training was expected to be of neural origin. In particular, the role of agonist/antagonist muscle activity in the modulation of joint stiffness has been hypothesized to be the responsible for acute adaptive responses.15 In this study different vibration frequencies were shown to have different effects on knee joint stiffness since the change in vertical jumping ability was parallel to the change in hamstrings’ flexibility. During vibration the body and the skeletal muscle undergo to small changes in muscle length. The peculiar characteristics of the vibratory stimulus determine an activation of Ia afferent fibers.16 Mechanical vibrations applied to the muscle itself or the tendon elicit a reflex muscle contraction named tonic vibration reflex (TVR).17 This reflex contraction is caused by an excitation of muscle spindles leading to an enhancement of the activity of the Ia loop.18, 19 Facilitation of the excitability of spinal reflexes has been shown to be elicited through vibration to quadriceps muscle.20 Lebedev and Peliakov21 also suggested that vibration may elicit excitatory flow through short spindle – motoneurons connections in the overall motoneuron inflow. The neural circuitry involved in the tonic vibration reflex has been quoted to be similar to the one observed for the tendon tap reflex. It then involves the activation of the homonimous motor units and the decrease in excitability of the motor neurons innervating the antagonist muscle through the reciprocal- inhibition circuit. Since no EMG measurement was performed in this study, it was not possible to measure the actual effect of vibration on agonist and antagonist muscles acting on the knee joint. However, the changes in vertical jumping ability and the correspondent changes in hamstrings flexibility seem to suggest that vibration exercise is capable of acutely affect joint stiffness. 
 
Vibrations are perceived not only by spindles, but also by the skin, the joints and secondary endings. All those structures contribute to the facilitatory input to the γ-system 22, 23 which in turn affects sensitivity of the primary endings. Hollins and Roy 24 found that sinusoidal stimuli ranging from 10 to 100 Hz with a small amplitude applied to the left index fingerpad were perceived by Meissner and Pacinian afferents and were able to trigger spindle activation. The modulation of neuromuscular response to vibration is then not only to be referred to spindle activation, but to all the sensory systems in the body. Various parameters can affect the synergies in the sensory system and determine specific responses. Vibration is thought mainly to inhibit the contraction on antagonist muscles via Ia inhibitory neurons.25 However there is also some evidence that vibrations can produce also coactivation. Rothmuller and Cafarelli 16 applying vibrations to the patellar tendon and measuring biceps femuris coactivation have observed this phenomenon. Jones and Hunter 26 also found an increased coactivation when applying vibrations. This phenomenon has been attributed to central mechanisms increasing presynaptic inhibition of Ia afferents transferring the inhibition to antagonist motoneurons.16 This has been observed when vibrations were applied in fatiguing conditions or when vibration was causing fatigue. 
 
Individual fitness status should also be considered when developing vibration exercise protocols. As supportive evidence, welltrained individuals showed an acute improvement in force-generating capacity 2, 3 and untrained subjects showed an acute decrease following similar vibration exercise protocol. 11 In our study, untrained individuals showed acute improvement in vertical jumping ability and hamstrings flexibility following low frequency vibration exercise and acute decrease in vertical jumping and hamstrings’ flexibility following high-frequency vibration exercise. The muscle-tendon complex acts as a low pass filter and is able to attenuate vibration transmission to the spindles. Those capabilities of the musculotendinous units have been clearly identified in the lower limbs while running. In fact, impact forces during running have been found to produce vibrations, which are transmitted to the body at a frequency component between 10 and 20 Hz.27 The soft tissues of the lower limb damp those vibrations coming from heel contact changing their stiffness. The adjustment of the stiffness of the lower limbs based upon the shock wave received is also based on the sensory receptors in the muscle itself, in the tendons (Golgi tendon organs), but also in the joints, ligaments and in the skin. In our opinion the “muscle tuning” hypothesis underlined by Nigg and Wakeling 27 and Wakeling and Nigg 28 is to be considered also as the possible adaptive response to the application of vibrations. Different individuals can adapt to different vibration frequencies since they possess different bandwith properties of their spindles, different amounts and location of mechanoceptors and proprioceptors, different viscoelastic properties of the muscle tendon complex and different percentages of type II fibers. Previous authors have reported that a frequency below ~20 Hz induces muscle relaxation, whereas at frequencies above ~50 Hz severe soreness may emerge in untrained subjects.9 The enhancement of performance observed with low frequency stimulation could be due to several aspects. First, it is possible that the low frequency stimulation used in our study was not strong enough to cause muscle fatigue and was triggering a limited TVR. Also, relaxation of hamstrings muscles, as shown by an increase in flexibility, would have facilitated vertical jumping ability, which is characterised by high-speed knee joint rotation. Second, it is likely that the high frequency treatment elicited a strong TVR, increasing the neuromuscular activation of the lower limbs in order to damp the vibratory waves transmitted to the body. Stressful vibratory stimulation could in fact increase co-contraction of the hamstrings. Our results seem to support this view. As previously suggested,15 when the vibration stimulus does not produce fatigue and is of relatively short duration can determine an increased excitatory state of the CNS and facilitate force-generating capacity in humans. On the other hand, when the opposite occur (vibratory stimulus is too stressful causing fatigue), force generating capacity is impaired. Considering the background of our subjects (untrained individuals) it should not be far fetched to suggest that a good progression program with vibration exercise should start with the use of lower frequencies of stimulation. Moreover further studies are needed in order to elucidate the exact neurophysiological mechanisms involved in the adaptive responses to vibration exposure in different populations. Vibration exercise it’s a novel form of exercise and only few studies have been so far conducted on combining different frequencies/amplitudes of stimulation. The results of our study suggest that untrained individuals are able to increase force-generating capacity acutely with low-frequency vibration stimulation. The main conclusion of the present study suggests that future work is needed in order to develop safe and effective protocols for vibration exercise in different subjects and in different muscle groups. 
 
 
Electromyography Activity of Vastus Lateralis Muscle
Power Plate Studies
ABSTRACT
 
The aim of this study was to analyze electromyography (EMG) responses of vastus lateralis muscle to different whole-body vibration frequencies. For this purpose, 16 professional women volleyball players (age, 23.9 ± 3.6 years; height, 182.5 ± 11.1 cm; weight, 78.4 ± 5.6 kg) voluntarily participated in the study. Vibration treatment was administered while standing on a vibrating platform with knees bent at 1008 (Nemes Bosco-system, Rome, Italy). EMG root mean square (rms) and was recorded for 60 seconds while standing on the vibrating plate in the following conditions: no vibrations and 30-, 40-, and 50-Hz vibration frequencies in random order. The position was kept for 60 seconds in each treatment condition. EMGrms was collected from the vastus lateralis muscle of the dominant leg. Statistical analysis showed that, in all vibration conditions, average EMGrms activity of vastus lateralis was higher than in the no-vibration condition. The highest EMGrms was found at 30 Hz, suggesting this frequency as the one eliciting the highest reflex response in vastus lateralis muscle during whole-body vibrations in half-squat position. An extension of these studies to a larger population appears worthwhile to further elucidate the responsiveness of the neuromuscular system to whole-body vibrations administered through vibrating platforms and to be able to develop individual treatment protocols.
 
Key Words: muscle tuning, neuromuscular responses
 
Introduction
 
Mechanical vibrations applied to the muscle belly or tendons have been shown able to elicit reflex muscle contractions (12). This neuromuscular response has been named ‘‘tonic vibration reflex’’ (TVR) and has been shown to be mediated by mono- and polysynaptic pathways (8, 19). Muscle spindle Ia reflexes have been indicated as the major determinant of this vibration-induced neuromuscular activation (5) leading to the TVR. Recent observations have shown the possibility of utilizing vibrations as a training tool in athletic settings. In fact, neuromuscular performance has been enhanced through the administration of vibration treatment (2). These improvements have been attributed to an enhancement of neural factors determining neuromuscular performance: recruitment, synchronization, inter- and intramuscular coordination and also proprioceptors’ responses. In this connection, it should be noticed that vibrations have been shown to be effective in inducing improvements in vertical jumping ability (3) and in mechanical power of lower limbs in elite athletes (4). Moreover, studies conducted by Issurin et al. (13, 14) have shown increases in explosive strength and flexibility in athletes. Vibrations applied to the arm showed enhancement of mechanical power and an increase in neuromuscular efficiency as indicated by a decrease in EMG/power ratio supporting the evidence that vibrations represent a strong stimulus for the neuromuscular system (2). Furthermore, EMG activity during vibrations has been shown to reach values higher than 200% of the baseline in arm flexor muscles (2). Previous studies have found vibrations determining an increased EMG activity in the muscles undergoing the vibration treatment (15, 7).
 
Vibrations are starting to be used as an alternative training means for enhancing strength/power characteristics. However, it should be pointed out that there is a lack of information on the effectiveness of different vibration frequencies on neuromuscular performance. Moreover, it should also be considered that currently there is no methodology able to identify the individual vibration load that an individual can sustain. Muscle activation during vibration can be monitored recording the EMG signal of the target muscles. With this tool, it is in fact possible to determine muscle activity in a given task. In light of the above, it is possible to affirm that EMG can be used to provide an indication of the muscle activity determined by vibration. In fact, the EMG signal can be used to measure the severity of muscle activation following the application of vibration. The aim of this study was to analyze EMG responses in the vastus lateralis muscle while standing on a vibrating plate producing oscillations of different frequencies to verify the hypothesis that different vibration frequencies determine different neuromuscular responses.
 
Methods
 
Subjects
 
Sixteen professional women volleyball players volunteered as subjects for the present study (age, 23.5 ± 4.6 years; height, 183.4 ± 8.4 cm; weight, 75.1 ± 7.4 kg). All of the subjects had competed for several years at a high level and were regularly training at the time of the experiment. Full advice was given to the volunteers regarding the possible risk and discomfort that might be involved, and all the subjects gave their written informed consent, approved by the ethical committee of the Italian Society of Sport Science, to participate in the experiment. Subjects with a previous history of fractures or bone injuries were excluded from the study.
 
EMG Analysis
 
The signals from the vastus lateralis of the dominant leg were recorded with bipolar surface electrodes (interelectrode distance, 1.2 cm) including an amplifier (gain, 600; input impedance, 2 GV; CMMR, 100 dB; band-pass filter, 6–1500 Hz; Biochip, Grenoble, France) fixed longitudinally over the muscle belly. The MuscleLab converted the amplified EMG raw signal to an average root-mean-square (rms) signal via its built-in hardware circuit network (frequency response, 450 kHz; averaging constant, 100 milliseconds; total error, ±0.5%). The EMGrms was expressed as a function of the time (millivolts or microvolts). EMG cables were secured with an appropriate setup to prevent the cables from swinging and from causing movement artifact. A personal computer (PC Celeron 400) and the MuscleLab software were used to collect and store the data. The EMGrms was collected during each repetition, lasting 1 minute each, of isometric half squat. In a previous study, the reliability of the EMG measurements was shown to be 0.91 (2).
 
Treatment Procedures
 
Subjects were exposed to a vibration treatment (VT) using a vibration platform called Nemes Bosco-system (OMP, Rieti, Italy). The amplitude allowed by the vibration platform was (peak-to-peak) 10 mm. The subjects were exposed randomly to three different VTs. The frequencies used in the experiment were 30, 40, and 50 Hz. Each vibration treatment lasted 60 seconds, with 60 seconds of rest allowed between each VT frequency. The subjects were asked to stand in half-squat position on the vibration platform (knee angle 1008) as indicated in previous studies (3). The EMG signal was collected from the vastus lateralis muscle during the 60-second duration of the testing position. A total of 4 sets lasting 60 seconds each were performed with 60 seconds of rest between sets allowed. EMGs were recorded for 5 seconds before starting the vibration treatment to verify the absence of residual muscle activity. If the EMG was different from the baseline measurement, a further 30 seconds of rest were allowed. The subjects then performed the isometric half squat in the following conditions: no vibrations, and randomly 30-, 40-, and 50-Hz vibration frequency.

 
 
Electromyography root mean square (EMGrms) average values of vastus lateralis recorded during different vibration frequencies.


 
Statistical Analyses
 
Ordinary statistical methods were employed, including means (X) and standard deviation (±SD). Average EMGrms values for vastus lateralis were considered for analysis. To analyze differences in EMG between vibration frequencies, a repeated measures ANOVA was computed to identify significant differences for the dependent variables. Significant F values were followed by paired t-tests for within- and between-group comparisons. Significance was set at p <= 0.05.
 
Results
 
Whole-body vibration treatment lead to an increase of EMGrms activity of vastus lateralis muscle as compared with baseline values (p < 0.001) collected in the no-vibration condition (see Figure 1). The highest EMGrms activity was found at 30-Hz vibration frequency (134%, p < 0.001). Between-treatment comparisons showed statistically significant differences between 30- and 50-Hz frequencies (20%, p < 0.05) and 40- and 50-Hz frequencies (10%, p < 0.05). EMGrms activity between 30- and 40-Hz vibration frequencies did not show any statistically significant difference (9%, n.s.).
 
Discussion
 
The results of the present study demonstrated an increased EMG activity in vastus lateralis muscle at various vibration frequencies when subjects were standing on a vibration platform. An increase in EMG has been observed in quadriceps muscle undergoing vibrations and it has been attributed to a facilitation of the excitability of spinal reflex (7). Based on these findings, it was possible to verify that whole-body vibrations transmitted through a vibrating platform in halfsquat position were able to determine a higher EMG activity compared with the nonvibrating condition. This effect has been related to excitation of primary endings of muscle spindles and activation of a-motoneurons as indicated elsewhere (i.e., 12, 16).
 
In our opinion, two factors could account for the observed increase in EMG activity: i) the initial length of analyzed muscles and ii) the frequency of vibratory stimulation. In fact, it is already known that vibratory stimulation is more effective in stretched muscles (21, 9). Vibration sensitivity of human muscle spindles has also been demonstrated in single human spindle afferent by Burke and Gandevia (5). These observations support the utilization of the half-squat position on the vibrating platform as an effective position for triggering vastus lateralis stimulation. Escamilla et al. (10) reported that the two vasti muscles produce 40–50% more activity than the rectus femuris during half squat. Moreover, compared with each other, the vastus medialis and vastus lateralis produce approximately the same amount of activity (10, 11, 24). The position chosen in the experiment could then be considered optimal for stimulating the vastus lateralis muscle because of the lengthened position and the activation relative to the quadricep muscles, as suggested elsewhere (1).
 
Vibrations-induced increases in EMG activity and the consequent degree of motor unit synchronization have been shown to be dependent on the vibration frequency (6, 17, 19, 23). This was not observed in our experiment because the highest EMGrms activity was found when the frequency was 30 Hz. However, previous observations at constant displacement amplitude have shown that monosynaptic inhibition does not vary with vibration frequency (18). The missing EMG augmentation with vibration frequency may be due to inhibitory mechanisms mediated by mechanoceptors and skin receptors, which have been shown to be activated during whole-body vibrations and contribute to the EMG activity (22). The results of our study suggest that, in the specific position used, the frequency able to cause the highest EMG response in vastus lateralis muscle was 30 Hz. In our opinion, vibrations are strong perturbations that are perceived by the central nervous system, which modulates the stiffness of the stimulated muscle groups. The reflex muscle activity could then be considered a neuromuscular tuning response to minimize soft-tissue vibrations. These responses are individual and probably could be population- specific and could be based on mechanical and reflex factors. Natural frequencies of muscle groups in athletes’ legs have been reported to be between 5 and 65 Hz (20); the input frequencies used in our study are in this range and it suggests that individual responses could be related to individual capabilities in damping external perturbations to avoid resonance effects. These adaptations have been hypothesized during running in humans (20), and our opinion is that the same principles apply to vibrations superimposed with vibrating plates.
 
In conclusion, this study indicates, first, that standing on a vibrating platform in half-squat position elicits higher EMG responses in vastus lateralis muscle as compared with the same position without vibrations being transmitted. Second, EMG recordings could represent a means for individualizing training protocols for whole-body vibrations. Further studies are needed for elucidating the mechanisms determining individual responses and the effectiveness of individualized treatments on neuromuscular performance.
 
Practical Applications
 
Vibrations exercise has been shown to be effective in enhancing strength/power capabilities (i.e., 2–4, 13, 14). However, it should be pointed out that there is no current knowledge about effective exercise protocols or measurements to which to refer when prescribing a vibration exercise program. One way for individualizing vibration treatments could be the use of surface EMG to assess muscle responsiveness to different vibration frequencies. In fact, the results of this study support the idea that vibrations elicit higher EMG activity compared with isometric conditions. Also, different vibration frequencies elicit different EMG responses in the stimulated muscles.
 
This new technique could be used separately or in combination with conventional strength-training routines. In particular, due to the effects of vibration on stretch reflexes, it can be suggested for use in a complex training-type routine in place of plyometric drills. Vibration in fact stimulates reflex muscle responses due to the small and quick changes in length of the muscle-tendon complex and the small and fast joint rotations associated with the oscillatory motion. Those stimuli are similar to the ones experienced during plyometric exercise and place less stress on joints due to the reduced impact load. Future studies are needed in order to develop safe exercise procedures on vibrating platforms and in order to understand the effectiveness of different vibration exercise protocols.
 
 
Effect of four-month vertical whole body vibration on performance and balance
Power Plate Studies
Introduction
 
Mechanical vibration has recently aroused large interest because it has been hypothesized that a low-amplitude, high-frequency stimulation of the whole body could positively influence many risk factors of falling and related fractures by simultaneously improving muscle strength, body balance, and mechanical competence of bones (3–5,9,10,21,23–28,35). There is, however, very little scientific evidence about the effects of whole body vibration on these parameters. Bosco et al. showed that a single vibration bout resulted in a significant temporary increase in muscle strength of the lower extremities (5) and arm flexors (4). They also studied the effects of 10-d vibration on muscular performance of physically active subjects and showed that whole body vibration applied for 10 min·d-1 induced an enhancement in explosive power (3). Runge et al., in turn, showed that whole body vibration could enhance muscle performance in elderly people (2- month training program three times a week at the frequency of 27 Hz) (29). There is also some preliminary evidence that vibration-loading could stimulate trabecular bone formation and prevent postmenopausal and ovariectomy-induced bone loss (10,24,26).
 
Despite the above noted preliminarily positive findings and wide use of different vibration devices among athletes, conclusive evidence on the efficacy and safety of vibration training is lacking. This lack of data is especially clear concerning the long-term effects. The purpose of this study was, therefore, to investigate the effects of a 4-month whole body vibration intervention on muscle performance and body balance of young, healthy volunteers, using a randomized controlled study design.
 
MATERIALS AND METHODS
 
Subjects and Study Design
 
Fifty-six young, healthy, nonathletic volunteers (21 men and 35 women aged 19–38 yr) participated in the study. Half of the subjects were randomized to the vibration group and half to the control group. The men and women were randomized separately into the groups so that number of men and women would be approximately equal in both groups. The vibration protocol consisted of a 4-month whole body vibration training (see below). The performance tests were done at baseline (before randomization) and at 2 and 4 months.
 
The exclusion criteria from the study were: any cardiovascular, respiratory, abdominal, urinary, gynecological, neurological, musculoskeletal, or other chronic disease; pregnancy; prosthesis; medication that could affect the musculoskeletal system; menstrual irregularities; and regular participation in any exercise-inducing impact-type loading on the skeleton more than three times a week.
 
The subjects completed a questionnaire detailing their physical activity and calcium intake (from a 7-d calcium intake diary) (36) at the beginning of the study and at 2 and 4 months. All participants gave their informed written consent before enrollment, and the study protocol was approved by both the Institutional Review Board and the Ethics Committee of the UKK Institute.
 
Vibration Loading
 
Vibration loading was carried out in a standing position on a whole body vibration platform (Kuntota¨ry, Erka Oy, Kerava, Finland) and the subjects were asked to train with it 3 to 5 times a week. The duration of daily stimulus was 4 min. While standing on the platform, the subjects repeated four times a 60-s light exercise program according to instructions prepared earlier. The rationale of the exercise program was to provide a multidirectional vibration exposure on the body and make the standing on the platform less monotonous in a way that would be feasible for a long-term intervention trial. The program comprised of light squatting (0–10 s), standing in the erect position (10–20 s), standing in a relaxed position with slightly flexed knees (20–30 s), light jumping (30–40 s), alternating the body weight from one leg to another (40–50 s), and standing on the heels (50–60 s).
 
During the 4-min vibration exposure, the vibration frequency increased in one min intervals. During the first two weeks, the duration of the loading was 2 min, and the frequency of vibration was 25 Hz for the first minute and 30 Hz for the second minute (the practice period). During next 1.5 months, the duration of the vibration loading was 3 min and frequency 25 Hz/60 s +30 Hz/60 s +35 Hz/60 s. During the remaining 2 months, the length of exposure was 4 min, and the frequency was 25 Hz/60 s +30 Hz/60 s + 35 Hz/60 s +40/60 s. The peak-to-peak amplitude of the vertical vibration was 2 mm. Considering the amplitude and the sinusoidal nature of the loading, the theoretical maximal acceleration was some 2.5 g (where g is the Earth’s gravitational field, or 9.81 ms2) with 25 Hz loading, 3.6 g with 30 Hz loading, 4.9 g with 35 Hz loading, and 6.4 g with 40 Hz loading.
 
Performance Tests
 
At the beginning of each test session, a 4-min warm-up was performed on a bicycle ergometer (workload 40 W for women and 50 W for men). The subjects wore the same shoes during all three performance test sessions, and the order of the performance tests was the same in every test session (see below). Use of alcohol or strenuous physical activity was not allowed during the test-day nor the preceding day. Before each test, the subjects had one to two unintensive familiarization trials.
 
A vertical countermovement jump test was used to assess the lower-limb explosive performance capacity (2). The subject kept hands on the pelvis. The tests were performed on a contact platform (Newtest, Oulu, Finland), which measures the flying time. The obtained flight time (t) was used to estimate the height of the rise of body center of gravity (h) during the vertical jump (i.e., h =gt2/8, where g =9.81 m·s-1) (2). The median value of three measurements was used as the test score.
 
A postural sway platform (Biodex Stability System, New York, NY) was used to assess static body balance (32). The subjects stood on a labile platform on both legs, with eyes opened and arms beside the trunk. The platform provides eight different stability-levels (level 8 is virtually stable and level 1 is the most labile). As a test, we employed a 40-s protocol in successive 10-s intervals [level 5 (0–10 s), level 4 (10–20 s), level 3 (20–30 s), and level 2 (30–40 s). This system provides a numerical stability index that reflects the body sway variation around the body’s center of gravity so that the lower the index, the higher the level of stability (32). Each subject’s feet position coordinates on the platform were recorded after the first stability measurement, and the same coordinates were used throughout the study to obtain consistency between the tests. The mean value of two stability indices was used as the test score.
 
Grip strength was measured using a standard grip strength meter (Digitest, Muurame, Finland). The median value of three readings was used as the test score.
 
Maximal isometric strength of the leg extensors was measured with a standard leg press dynamometer (12). The subjects sat on the dynamometer chair with their knees and ankles at an angle of 90° of flexion while pressing maximally against strain gauges (Tamtron, Tampere, Finland) under their feet. The isometric strength was recorded for three maximal efforts, and the median value of three readings was used as the test score.
 
A shuttle run test over a 30-m course was used to assess the dynamic balance or agility (1). The subjects were asked to run as fast as possible six times between markers placed four meters apart, to touch the floor after each 4-m run, and finally to run a 6-m course over the finish line. A single performance was done and the running time was recorded with photoelectric cells.

Safety
 
Possible side effects or adverse reactions were asked from the subjects of the vibration group monthly and from the control group in 2-month intervals. The subjects also had the liberty of consulting the responsible study physician whenever needed.
 
Basic characteristics of the vibration and control groups.
Statistical Analysis
 
Means and standard deviations are given as descriptive statistics. The 2-month and 4-month effects of the whole body vibration on physical performance and balance were defined as absolute and relative mean differences [with 95% confidence intervals (CI)] between the vibration and control groups, respectively. The relative differences were achieved through log-transformation of the variables. The time-effect at 2 and 4 months was determined by one-way ANCOVA, using the baseline values as the covariate. In all tests, P =0.05 was considered significant.
 
RESULTS
 
Twenty-six subjects in the vibration group and 26 controls completed the study without side effects or adverse reactions. Two participants in the control group withdrew from the study because of loss of interest, and two participants in the vibration group withdrew because of musculoskeletal problems that were independent from the vibrationloading (the first one for rib fracture; the second one for an orthopedic operation). The basic characteristics of the 52 subjects are given in Table 1.
 
The reported mean vibration-training frequency was 3.1 (±0.9) times per week. Because there were no gender differences in the time-effect at the 2-month and 4-month tests, the data of women and men were pooled and analyzed together.
 
Muscle Performance and Body Balance
 
Power and strength tests.
 
The vertical jump height increased an average of 2.0 cm after 2 months vibration as compared with a mean decrease of 0.6 cm in the control group, resulting in a significant 10.2% net benefit (95% CI, 5.6–15.1%, P =0.000) in the vibration group. At the 4-month test, jump height had increased 2.5 cm (from the baseline) in the vibration group and 0.3 cm (from the baseline) in the control group, resulting in a significant 8.5% net benefit (95% CI, 3.7–13.5%, P =0.001) in the vibration group (Table 2 and Figure 1A).
 
Isometric lower limb extension strength improved an average 11.2 kg after the 2-month vibration-intervention while in the control group a mean increase was 4.8 kg, resulting in a statistically significant 3.7% net benefit (95% CI, 0.3–7.2%, P =0.034) for the vibration group. At the 4-month test, this net benefit had diminished to 2.5% (P = 0.25) (Table 2 and Fig. 1B). In this context it must be noted that the lower limb extension strength of one control subject was clearly higher than that of the other control subjects, thus increasing the standard deviations in the control group (Table 2). This had, however, no effect on the absolute or relative mean between-groups differences. As expected, in neither group were changes observed in the grip strength at the 2- and 4-month tests (Table 2 and Fig. 1 C).
 
Performance and balance tests.
 
There were no differences at the 2- and 4-month shuttle run tests between the vibration and control groups (the mean between-groups net difference -0.5% at both time points, P -0.52 and P -0.57, respectively) (Table 2 and Fig. 1D). Neither effect was observed in the postural sway at the 2-month or 4-month tests (Table 2 and Fig. 1E).
 
DISCUSSION
 
This randomized controlled study showed that a 4-month whole body vibration-loading was safe to use and induced a significant 8.5% mean increase in the jump height of young healthy adults. This improvement was already seen after 2 months of the vibration. Lower limb extension strength was also enhanced by the 2-month vibration-period. This increase, however, slowed down by the end of the intervention, and at 4 months the difference between the groups was no more statistically significant, mostly due to increased extension strength in the control group (learning effect). Concerning the dynamic and static body balance, the 4-month whole body vibration-intervention showed no effect.
 
Effects of resistance training on neuromuscular properties of skeletal muscle are well known (6,13–18), and their knowledge may help to interpret and understand the above noted vibration findings. First, structural changes within a skeletal muscle are of great importance when adapting to strength training. However, voluntary strength performance is determined not only by intramuscular factors but also by the extent of neural activation, since training-induced changes in the nervous system (neural adaptation) allow more complete activation of the prime movers of a specific movement and better coordination of the activation of the relevant muscles, both of which result in a greater force in the intended direction of movement (6,30).

The performance and balance test parameters at baseline and after 2-month and 4-month whole body vibration intervention
 
The first adaptation mechanism of a skeletal muscle to resistance training is neural (6,18,30). Changes in the neural factors in response to training occur within a few months, whereas changes in the morphological structure of the muscle take longer (from several months to years). Specific adaptations to training depend much on the training program employed (6,30,31). In addition to pure maximal strength, explosive power is an important factor in several sport activities, and various stretch-shortening cycle (SSC) exercises (e.g. jumping or plyometric exercises) have been used to improve this performance trait. The exact mechanism by which the explosive power training can enhance neuromuscular activation is not known, but there are several possible explanations which could cause this enhancement, e.g., adaptation of certain reflex responses, increase in motor unit synchronization, co-contraction of the synergist muscles, or increased inhibition of the antagonist muscles. Strength and power training may also increase the ability of motor units to fire briefly at very high rates, which may induce an increase in the rate of force development even if the peak force does not necessarily increase (6,18,30).

The percentage changes in the power, strength, performance, and balance tests after the 2-month and 4-month vibrations. Mean and 95% confidence interval. * Indicates P < 0.05.
 
Whole body vibration-induced improvements in muscle performance (3) have been suggested to be similar (and occur via similar pathways) to those after several weeks of resistance training (4,7,15). During a whole body vibration loading, skeletal muscles undergo small changes in muscle length, most likely since mechanical vibration is able to induce a tonic excitatory influence on the muscles exposed to it (33). In other words, vibration elicits a response called “tonic vibration reflex,” including activation of muscle spindles, mediation of the neural signals by 1a afferents (11), and finally, activation of muscle fibers via large apha-motoneurons. The tonic vibration reflex is also able to cause an increase in recruitment of the motor units through activation of muscle spindles and polysynaptic pathways (8).
 
In this study, neurogenic enhancement or changes in the morphological structure of the muscles could not be assessed directly because the study protocol included neither EMG recordings nor muscle biopsies. However, on the basis of the evidence mentioned above, it is likely that the given whole body vibration training elicited neural adaptation. This was also supported by the results of the study; i.e., the quickly and clearly enhanced jump height suggested that neural adaptation did occur in response to the vibrationintervention. In addition, the lower-limb extension strength increased only after 2 months of vibration, thus also referring to neural potentiation. The rate of increase in the lower limb extension strength and difference between the intervention groups, however, diminished by the end of the 4-month intervention. This could be explained by general muscular adaptation to the vibration program. Further improvement in the extension strength might have required a greater change in the training stimulus.
 
When interpreting the results of the current study (the rise in vertical jump height), one has to remember that the training group subjects also did a light exercise program during the 4-min vibration exposure (see Materials and Methods), and thus, one could suspect that the improvement in the jump height was because of this exercise. However, it was very unlikely that these exercises were behind the clear rise in the jump height in that the exercises were very light.
 
 
Effect of a vibration exposure on muscular performance and body balance
Power Plate Studies
Summary
 
This randomized cross-over study was designed to investigate the effects of a 4-min vibration bout on muscle performance and body balance in young, healthy subjects. Sixteen volunteers (eight men, eight women, age 24-33 years) underwent both the 4-min vibration- and sham-interventions in a randomized order on different days. Six performance tests (stability platform, grip strength, isometric extension strength of lower extremities, tandem-walk, vertical jump and shuttle run) were performed 10 min before (baseline), and 2 and 60 min after the intervention. The effect of vibration on the surface electromyography (EMG) of soleus, gastrocnemius and vastus lateralis muscles was also investigated. The vibration-loading, based on a tilting platform, induced a transient (significant at the 2-min test) 2.5% net benefit in the jump height (P=0.019), 3.2% benefit in the isometric extension strength of lower extremities (P=0.020) and 15.7% improvement in the body balance (P=0.049). In the other 2-min or in the 60-min tests, there were no statistically significant differences between the vibration- and sham-interventions. Decreased mean power frequency in EMG of all muscles during the vibration indicated evolving muscle fatigue, while the root mean square voltage of EMG signal increased in calf muscles. We have shown in this study that a single bout of whole body vibration transiently improves muscle performance of lower extremities and body balance in young healthy adults.

Introduction
 
Mechanical stimulation in a form of vibration has recently aroused a great deal of interest in the fields of exercise physiology and bone research (Rubin & McLeod, 1994; Rubin et al., 1995, 1998, 2001a, b; Flieger et al., 1998; Bosco et al., 1999a, b; Falempin & Albon, 1999; Rittweger et al., 2000). It has been hypothesized that a low amplitude, high frequency mechanical stimulation of human body is a safe and efficient way to improve muscle strength, body balance and mechanical competence of bone.
 
Notwithstanding the preliminarily positive experimental and clinical results (Rubin et al., 1994, 1995, 1998, 2001a, b; Flieger et al., 1998; Bosco et al., 1999a, b; Falempin et al., 1999; Rittweger et al., 2000), conclusive evidence regarding the efficacy and safety of vibration in humans is lacking. The purpose of this study was therefore to investigate with a randomized controlled, within-subject design the effects of a single, 4-min vibration bout on healthy, young volunteers' muscle performance and body balance.
Materials and methods

Subjects

 Sixteen young healthy volunteers (eight men and eight women, 24-33 years of age) participated in the study. Their body mass ranged from 66 to 83 kg for males and from 51 to 70 kg for females and height ranged from 175 to 190 cm for males and from 156 to 178 cm for females. The exclusion criteria were: any cardiovascular, respiratory, abdominal, urinary, gynaecological, neurological, musculoskeletal, or other chronic diseases; pregnancy; prosthesis; medication that could affect the musculoskeletal system; menstrual irregularities and regular participation in impact-type exercise more than three times a week. All participants gave their informed written consent before enrolment to the study, and the protocol was approved by the Institutional Review Board and Ethics Committee of the UKK Institute.

Study setting

 All subjects were familiarized with the whole body vibration protocol and all outcome measurements about 1 week before the actual study tests. The tests were shared between two different, consecutive days to avoid fatigue and thus possible contamination of the results, i.e. in all subjects both the vibration- and sham-interventions were carried out twice in conjunction with different outcome measurements (Fig. 1). In each subject, the distance between the vibration- and shaminterventions was 1-2 weeks.
At the beginning of each study test session, a 4-min warm-up was performed on a bicycle ergometer (workload in W=1.2 body weight in N). During the tests and interventions, subjects wore thin-soled gymnastic-type shoes. Four minutes of cooling down on the cycle ergometer also followed each test session (Fig. 1). Use of alcohol or strenuous physical activity were allowed neither during the day before the test session, nor the testing day.
Study Protocol
 
All subjects thus received both the vibration intervention and sham intervention and were randomly assigned to start with either the vibration- or sham-intervention (Fig. 1) in order to eliminate the infiuence of a learning curve on the results. Both interventions were carried out in a standing position on the vibration platform (a prototype of Galileo, 2000, Novotec Maschinen GmbH, Pforzheim, Germany), with (vibration-intervention) or without (sham-intervention) the whole body vibration. The duration of both interventions was 4 min. While standing on the ends of the rigid lever arm of the platform (each foot kept 0.28 m away from the centre of the platform) the subjects repeated four times a 60-s light exercise programme according to instructions shown by the investigator. The rationale of the exercise programme was to guarantee a multidirectional, balanced vibration loading on the body and make the standing on the platform less monotonous. The programme comprised of light squatting (0-10 s), standing in the erect position (10-20 s), standing in a relaxed position the knees in a slight fiexion (20-30 s), light jumping (30-40 s), alternating the body weight from one leg to another (40-50 s), and standing on the heels (50-60 s). During the vibration intervention, the vibration frequency increased in 1 min intervals: 15 Hz for the first minute, 20 Hz for the second minute, 25 Hz for the third minute and 30 Hz for the last minute. The peak-to-peak amplitude of vibration at the end of the 0.72 m long tilting platform was 10 mm. Considering the amplitude at the 0.28 m site (where the feet were kept) and the sinusoidal nature of loading, the theoretical maximal acceleration was nearly 3.5 g (where g is the Earth's gravitational field or 9.81 m s-2) with 15 Hz loading, 6.5 g with 20 Hz, 10 g with 25 Hz, and 14 g with 30 Hz, respectively.

Performance tests
The baseline performance measurements were started 2 min after the warm-up. Ten minutes after the baseline measurements, subjects were exposed to either the above-described vibration- or sham-intervention. The same performance measurements were done again at 2 and 60 min after the 4-min vibration- or sham-intervention (Fig. 1).
 
The performance tests were shared between 2 days to avoid potential contamination of results due to fatigue. On day 1, stability platform test, measurements of grip strength and isometric extension strength of lower extremities were carried out. On day 2, tests consisted of tandem-walk, vertical jump, and shuttle run tests. The measurements were always done in the same order. Day-to-day reproducibilities (expressed as a rootmean- square coefficient of variation CV%rms) of the performance tests were determined using the duplicate baseline data measured before the vibration- and sham-interventions and are given in the `Results' section (see below).
A postural sway platform (Biodex Stability System, New York, NY, USA) was used to assess the body balance (Schmitz & Arnold, 1998). The subjects stood on a labile platform on both legs, eyes opened and arms beside the trunk. The platform provides eight different stability-levels: level 8 is virtually stable and level 1 is the most labile. As a test, we employed a 40-s protocol in successive 10 s intervals: level 5 (0-10 s), level 4 (10-20 s), level 3 (20-30 s), and level 2 (30-40 s). The system provides a numerical stability index, which refiects the body sway variation around the projection of the centre of gravity of the body (centre of foot pressure) so that the lower the score of the test the better the stability (Schmitz et al., 1998). Each subject's feet position coordinates on the platform were recorded after the first stability measurement and the same coordinates were used throughout the study to obtain consistency between the tests. The mean value of two stability indices was used as the test score. Before each test, the subjects had one to two familiarization trials.
 
Grip strength was considered as a reference test that was expected not to be affected by the vibration- or sham intervention It was measured using a standard grip strength meter (Digitest, Muurame, Finland). The median value of three readings was used as a test score.
Maximal isometric strength of the leg extensors was measured with a standard leg press dynamometer (Heinonen et al., 1994). The subjects sat on the dynamometer chair with their knees and ankles at an angle of 90° of fiexion while pressing maximally against strain gauges (Tamtron, Tampere, Finland) under their feet. The isometric strength was recorded for three maximal efforts, and the median value of three readings was used as the test score.
 
A tandem walk test along a 6-m line was used to assess the dynamic balance (Nelson et al., 1994). The subjects were instructed to place one foot behind the other, each time making sure that the tip of the foot was in contact with the heel of the other. The subjects were told to walk backwards as fast as possible while avoiding any mistakes. The time of a successful performance was measured with a stopwatch. The median value of three readings was used as a test score.
 
A vertical countermovement jump test (hands kept on the pelvis) was used to assess the lower-limb explosive performance capacity (Bosco et al., 1983). The tests were performed on a contact platform (Newtest, Oulu, Finland), which gives the time the subject is on air in milliseconds. The obtained `fiight' time (t) was used to estimate the height of the rise of body centre of gravity (h) during the vertical jump, i.e. h=gt2/8, where g=9.81 m s-2. The median value of three measurements was used as a test score.
A shuttle run test over a 30-m course was used to assess the dynamic balance or agility (Baker et al., 1993). The subjects were asked to run as fast as possible six times between markers placed 4 m apart and touch the fioor after each 4-m run, and finally run a 6-m course over the goal line. A single performance was done and the running time was recorded with photoelectric cells in milliseconds.
 
Electromyography (EMG) measurements
 Bipolar surface EMG from soleus, gastrocnemius and vastus lateralis (of the quadriceps) muscles was recorded by a dedicated differential amplifier (Myosystem 1008, Noraxon, Oulu Finland; input impedance >1 MW, gain 1000, and 3-dB bandwidth 20-350 Hz) during the 4-min bout of vibrationintervention. Disposable electrodes were located on the muscle bellies approximately in the midway between the centre of the innervation zone and the further tendon. Before attaching the electrodes, the skin was carefully shaved, rubbed and cleaned with alcohol. Good contact of electrodes was further secured with an adhesive tape.
The EMG signals were digitized at a sampling frequency of 1 kHz (DT2801 12-bit A/D-converter, Data Translation, Marlborough, MA, USA) during the 4 min periods and stored for further analysis with a dedicated software (NST, Noraxon, Oulu, Finland). A 1024-point Fast Fourier Transform was used to determine the power spectrum of given EMG signals. Four separate spectra were determined in 1 s intervals over a 4-s
period in the middle of the relaxed standing phase (only stabilizing muscle activity present), and the average of these spectra was determined. The EMG signal quality was visually checked before the spectrum analysis. From this average spectrum, a representative mean power frequency (MPF in Hz) and root mean square voltage (RMS, in mV) of the EMG signal were calculated for each minute of intervention and these variables used as test outcomes.
 
Statistical analysis
 Mean, standard deviation (SD), and 95% confidence interval (95% CI) are given as descriptive statistics.
The 2 and 60-min effects of whole body vibration on individual physical performance were defined as relative differences between the changes in the given test outcome observed after the vibration (V)- and sham (S)- interventions. The relative differences were achieved through log-transformation of the variables. The time-effect at 2 and 60 min was determined by one-way analysis of variance (ANOVA) with repeated measures.
 
Repeated measures ANOVA was also used to estimate the time-effect on EMG variables (MPF and RMS) during vibrationinterventions.
The associations between the mean power frequency and root mean square minute-values were analysed by the Pearson's correlation coefficients.
 
Results
 
All subjects completed the study without any objective sideeffects. Neither subjective adverse reactions nor exhaustive fatigue were reported after the 4-min vibration bout. Most of the subjects reported that the whole body vibration was `stimulating' for the lower extremities.
As response to sham- or vibration intervention showed no gender differences, the data of women and men were pooled and analysed together.

Muscle performance and body balance

 The day-to-day reproducibility (CV%rms) was 2.3% for the isometric extension strength of lower extremities, 2.5% for the vertical jump, 3.6% for the grip strength, 17.5% for the stability platform, 8.2% for the tandem walk and 1.8% for the shuttle run.
 
Strength tests
Isometric lower limb extension strength increased 2.0 kg at 2 min after the vibration-intervention as compared with a mean decrease of 3.4 kg after the sham-intervention resulting in a statistically significant 3.2% net benefit (P=0.02) for the vibration (Table 1 and Fig. 2a). At 60 min after the vibrationintervention the benefit diminished (2.4%, P=0.11).
 
The vertical jump height increased 0.7 cm at 2 min after the vibration-intervention as compared with an unchanged value after the sham-intervention resulting in a significant 2.5% net benefit (P=0.019) for the vibration (Table 1 and Fig. 2b). The effect disappeared completely by 60 min after the intervention
 
As expected, no effect was observed in the grip strength at 2 and 60 min after the vibration-intervention (Table 1 and Fig. 2c). The responses were virtually identical after sham- or vibration-intervention.

Stability tests
The net benefit of the vibration was 15.7% in the score of the stability platform at 2-min test (vs. sham-intervention, P=0.049) (Table 1 and Fig. 2d). No effect was observed in the 60-min test nor in the other balance and performance-tests (Table 1 and Fig. 2e-f).
 
EMG
Mean power frequency
Mean power frequency of the soleus muscle activity decreased systematically during the 4-min vibration, the 4-min values being on average 18.8% lower than the 1-min values. The magnitude of decrease in MPF during vibration was also statistically significant (P<0.001) (Table 2 and Fig. 3a). A similar pattern was observed in MPF of the gastrocnemius muscle activity. The MPF decreased throughout the vibration (P<0.001) and the 4-min values were on average 18.3% lower than the 1-min values (Table 2 and Fig. 3b).
The decrease in the MPF of the vastus lateralis muscle activity was not so evident and systematic as that in the soleus and gastrocnemius muscles during the first 3 min, but during the last minute of vibration-intervention a rapid 8.6% decrease occurred (P<0.001) (Table 2 and Fig. 3c).

Root mean square voltage
Root mean square voltage of the soleus and the gastrocnemius muscle EMG activity increased during the 4-min vibrationintervention, the 4-min values being on average 21.6% (P<0.001) and 35.2% (P=0.004) higher than the 1-min values, respectively (Table 2 and Fig 3a, b). Root mean square voltage of the vastus lateralis EMG activity was quite stable over the entire 4-min vibration-intervention and showed no statistically significant time-effect (Table 2 and Fig. 3c).

The performance test parameters after the 4-min sham- and vibration-interventions. Mean (SD) values and mean (95% CI and P-value) between-groups net differences for the relative change by time.



Mean power frequency (Hz) and root mean square voltage (mV) of the EMG signal of the soleus-, gastrocnemius- and vastus lateralis muscles as determined from the surface EMG during the 4-min vibration-loading. Mean (SD) and P-values.
For analysing the relationship between the mean power frequency and root mean square minute-values, Pearson's correlation coefficients were calculated for each subject, and a mean correlation coefficient was calculated. The mean of these individual correlation coefficients was -0.79 for the soleusmuscle, -0.83 for the gastrocnemius-muscle and -0.61 for the vastus lateralis muscle. This indicated a remarkable negative correlation between MPF and RMS values during vibration.

The changes in the mean power frequency (MPF) and root mean square voltage (RMS) values of the EMG recordings of the (a) soleus, (b) gastrocnemius, and (c) vastus lateralis muscles during the vibration-loading. Mean and SD. Trends in MPF values were linear (P<0á001) in all muscles, whereas linear trends in RMS were statistically signi®cant in the soleus- and gastrocnemius-muscles (P<0á001 and P<0á002, respectively), but not in the vastus lateralis muscle (see Table 2).
Discussion
 
We showed in this randomized cross-over study that in healthy young adults a single, 4-min vibration-loading induced a significant, transient increase in the isometric extension strength of the lower extremities, jump height, and body balance. These effects were observed 2 min after the vibration, but had disappeared more or less completely 1 h later. Although the improvements in these performance parameters were quite small, the systematic nature of these responses was clear. Thus, it was evident that the immediate effects of a short-bout vibration were beneficial for physical performance.

It has been shown that mechanical vibration exert a tonic excitatory infiuence on the muscles exposed to it. Vibration applied directly to muscle belly or tendon (at the frequency of 10-200 Hz) or to whole body (1-30 Hz) has been shown to elicit a response named `tonic vibration refiex' (TVR) (Hagbarth & Eklund, 1985, Seidel, 1988). The vibrationinduced TVR involves activation of muscle spindles, mediation of the neural signal by 1a afferents (Hagbarth, 1973), and activation of the muscle fibres via large a-motor neurones. The TVR induced by the vibration is also capable of causing an increasing recruitment of motor units via activation of muscle spindles and polysynaptic pathways (De Gail et al., 1966), which is seen as a temporary increase in the muscle activity. However, a long-term irritation of the muscle-spindles by vibration leads ultimately to muscle fatigue (Eklund, 1972; Martin & Park, 1997). This, in turn, is seen as a reduction of EMG activity, motor unit firing rates, and contraction force.
We initially anticipated that whole-body loading via vibration is fatiguing. However, the subjects experienced the vibration loading stimulating rather than fatiguing. This subjective opinion was also corroborated by the objective measurements (2 min performance tests). The improved strength and power of the lower extremities and the improved body balance after the vibration intervention (Fig. 2a, b, d) suggests that neurogenic adaptation may have occurred in the muscles of the lower extremities in response to vibration.
 
Although the participants did not subjectively experience the vibration fatiguing, and neither was any apparent fatigue-effect seen in the performance measurements, the EMG analysis showed a significant reduction of the MPF during the vibration-intervention. A reduction in MPF is generally considered a sign of muscle fatigue (Viitasalo & Komi, 1977; Petrofsky et al., 1982; Dowling, 1997; Jurell, 1998). The muscle fatigue identified by the spectral analysis of EMG was more distinct in the calf muscles (soleus and gastrocnemius) than in the vastus lateralis of the quadriceps muscle (Fig. 3). In contrast to the reduction of the MPF in both calf and thigh muscles, the root mean square voltage of EMG increased in the former muscles during the vibration (Fig. 3). This finding suggests that it may have been necessary to recruit more motor units in the calf muscles to compensate the more pronounced fatigue present in these muscles during the vibration while in the thigh muscles such a response was not needed. Perhaps, a longer bout of vibration might have resulted in a similar response in the activity of the vastus lateralis muscle as well. These findings, improved results of the strength tests but the decreased EMG activity, suggest that our vibration stimulus was long enough to stimulate the muscles of the lower extremities, but too short to induce significant muscle fatigue. One may suspect whether this effect is specific for vibration stimulus only; that is, would it have been possible to get a similar effect by other forms of stimulating physical activity, such as a normal warming-up manoeuvre. In order to eliminate this possibility, the subjects performed exactly the same warm-up on a bicycle ergometer and performed the same exercise protocol while standing on the platform during both the vibration- and sham-interventions. It is also recalled that the order of these interventions was randomized so that the learning curve bias could be minimized.
 
When considering the possible effects of vibration loading on bone, it is possible that these effects are transferred to bone via vibration-induced muscle activity. Actually, according to the literature, it has been proposed that even extremely small strains induced by very low acceleration (g=0.3, much smaller than that used in our experiment) may be effective determinants of bone morphology (Rubin et al., 2001a, b). On the other hand, our observations on the muscle fatigue are also of interest. The reduction in MPF indicated that in response to even a short bout of vibration, the muscles of the lower extremity tend to fatigue. This may indicate that with continuing vibration a larger proportion of the incident vibration energy is directed to bones, instead of being absorbed by muscle tissue (Yoshikawa et al., 1994; Millgrom et al., 1999).
The above noted findings suggest that vibration is a potentially efficient training stimulus and future studies should focus on evaluating the long-term effects of whole body vibration on body balance and muscle performance, and, as a broader objective, on bone structure and strength.
 
Whole-body vibration exercise leads to alterations in muscle blood volume
Power Plate Studies
Summary
 
Occupationally used high-frequency vibration is supposed to have negative effects on blood flow and muscle strength. Conversely, low-frequency vibration used as a training tool appears to increase muscle strength, but nothing is known about its effects on peripheral circulation. The aim of this investigation was to quantify alterations in muscle blood volume after whole muscle vibration - after exercising on the training device Galileo 2000 (Novotec GmbH, Pforzheim, Germany). Twenty healthy adults performed a 9-min standing test. They stood with both feet on a platform, producing oscillating mechanical vibrations of 26 Hz. Alterations in muscle blood volume of the quadriceps and gastrocnemius muscles were assessed with power Doppler sonography and arterial blood flow of the popliteal artery with a Doppler ultrasound machine. Measurements were performed before and immediately after exercising. Power Doppler indices indicative of muscular blood circulation in the calf and thigh significantly increased after exercise. The mean blood flow velocity in the popliteal artery increased from 6.5 to 13.0 cms -1 and its resistive index was significantly reduced. The results indicate that low-frequency vibration does not have the negative effects on peripheral circulation known from occupational high-frequency vibration.
 
Keywords: arterial blood flow, muscle contraction, tissue blood flow, vibration.
 
Introduction
 
As early as in 1949, Whedon et al. (1949) reported the positive effect of passive exercise, by means of an oscillating bed, on metabolic abnormalities in plasterimmobilized patients. In an experimental study it has been shown that the application of 50 Hz, 10 g vibration for 2-5 h daily increased the cross-section of muscle fibres and reduced the fat content of muscle tissue (Hettinger, 1956). A randomized study showed that, in athletes, 3 weeks of strength training (sitting bench-press) with superimposed vibratory stimulation led to an almost 50% increase in the one-repetitionmaximum compared with an average gain of 16% with conventional training and no gain for the control group (Issurin et al., 1994). On the other hand, investigating forestry operators, Bovenzi et al. (1991) showed that a loss in grip strength may occur after prolonged occupational vibration exposure. Workers who use hand-held vibrating tools may also experience finger blanching attacks as a result of episodic vasospasm in the digital vessels (Bovenzi & Griffin, 1997). An experimental study with rats attached to a vibrating table (80 Hz, 32 m s2) 5 h daily for 2 days indicated that vibration may lead to muscle injury (Necking et al., 1996).
 
The power Doppler sonography technique allows quantification of relative moving blood volume (Rubin et al., 1995). MR imaging and conventional colour Doppler imaging correlate well with other physiological measures of exercise-induced changes in blood flow (Hirsch et al., 1995; Pena et al., 1996). Fleckenstein et al. (1988) showed that a few minutes of muscle activity led to signal intensity changes on MR images, which correlated moderately with the level of exertion.
When standing on a vibrating platform, one tends to attenuate the imposed vibration and misalignment of stance by physical activity. The rhythmic muscle contractions evoked by standing on a vibrating platform may be beneficial in counteracting the lack of other physical exercise, but its effects on peripheral circulation are not thoroughly examined yet.
 
So far, most studies investigating the effect of vibration on blood flow have used frequencies common among tools used in industry which generally means 80-100 Hz (LundstroÈm & BurstoÈm, 1984). In this study this range of frequency is summarized as `high frequency' and frequencies below that are called `low frequency'. A comparison of different magnitudes (22 and 87 m s2) and frequencies (31.5 and 125 Hz) revealed that the high-frequency vibration stimulus produced a greater reduction in finger blood flow (Bovenzi & Griffin, 1997). The authors conclude that the digital circulatory response to acute vibration depends upon the magnitude and frequency of vibration.
 
The aim of this study was to determine whether standing on a vibrating platform that moves up and down at a frequency of 26 Hz and with an amplitude of 3 mm has negative effects on blood flow, as is known from occupational studies investigating highfrequency vibration.
 
Methods
 
Subjects
 Healthy volunteers between 25 and 35 years of age were allowed to participate in the study. With respect to regular physical activity they were required not to have a sedentary lifestyle but also not to engage in regular strenuous physical activity, especially weightlifting exercises. Informed consent was given by all participants and the protocol conformed to the Declaration of Helsinki.
 
Training
 Prior to the experiment, the subjects' age, height and weight were recorded. The level of regular occupational and recreational physical activities were assessed according to the American Heart Association (1975).
 
The subjects were exposed to whole-body vibration using the Galileo 2000 device (Novotec GmbH, Pforzheim, Germany). They stood on a platform fixed on a sagittal axle which alternately pushes the right and left leg upwards and downwards at a frequency of 26 Hz (amplitude =3 mm, peak acceleration = 78 m s2). Three sets of different positions were used.
 
During the first set, the subjects stood with their legs straight and their forefeet parallel to each other on the platform. The second bout was performed with the entire feet standing on the platform and moderately (60-70°) bent knees. Position 3 was the same as position 2 but the legs were rotated externally by about 30° and the knees were bent by about 60-70°.
 
Each of the three positions was held for 3 min and the exercise was continued without break between the positions. Thus, the total work out was 9 min. The subjects stood barefoot in order to avoid footwaredependent attenuation of the vibrations. To avoid biorhythmic changes, all subjects performed the experiment at the same time of the day between 10 a.m. and 12 p.m.
 
Outcome measurements
 Heart rate was monitored with suitable devices (Polar beat, Polar Electro Oy, Kempele, Finland) and blood pressure was measured using conventional manometer technique (Heintel Rudolf, GesmbH, Vienna, Austria). Using a diagnostic ultrasound machine (Ultramark 9, ATL Advanced Technology Laboratories, Inc., Bothell, WA, USA) with colour Doppler and power Doppler, relative moving blood volume was quantified according to Newman's method (Newman et al., 1997). During the measuring procedure the subjects were standing. First, measuring points were marked on the skin. For evaluating the gastrocnemius muscle, the point of axial measurement was marked on the dorsal aspect of the calf 15 cm distal to the right knee. For evaluating the quadriceps muscle, a point on the ventral aspect of the thigh 20 cm proximal to the right knee was marked. The relative moving blood volume of these two muscles was quantified with a power Doppler method. The number of distinctly visualized vessels in the colour box with a diameter of 2 mm or more were assigned a point value: 1 (five of fewer vessels), 2 (6-10 vessels), 3 (11-15 vessels), 4 (16-20 vessels) or 5 (>20 vessels) (Newman et al., 1997). The blush score, defined by three or more vessels with contiguous margins at some point, was assigned a value 0 (no contact), 1 (three or more vessels in contact but <50% of the vessels within the portion of the colour box overlapping the gastrocnemius or quadriceps muscles, respectively) or 2 (>50% vessel contiguity in visualized gastrocnemius or quadriceps muscles for the portions of the gastrocnemius or quadriceps muscles, respectively, in the colour box) (Newman et al., 1997). Blood flow in the popliteal artery was measured along the axis of the vessel by means of Doppler sonography, the speed of flow was registered and the resistive index (Pourcelot, 1974) calculated.
 
All outcome measurements were assessed before and immediately after 9 min of exercise.
 
Statistical analysis
 For all parameters, descriptive analysis was performed. The mean of all pre- and post-training parameters was compared with Student's paired t-test and the t-value was used as a parameter of choice. The Statistics Package for Social Sciences (SPSS Inc., Chicago, IL, USA) was used and P-values <0.05 were considered statistically significant.
 
Results
 
Twenty subjects - eight women and 12 men - voluntarily participated in the study. Anthropometric data are shown in Table 1. The participants were physically active but did not engage in strength or strenuous power training. In general, they did not perform more than 8-9 metabolic equivalents (METs) (one MET is the metabolic requirement under basal conditions, which is equal to the metabolic rate) within 1 week.
 
Heart rate values, systolic and diastolic blood pressures after exercise did not show a statistically significant change compared with baseline (HR: 87.6 - 11.7 and 82.8 - 9.4 beats min-1, RRsyst: 122.0 - 10.3 and 126.5 - 11.6 mmHg, RRdiast: 79.5 - 8.9 and 81.0 - 7.7 mmHg, respectively).
 
In all of the exercising subjects a redish change in skin colour (erythema) of the foot and calf was subjectively visible. Figure 1(a) shows the number of distinctly visualized vessels with a diameter of at least 2 mm which were sonographically visualized immediately before and after the work out. Before and after the work out the Power Doppler Index (PDI) was 1.5 (0-6) [median (min-max)] and 3 (0-6) for the quadriceps muscle and 1 (0-3) and 2 (1-3) for the gastrocnemius muscle, respectively. The number of distinctly visualized vessels with a diameter of at least 2 mm depicted a statistically significant increase (P = 0.005 and 0.0006 for the quadriceps and gastrocnemius muscles, respectively). Figure 1(b) shows the sonographically determined blush scores of these muscles. Pre- and post-exercise values of the PDI were 0.5 (0-2), 1 (0-2) and 0.5 (0-2), 1 (0-2) for the quadriceps muscle and the gastrocnemius muscle, respectively. Comparing the PDIs revealed statistically significant increases (P = 0.02 and 0.0001 for the quadriceps and gastrocnemius muscles, respectively). For both parameters, the number of distinctly visualized vessels and the blush score, the t-value was higher for the gastrocnemius muscle indicating a higher difference between the values before and after the workout.
 
Table 2 shows the ultrasonic measurements of the popliteal artery. The systolic area of the popliteal artery remained unchanged. No statistically signifi- cant change in the maximal systolic and diastolic flow of the popliteal artery was found, but its mean speed of blood flow did show a statistically significant increase. On the other hand, the resistive index of the popliteal artery yielded a statistically significant reduction.
 
Discussion
 
A few minutes lasting stance on a vibrating platform leads to an increase in the relative moving blood volume of quadriceps and gastrocnemius muscles. Mean blood flow in the popliteal artery was also increased and its resistive index decreased.


 
According to the authors' opinion, trying to attenuate the imposed vibration on the body evokes rhythmic muscle contractions. However, this supposed muscular exercise did not alter the heart rate or blood pressure. It induced changes in peripheral circulation. The increased number of visualized vessels with a diameter of at least 2 mm reflects the exercise-induced widening of small vessels. Widening of the capillaries in the quadriceps and gastrocnemius muscles facilitates the passage of more molecules and, therefore, the blush score of these muscles increased. These findings are in line with Rittweger et al. (2000) who reported that, even if performed to exhaustion, cardiovascular effects of vibration exercise are mild. The authors also found an increase in the foot and calf blood flow assessed with the cutaneous laser Doppler flow. In this study, however, the Newman's method measuring the relative moving blood volume of the capillary system was used. With this method a differentiation between arterial and venous capillary loop is not possible. However, this does not matter as the speed of blood flow is approximately the same in the whole capillary region.
 
As expected, the systolic area of a relatively large vessel, the popliteal artery, did not change. Also, the maximal systolic and diastolic speed of blood flow were the same, but the mean speed of blood flow in this vessel increased. The most reasonable explanation may be the following: widening of the small vessels in the muscles reduces the peripheral resistance, which may increase the mean speed of flow in the popliteal artery. Additionally, the effect of thixotropism, a reclotting phenomenon, may also play a role. Vibration might reduce the viscosity of blood and thereby increase the mean speed of blood flow in the popliteal artery. The reduction in peripheral resistance probably also is the reason for the reduction in the resistive index of the popliteal artery.
 
Previously reported results regarding peripheral circulatory function during exposure to vibration are conflicting. However, studies investigating the effect of hand-held vibrating tools showing a vasoconstriction following exposure to vibration did not take the effect of grasping into account (Bovenzi et al., 1999). Our findings are in line with a vibration exposure test performed by Nakamura et al. (1995), who showed that the digital blood flow increased when the individual was exposed to vibration while grasping a handle compared with grasping alone. The authors thought that the negative correlation between digital blood flow and endothelin levels during vibration exposure suggested the following: a reduction in the release of the vasoconstrictor endothelin from smooth muscle into the vessel cavity during vibration leads to vasodilatation, possibly attributable to a local axon reflex


 
Electromyography (EMG) studies revealed that exposure to seated 5 Hz sinusoidal vibrations increased the development of muscular fatigue in comparison with sitting alone. Whole-body vibration induced vibration-synchronous EMG activity in the erector spinae muscle, which exceeded the activity without whole-body vibration (Seidel, 1988; Seroussi et al., 1989). These studies show that vibration induces muscle activation and therefore muscle training.
 
It has been suggested that muscle stimulation by vibration might improve the mechanical power of the lower limbs in elite athletes by means of neural adaptation (Bosco et al., 1998). One minute of mechanical vibration applied during arm flexion in isometric conditions enhanced the average power of the arm in international level boxers (Bosco et al., 1999a). In another investigation, Bosco et al. (1999b) showed that whole-body vibrations increased the average velocity, average force and average power in well-trained subjects.
 
The underlying mechanism of muscle activation via vibration may be that it activates Ia afferent fibres which are segmentally connected to the corresponding a-motor neurone (Rothmuller & Cafarelli, 1995). Additionally, it has been shown that the activation of the muscle spindle receptors is not only limited to the muscle the vibration is applied to, but also affects the neighbouring muscles (Kasai et al., 1992).
 
Nevertheless, some published investigations report that vibration has a negative outcome on muscle strength. It has been found that the hand-grip force of vibration-exposed forest workers using chain saws was diminished in comparison with controls (Bovenzi et al., 1991). In rats, vibration held at a constant frequency of 80 Hz and a constant acceleration of 32 m s-2, 5 h daily, during five consecutive days led to different degrees of degeneration of muscle fibres in some muscles (Necking et al., 1996). It was postulated that changes in the size of muscle fibres were the first indication of vibration-induced muscle injury.
 
The crucial points concerning a positive or negative outcome in vibration studies on peripheral circulation and muscle strength seem to be the frequency and amplitude of vibration as well as the duration of exposure. The results of the study indicate that a short-term exposure to whole-body vibration of 26 Hz does not have the negative effects known from long-term exposure to high frequency.
 
 
Acute physiological effects of exhaustive whole-body vibration exercise in man
Power Plate Studies
Summary
 
Vibration exercise (VE) is a new neuromuscular training method which is applied in athletes as well as in prevention and therapy of osteoporosis. The present study explored the physiological mechanisms of fatigue by VE in 37 young healthy subjects. Exercise and cardiovascular data were compared to progressive bicycle ergometry until exhaustion. VE was performed in two sessions, with a 26 Hz vibration on a ground plate, in combination with squatting plus additional load (40% of body weight). After VE, subjectively perceived exertion on Borg's scale was 18, and thus as high as after bicycle ergometry. Heart rate after VE increased to 128 min-1, blood pressure to 132/ 52 mmHg, and lactate to 3.5 mM. Oxygen uptake in VE was 48.8% of VO2max in bicycle ergometry. After VE, voluntary force in knee extension was reduced by 9.2%, jump height by 9.1%, and the decrease of EMG median frequency during maximal voluntary contraction was attenuated. The reproducibility in the two VE sessions was quite good: for heart rate, oxygen uptake and reduction in jump height, correlation coefficients of values from session 1 and from session 2 were between 0.67 and 0.7. Thus, VE can be well controlled in terms of these parameters. Surprisingly, an itching erythema was found in about half of the individuals, and an increase in cutaneous blood flow. It follows that exhaustive whole-bodyVE elicits a mild cardiovascular exertion, and that neural as well as muscular mechanisms of fatigue may play a role.
 
Keywords: energy turnover, exercise physiology, osteoporosis, sports, training.
 
Introduction
 
Vibration exercise (VE) is a type of exercise that has recently been developed for the prevention and treatment of osteoporosis. It elicits neuromuscular 1training reflectorily, without much effort and in short periods. In ovariectomized rats, VE has been reported as a successful countermeasure against loss of bone mineral (Flieger et al., 1998). Moreover, it is conceivable that, depending on the frequency of vibration, VE renders specific training of type II muscle fibres possible. At present, several chronic training studies are being conducted in various fields, including sports and training sciences, geriatrics and treatment of osteoporosis (Rubin et al., 1998; Wilhelm et al., 1998; Bosco et al., 1999).
 
We currently work with a prototype, in which a platform vibrates around a horizontal rotation axis. Exercise is usually performed with both legs, the feet posed equidistant on either side of the rotation axis. Hereby, extensor and flexor contractions alternate continuously in the left and the right legs. There is no direct vertical acceleration to the body's centre of gravity. This reduces passive forces to the joints, but elicits reflexes to stabilize the body posture.
 
In previous experiments,wehave ascertained thatVE elicits muscle contractions by recording an electromyogram (EMG). Moreover, oxygen uptake and hence metabolism typically increases during VE with 26 Hz by about 5 mlO2 min-1 kg-1 body weight, as compared to squatting without vibration (unpublished data). The present study was performed to explore the limits, i.e. the exertion and fatigue effects of exhaustive VE, and how far these limits are reproducible in subsequent exercise sessions. As a comparison, aerobic capacity was determined by progressive bicycle ergometry. Variables of interest were (i) gas exchange and lactate, (ii) heart rate and blood pressure, and (iii) neuromuscular function and fatigue. In addition, skin blood flow was assessed, since in previous experiments some subjects developed an erythema over the activated muscles.
 
Materials and methods
 
Subjects and set-up
 
The study was approved by the ethics committee of the Freie UniversitaÈt Berlin (signature: GALI- 2LEO\PHYSIO\AKUT). The subjects were recruited 3through our centre by announcements on the campus. All subjects gave written informed consent before inclusion to the study. They came for three visits, with at least 8 days in between.
 
Forty persons decided to participate. One person was excluded for medical reasons, and two subjects dropped out in the course of the study: 16 females and 21 males were therefore included. The mean age was 23.5 years (SD 2.7 years, no significant difference between sexes). The female subjects were on average 168.9 cm tall (SD 5.0) and had a weight of 60.6 kg (SD 6.5). The males were 181.4 cm tall (SD 5.3) and weighed 75.2 kg (SD 8.3).
 
First visit (BIC)
 
Subjects were clinically investigated. A bicycle ergometry was performed, with increasing steps of 50 W over 3 min until exhaustion. Before (CTRL), during, immediately after (POST), and after 15 min recovery (REC), the following parameters were measured:
 
1 arterial blood pressure (Riva Rocci) in sitting position (POST: 60 s after termination), 2 ECG and heart rate, 43 O2 uptake and CO2 delivery, sampled at 0.1 Hz (Metamax, Cortex Biophysik, Leipzig), 4 subjectively perceived exertion, assessed by Borg's scale (Borg, 1976), and 5 blood lactate concentration from the finger tip (Accusport, Boehringer; CTRL & POST only).
 
The Metamax system has a resolution of 15 ml and an accuracy of 1.5% for volume measurement. The Zirkonium oxygen sensor and the infrared CO2 sensor have an accuracy of 0.1 vol%.
 
Second and third visit (VIB1 & VIB2)
 
On the second and third visits, exercise until exhaustion was performed on a vibration platform (Novotec, Pforzheim, Germany). The subjects stood on this platform with their feet at 15 cm distance from the rotation axis on either side. Vibration was with an amplitude a0 = 1.05 cm, a frequency of 26 Hz, and hence a peak acceleration of 147 m s-2, or 15 g.
 
The subjects bore an additional load fixed around the waist (40% of body weight in males; 35% in females because of their higher total body fat mass). After 30 s of simple standing, they started squatting, i.e. bending their knees in a 6 s cycle, 3 s down and 3 s up, as smoothly as possible.
 
When exhausted, the weight was removed, and the post-exercise (POST) values were immediately assessed. These were, in addition to those of the BIC visit:
 
1 jump height in three trials with 5 s intervals, with both hands places on the hips, the knees bent to 90° when started, and extended while in the air,
 
2 cutaneous laser Doppler flow (LDF) over the calf and over the foot during a 20 s period (Periflux3, Perimed, Sweden), and
 
3 for a period of 10 s, maximal voluntary contraction (MVC) of the knee extensors on the dominant side, torque and EMG were recorded (2 min after termination).
 
Assessment of arterial blood pressure was not possible during VE.
 
Signal and data analysis
 
The ECG and the EMG over the vastus lateralis muscle were recorded continuously during the whole experiment. Together with the LDF, torque chair and 6jump pad signals they were sampled at 1000 Hz after low-pass filtering (cut-off 350 Hz). The resolution of the Analog±digital (AD) board was 12-bit. The EMG was picked up with two Ag/AgCl electrodes 8(0.33 mm2), positioned with 30 mm distance over the vastus lateralis at 66% of the distance between the knee joint cleft and the spina iliaca anterior superior of the dominant leg.
 
The mean blood lactate concentration before BIC, VIB1 and VIB2 visits was 1.69 mM (SD 0.50, no significant difference). Hence, a lactate concentration ³3 mM was considered as elevated. From 3 min before the exercises (VIB1, VIB2 & BIC) until termination, O2 uptake and CO2 delivery were recorded. These signals yielded the resting and the peak values of O2 uptake, peak CO2 delivery and the respiratory quotient.
 
For the vastus lateralis EMG during MVC, spectral analysis (Hanning window) was conducted for periods of 200 ms with 100 ms overlapping period, yielding a power spectrum every 100 ms. From these spectra, the absolute power and the EMG median frequency were extracted (Kupa et al., 1995). Torque, power and median frequency were averaged for every 2000 ms.
 
Statistics were performed with SPSS software (PC version 7.5.2). Before the t test or the multiple t test was applied, variables were tested for normal distribution with the Kolmogorov±Smirnoff test and for homogeneity of variances with Bartlett's F test. Whenever data showed normal distribution, differences between groups were checked by one-way ANOVA and the t test with Bonferroni's correction for multiple comparisons. In all other cases, the Wilcoxon or Friedman tests were applied. Significance was assumed if P<0.05. Values for VIB1 and VIB2 sessions were compared by correlation analysis and regression analysis.
Results
 
All analyses were performed for females and males separately. For the sake of clarity, however, we detail gender differences only where they were significant.
 
The delay between squatting and the first POST jump was on average 10.9 s in VIB1 (SD 1.29) and 10.6 s in VIB2 (SD 1.26). The jump intervals were 4.87 s in VIB1 and 4.80 s in VIB2. No significant differences were found.
 
Exercise and cardiovascular data
 
Exercise and cardiovascular data are summarized in Table 1. Exercise time was 325 s in VIB1 and 362 s in VIB2. Subjectively perceived exertion was initially higher in VIB1 than in VIB2 (12.0 versus 10.6), and also higher in both VE sessions than in the BIC visit. At the termination of exercise, however, there was no difference, with a Borg value of about 18 meaning something between `very hard' and `very, very hard' exercise.

Exercise Data during bicycle ergometry
 
As shown in Table 1, heart rate increased signifi- cantly less in VE than in bicycle ergometry (127.5 and 128.6 versus 171.4 min±1). Control values were higher before VE than before bicycle ergometry. No difference was found in the recovery values. Similar to heart rate, the O2 uptake was significantly lower during VE than during bicycle ergometry (21.9 and 22.1 versus 44.8 ml min-1 kg-1). Again, control values were higher in VE. All subjects tested had a significant increase of lactate concentration after bicycle ergometry, indicating significant exertion. Hence, we identified the maximal specific O2 uptake (VO2max): 48.8% of VO2max was reached in VIB1, and 49.3% in VIB2 (Table 1). High values of up to 81.1% 10of VO2max were observed in two former male Judoka. The mean lactate values after VE were significantly lower than during bicycle ergometry, but still 20 out of 37 had a lactate concentration higher than 3 mM in VIB1, and 27 in VIB2.


 
Systolic blood pressure increased significantly during bicycle ergometry and VE (Table 1). Again, the rise in VIB1 and VIB2 was not as large as in BIC (131.6 and 134.9 versus 147.6). Interestingly, the diastolic blood pressure was decreased after VE, but not after bicycle ergometry. Blood pressure returned to control values after 15 min recovery.
 
For individual subjects, heart rate and oxygen uptake values of the VIB1 and VIB2 sessions were correlated with each other (Fig. 1). The correlation coefficients of the end-exercise values of VIB1 and VIB2 were 0.68 for heart rate and 0.70 for VO2 (Fig. 1). No obvious relation was seen between the residuals of heart rate (VIB1/VIB2) and oxygen uptake (VIB1/VIB2).
 
The data for cutaneous LDF were not normally distributed. The Friedman test yielded a significant increase of the LDF signal after VE over both calf and foot (see Table 2). After recovery, the values had returned to normal. Without quantification, we report that a number of subjects, particularly women and particularly in the VIB1 visit, showed considerable erythema on their legs, often sharply delineated like stockings. In many cases, the erythema was paralleled by oedema over the foot and the tibia (see Fig. 2). Oedema and erythema also occurred independently of each other. Moreover, many subjects spontaneously reported itching of the leg (not the foot sole) after about 1±2 min of VE.

 
Heart Rate and specific oxygen uptake at the termination of vibration exercise graph
 
Neuromuscular data
 
Basal values of jump height differed significantly between females and males. In both groups, a reduction was observed after VE. This was more pronounced in males, where the first and second POST jumps were reduced by 10%. In females, only the first POST jump in VIB2 was significantly lowered (see Fig. 3). No reduction of jump height was found in the third jump

Erythem and oedema of the foot.
 
Reduction of jump height (DJH) was computed as the mean jump height in CTRL minus first jump POST. A significant correlation was found between DJH in VIB1 and VIB2 (r =0.67, no significant offset, see Fig. 4). Multiple linear regression analysis did not reveal any relation between DJHVIB1 and POST blood lactate concentration, maximal specific O2 uptake, duration of exercise, or CTRL mean jump height (P>0.25 in all cases).
 
After VE, a significant reduction of knee extension torque by about 10% was observed in the males 11during the first 2 s, but not during the last seconds (see Table 3). However, the EMG frequency in POST was increased in the first as well as in the last two seconds, if compared to CTRL and REC. In the females, the same tendency was observed without reaching the level of significance.
 
To account for the time course of torque and EMG median frequency during MVC, the differences between the 0-2 s and 8-10 s values were computed. These differences were significantly lower in POST than in CTRL in both sexes, i.e. torque and median frequency during MVC decreased less after VE than before (P<0.05, ANOVA).
From the 0-2 s values of EMG median frequency and torque, the reduction (DMF and Dtorque) was calculated in the same way as for reduction of jump height (DJH). Dtorque was weakly correlated with DJH, and DMF with POST lactate. DMF showed a correlation to lactate, particularly at lower lactate levels.
 
Laser Doppler flow of the skin

 
Discussion

jump height before vibration exercise
 
Treatment and prevention of osteoporosis by physical exercise is a new therapeutic concept (Calmels et al., 1995). While several studies have demonstrated that it is applicable (Braith et al., 1996; Heinonen et al., 1996; Heinonen et al., 1999), there definitely is potential for improvement. The present investigation was conducted to explore a novel method, vibration 12exercise, at its extreme.
 
Generally, the subjects became acquainted very rapidly with this exercise. By the second VE visit, they were standing confidently and safe on the platform. This is mirrored in the data: exercise time was longer in VIB2 than in VIB1, and the Borg values at the beginning of exercise were lower in VIB1 (approximately 12) than in VIB2 (approximately 10.5). At the termination of exercise, i.e. after about 5 min, the subjects appeared to be quite as exerted by VE as after 12 min bicycle ergometry (see Table 1).
Reduction of jump height after vibration exercise
 
O2 uptake reached only about 50% of VO2max, almost uniformly in VIB1 and VIB2. Likewise, heart rate rose to about 130 in females, which is about the value expected for 50% of VO2max (Rowell, 1971). This seems to rule out possible additional stimulating effects on the cardiovascular control system (McCloskey et al., 1972; Schulz et al., 1983). For technical reasons, CTRL heart rate and oxygen uptake in VIB1 and VIB2 were recorded while the subjects were standing, which accounts for their elevation compared to CTRL in BIC. Systolic arterial blood pressure, which could not be measured during VE, was found to have increased after it, but less so than after bicycle ergometry. In contrast, diastolic blood pressure had decreased only after VE.
 
The fatigue in VE therefore appears to be caused not by insufficiency of cardiac output (as in exhaustive bicycle ergometry), but rather occurs in the neuromuscular system. Lactate never increased in VE as much as in bicycle ergometry. But even if the blood concentration was low, some parts of the musculature 13may accumulate lactate, which then is signalled via 14muscle metaboreceptors and leads to subjective exertion.
 
In both VE visits, a reduction of jump height (DJH) was observed 10 s after termination of VE, which was basically recovered from within 20 s. DeltaJH was not dependent on initial jump height, POST VE lactate concentration, or peak specific O2 uptake, and it was intra-individually stable in repeat visits. Therefore, DJH seems to express individually typical information. Similar r-2 values were found for VIB1 and VIB2 values of peak specific oxygen uptake and heart rate. The residuals of these regressions, however, were not correlated with each other. This indicates that different subjects depict different, but intra-individually typical response patterns, which is of importance for training regimes.
 
MVC was performed 2 min after termination of VE. Usually, the EMG frequency and force decline during sustained maximal voluntary contraction (Sandercock et al., 1985). This was seen in CTRL, but not in POST, where EMG median frequency hardly decreased during MVC, and torque even showed a tendency to increase. In other words, in POST, less force was produced at a higher median frequency, but with less tendency to decline during sustained contraction.
 
Taking these finndings together, it becomes clear that in VE, at least two mechanisms of fatigue play a role: a rapid one (evidence: recovery of DJH within 20 s), and a slow one (evidence: Dtorque and DMF in MVC). DeltaMF was correlated with lactate and could be partly explained by muscle fatigue. The facts that (i) subjective fatigue could occur without increased lactate, (ii) correlation of DeltaJH with lactate and DeltaMF is poor or lacking, and (iii) the recovery time of 20 s, hint at neural causes of the fast-recovering fatigue mechanism, at peripheral, spinal or higher levels.

Torque and EMG median frequency
 
These findings and conclusions are in line with studies of fatiguing by the tonic vibration response (TVR). The TVR is elicited by vibrating devices 16applied either to the muscle bellies or tendons. It is 17transmitted by activation of Ia afferents (Hagbarth, 1973), which activate, via large œ-motor neurones, mainly type II muscle fibres. It has been shown (i) that sustained TVR decreases voluntary force until 10-20 s after the end of vibration, (ii) that it is accentuated by preceding muscle exercise, and (iii) that it affects primarily the subject's ability to generate high firing rates in high-threshold motor units (Bongiovanni et al., 1990). A pre-synaptic inhibition or a transmitter depletion of the Ia afferents have been postulated. Recently, Ribot- Ciscar et al. (1998) have shown a third mechanism, namely a `fatiguing' of the Ia afferents themselves.
 
Some important differences between TVR and VE must be kept in mind: (i) in TVR, the usual frequencies applied are around 100 Hz or even higher, (ii) VE in this study was applied to the whole body and not to a single muscle, and (iii) VE was combined with slow, voluntary movements, which usually break the TVR.
 
An unexpected finding of interest is the reduced diastolic blood pressure after VIB1 and VIB2. Since heart rate and systolic pressure were higher than in CTRL, arterial vasodilation is the cause. It is not clear whether the diastolic hypotension emerges during VE, or only after it. In the latter case, vasodilation might occur in response to impeded muscular circulation, which in isometric contraction is known to occur above 60% maximal isometric force (Petrofsky & Hendershot, 1984).
 
It is another interesting question, whether the swelling and erythema are caused by vasodilation of supplying arteries via an increase of perfusion pressure. Interestingly, swelling and flare have also been observed after TVR at frequencies around 30 Hz, but not higher (Homma, 1973). Quantitatively, we assessed an increase in the LDF signal after VE. LDF measures blood flow, i.e. volume ´ speed. Increases of LDF were also observed without erythema, suggesting differential effects on cutaneous superficial and deeper arteries. Both, erythema and oedema were often limited to stocking-like areas, always starting at the bottom of the foot, i.e. closest to the vibrating platform. This renders a mechanic explanation likely. A well-known reaction of the skin to mechanical 18stimulation is dermographism: friction over the cutis leads to reddening and swelling (Wong et al., 1984). Because so frequently observed, the reaction in our subjects is definitely distinct from the scarce vibratory angioedema (Lawlor et al., 1989) and from acute pressure urticaria (Lawlor et al., 1991).
 
In brief, vibration exercise is a new strategy in eliciting muscular contraction by reflexes. It therefore may allow the combination of voluntary and involuntary muscle work. The present investigation has shown that, even
 
Acute and residual effects of vibratory stimulation
Power Plate Studies
Introduction:

Vibration applied to muscle or tendon induces a non-voluntary muscular contraction termed the `tonic vibration reXex’ (Eklund and Hagbarth, 1966). The voluntary impetus increases such a muscular contraction, and thus the maximum voluntary contraction can be facilitated (Matyas et al., 1986). Moreover, vibratory stimulation combined with a substantial voluntary eV ort was shown to elicit movement in neuromuscular patients who were unable to contract their paretic muscles (Hagbar th and Eklund, 1966). The technique is widely used in neurophysiology and physiotherapy (Granit, 1970; Bishop, 1974). Attempts to use vibratory stimulation in the training of athletes have been undertaken only recently (Nazarov and Spivak, 1987). A substantial increase in muscle strength was observed after 3 weeks of vibratory stimulation strength training when compared with regular strength training (Issurin et al., 1994).
 
Explosive strength, or the ability to develop force within a very short time, is of primary importance in many sports. Typical exercises for explosive strength training are characterized by fast muscular contractions with an external load of about 50± 70% of maximal strength (Vrijens, 1990). The immediate eV ect of such exercises can be assessed by the power which an athlete can generate in a movement. Several additional training techniques have been used to accentuate power training: the quick release technique, pre-stretching of active muscles before contraction, electrical stimulation and biofeedback. The objectives of these techniques are to improve upon previous achievements, to facilitate motor learning eV ects and to enhance muscular capacity (Torrey, 1985). Based on the results of a previous study (Issurin et al., 1994), it is likely that similar outcomes may also be achieved using vibratory stimulation.
The physical characteristics of the two groups of athletes (mean ± s)

 
Vibratory stimulation of the muscle tendon evokes an excitation of muscle sense organs (Brown et al., 1967). It has also been suggested that vibratory stimulation activates central nervous organization which is responsible for neuromotor control (Granit, 1970).
 
Another suggestion made recently concerns the dif- Wculty in achieving full muscle activation by voluntary eV ort during dynamic exercise when large muscle groups are involved (James et al., 1995). It is possible that, owing to vibration, the muscles will be par tially activated and their mobilization at the beginning of the eV ort will be faster. Therefore, it could be hypothesized that this additional vibratory excitation will stimulate the appropriate muscle group activation and the power exertion in explosive strength exercises. Moreover, an increased excitability of peripheral sense organs and the central nervous system may have a positive eV ect on the subsequent contractions. From an ethical point of view, vibratory stimulation exercises should be viewed as belonging to the group of so-called `non-conventional training’ methods, such as electrical muscle stimulation, velocity-assisted exercises (Maglischo, 1982) and computerized training machines (Torrey, 1985). Thus, superimposed vibration to the muscle may enhance its contraction (acute eV ect) or elicit post-stimulation facilitation (residual eV ect). The aim of this study was to establish the acute and residual eV ects of vibratory stimulation in explosive strength exercises.

 
Methods:
 
Participants:
Altogether, 28 male athletes aged 18± 42 years volunteered to participate in the study. They were divided into two groups (Table 1). The Wrst group consisted of athletes from the Israeli national judo, wrestling, weightlifting, gymnastics and track and Weld teams. These athletes regularly engaged in highly intensive power training. The second group consisted of amateur athletes par ticipating in club or college sports, such as basketball, volleyball, judo, weightlifting, body-building, boxing and track and Weld. The amateur athletes were also engaged in power exercises but not as extensively as their elite counterparts (2± 4 times a week). Because all of the athletes were familiar with power exercises, they were able to perform several repetitions with maximal eV ort and high reproducibility (see Table 2). This was one reason why elite and highly qualiWed athletes were enrolled as par ticipants.
The study was approved by the local ethics committee and informed consent was obtained from the par ticipants before the study began.
 
Instrumentation and tests:
The athletes performed bilateral biceps curl exercises in a sitting position on a `Schnell’ dynamic bilateral biceps machine (Schnell, Germany, D.B. Pat. 2213440). They were secured to the machine by pads placed at the elbow, chest and back (Fig. 1). The pulling action began from a position of maximal forearm extension and Wnished with the elbow at an angle of 90° (1.57 rad). The athletes were instructed to perform each repetition as quickly as possible.
 
The superimposed vibration during the exercise was transmitted to the muscles by a specially designed vibratory stimulation device (Issur in et al., 1994). It consists of an electromotor with a speed reduction and eccentric wheel. The load is held by a cable which is passed through the eccentric wheel via the pulleys (Fig. 1). The eccentric rotation elicited peak-to-peak oscillations of 3 mm with a frequency of 44 Hz. After vibration damping owing to cable transmission, the acceleration on the handle was about 30 m ´ s- 2 (RMS). Vibration from the two-arms handle was transmitted through the contracting muscles involved in the pulling action.
The power of the active phase of exercise was measured using a `Power Teach’ instrument (GE Sport S.A.S., Rome, Italy). Two probes were installed on the counterweight frame. The locations of the probes were established during the warm-up; the lower probe was placed 2 cm above the counterweight start position and the upper probe was placed opposite the Wnal counterweight position. Therefore, the probes covered the complete range of movement. The distance between the probes and the counterweight was transfer red to a microcomputer before the primary task. A magnetic element was Wxed to the counterweight. When the counterweight and magnet moved through the probes, electrical signals were generated and the time between the signals from the lower and upper probes was recorded. The mean power was computed as a product of force and velocity. The power of each repetition was shown to the performer on-line. After each set of exercises, the maximal and mean values were automatically recorded and displayed on the screen to an accuracy of 1 W.
The bilateral biceps curl exercise and instrumentation.

 
Anthropometric measures included the determination of height, body mass and bicep girth (i.e. mid-upper arm circumference), according to Tittel and Wutscherk (1972).
 
Study design:
Two separate series of biceps curl exercises were performed in random order by each athlete. Each series consisted of three sets with three repetitions in each set. In one series, the exercise was performed with vibratory stimulation in the second set; in the other series, the exercise was performed without vibratory stimulation. The maximal and mean power values of three repetitions were recorded after each set.
The athletes performed a general warm-up for 5± 7 min, including indoor running (2± 3 min), general calisthenics (1± 2 min) and exercises for the upper extremities (2 min). They then performed 8± 10 repetitions of the biceps curl with a low to medium load (20± 40% of body weight) to adapt to the exercise and equipment.
Then, 3± 5 attempts were performed at increasing weight to determine the one-repetition maximum value. The athletes were then allowed to rest for 15 min, during which anthropometric measures were taken and informed consent was obtained.
A weight equivalent to 65± 70% of the one-repetition maximum value was selected. Two series of exercises were performed, with the interval between them allowing full recovery (8± 15 min); the duration of the rest period was determined by the athletes. The exercise rate within a set was approximately one repetition every 2 s; the period of rest between sets was 2± 3 min. The athletes were asked to perform each repetition with maximal eV ort.
 
Data analysis :
The acute eV ect of vibratory stimulation was assessed as the diV erence between the power values in the second set with vibratory stimulation and in the Wrst set without vibratory stimulation. Similarly, the residual acute eV ect was assessed as the diV erence between the power values in the third (after vibratory stimulation) and Wrst (before vibratory stimulation) sets. These diV erence values in the Wrst and second series were subjected to repeated-measures analysis of var iance with group (elite vs amateur athletes) as a between-participants factor. SigniWcance was accepted at P < 0.05. Paired t-tests and Pearson product± moment correlations were computed to establish diV erences and relationships between the two series for maximal and mean power.

 
Maximal and mean power and two series
 
Results :
 
The means and standard deviations of maximal and mean power in the Wrst set of each series were compared using paired t-tests (Table 2). No signiWcant diV erence between the two series was found for the elite or amateur groups. The test± retest correlation coeYcient between the two series was 0.97 for maximal power and 0.97 for mean power of the biceps curl exercises.
 
The repeated-measures analysis of variance showed that mode of exercise (with vs without vibratory stimulation) had a signiWcant eV ect for mean power (F1,26 = 59.2, P < 0.001) and for maximal power (F1,26 = 56.3, P < 0.001). Also, the group factor (elite vs amateur) resulted in a signiWcant eV ect for maximal power (F1,26 = 4.41, P < 0.04). These eV ects are shown in Figs 2 and 3.
 
In the elite athletes, vibratory stimulation resulted in an average gain in maximal power of 30.1 ± 15.3 W and in an average gain in mean power of 29.8 ± 16.6 W; these values correspond to increases of 10.4% and 10.2% respectively. The series without vibratory stimulation revealed a non-signiWcant decrease in these values of 1.1 and 2.6 W, respectively. In the amateur athletes, the gains in maximal and mean power owing to vibratory stimulation were 20.0 ± 16.9 and 25.9 ± 18.9 W respectively; these values correspond to increases of 7.9% and 10.7% respectively. The maximal and mean power decreased by 7.4 W without vibratory stimulation. We also observed that the immediate acute eV ect in maximal power was signiWcantly greater in the elite than in the amateur athletes (F1,26 = 7.32, P < 0.01).
 
Similar analyses of variance were applied to the mean and maximal power diV erences between the third and Wrst sets in the two modes of exercise (with vs without vibratory stimulation). Group (elite vs amateur exercise mode and the interaction eV ects were all nonsigniWcant (P > 0.05) (see Fig. 4). Therefore, vibratory stimulation in the second set resulted in an insigniWcant residual eV ect in the third set.

 
Discussion:
 
An increase in contraction strength induced by the tonic vibration reXex has been widely documented. Hagbar th and Eklund (1966), Johnston et al. (1970) and Arcangel et al. (1971) all reported that muscle force registered during isometric contractions increased because of local vibratory stimulation applied to the muscle or tendon. A similar result was noted by Armstrong et al. (1987), who administered 40 Hz superimposed vibration and registered an increase in grip force of 52%. These studies applied vibratory stimulation to muscles which contracted with low to intermediate levels of eV or t. Matyas et al. (1986) reported the facilitation of maximum voluntary contraction caused by 50 Hz tendon vibration in hemiplegic patients. Samuelson et al. (1989) reported a reduction in endurance of a maximal isometric contraction and a decrease in maximal force with 20 Hz superimposed vibration, in contrast to the results of the present study.
 
Three factors may be attributed to the acute vibratory stimulation eV ect: (1) the motor pool activation, (2) the frequency of vibratory stimulation and (3) the initial length of the stimulated muscles. Matthews (1966) and Brown et al. (1967) found that vibratory stimulation excites the primary aV erent endings of the muscle spindles which activate a-motoneurons. Unlike local vibratory stimulation, the low-frequency superimposed vibratory wave propagates from the distal links to muscles located proximally and activates a greater number of muscle spindles. Their discharge activates a larger fraction of the motor pool and recruits many previously inactive motor units into contraction.
There is evidence that an increase in vibration frequency evokes a proportional increase in muscle tension (Matthews, 1966). However, the high-frequency component of vibration is absorbed by soft tissues, whereas the low-frequency component propagates through the human body tissues (Pyykko et al., 1976). Therefore, on the one hand, the eV ect of vibratory stimulation depends on the frequency; on the other hand, low-frequency vibratory waves can only propagate through the kinetic chain to proximal muscle groups and activate them. It is likely that vibratory stimulation at a frequency of 40± 50 Hz may be optimal to combine two diV erent tasks: (1) transmission of vibration and (2) muscle activation before and during voluntary contraction (Issurin and Temnov, 1990).
 
It is known that stretched muscles are more sensitive to vibratory stimulation and contract more strongly (Eklund and Hagbar th, 1966; Johnston et al., 1970; Rohmert et al., 1989). In Samuelson and co-workers’ (1989) study, the superimposed vibration was administered during knee-joint extension in the sitting position with a knee angle of 90° (1.57 rad). Hence, the quadriceps muscle was not in a stretched position. This may be one reason why Samuelson et al. did not Wnd any facilitatory eV ect of vibration on maximum isometric contraction. Another reason may be the lower vibratory stimulation frequency of 20 Hz they used. In contrast, the present study was conducted with extremely stretched muscles before each repetition. This could be why we observed a power increase during vibratory stimulation.
Post-vibratory residual eV ects have also been widely documented in the literature. Arcangel et al. (1971) reported a substantial and signiWcant increase in the Achilles tendon reXex after 10 and 20 s tendon vibration. Cafarelli and Layton-Wood (1986) reported an improvement in force sensation in fresh muscles after short-term vibration. The reasons for such eV ects are probably associated with an increase in the sensitivity of the muscle receptors to excitation. Elevation of muscle temperatures resulting from the friction between vibrating tissues (Oliveri et al., 1989) and vibrationinduced increases in blood Xow (Wakim, 1985) may also contribute to the post-vibratory eV ect. In fact, the residual gain in power observed in this study was relatively small and not statistically signiWcant. Relatively short-term vibratory stimulation, as implemented in this study (6± 7 s), is probably not suYcient to aV ect subsequent muscle strength.
 
The diV erence in muscle response between the elite and amateur athletes was statistically signiWcant. The average gain in maximal power owing to vibratory stimulation was greater among the elite athletes. The reason for this marked diV erence may be associated with the higher sensitivity of muscle receptors and the central nervous system of elite athletes to additional stimulation.
 
In summary, the superimposed vibratory stimulation allowed a signiWcant facilitation of an explosive strength exertion. This approach may be useful in identifying the hidden reserves of an athlete and in augmenting an acute eV ect of power training
 
 
New trends in training science
Power Plate Studies

Introduction:

The adaptive responses of the human body to training stimuli have been investigated in depth over the past few years. Thanks to the research carried out in different parts of the world, we know that the adaptation to the training stimulus is related to the modification induced by the repetition of daily exercises, which are specific to the movements executed (Edington and Edgerton, 1976). These adaptations are related to the fact that human skeletal muscle is a specialised tissue, which modifies its overall functional capacity in response to regular exercise with high loads (McDonagh and Davies, 1984).

The above-mentioned findings all suggest that resistance exercise can be an effective means of enhancing muscular performance. In this context it should be noted that changes within the muscle itself constitute the most important adaptation to resistance exercise (Sale, 1988; Behm, 1995).

In fact, strength training responses have been shown to be mediated by both neurogenic and myogenic factors (Moritani and De Vries, 1979). Neural adaptations have been shown to be the first changes to occur in the muscle, permitting gains in muscle strength and power in the early stages of a resistance exercise programme in the absence of an increase in the cross-sectional area of the muscle (Behm, 1995; Costill et al. 1979). It has also been demonstrated that specific adaptations occur depending on the training programme implemented (Sale and McDougall, 1981).

Strength training can therefore be considered as a training stimulus, which produces specific adaptations in human skeletal muscles, based upon the protocol, utilised for training. The specificity of training effect from strength work has been underlined by many authors (Sale, 1988; Behm, 1995; Morrisey et al. 1995; Bandy et al. 1990) and the velocity specific effect has been highlighted as the most interesting outcome of resistance exercise programmes. However, even if the mechanisms underlining this velocity specific effect have not been clearly defined, most importance has been given to the neural adaptations such as improved co- ordination, increased activation of the prime mover muscles (Moritani and De Vries, 1979), recruitment and synchronisation.

The aim of most resistance training programmes for elite athletes is to improve the mechanical power output for a given movement, or to enhance speed. In thinking about a boxing punch, a handball throw, a volleyball spike, or a shot putt, these movements involve the exact timing of many muscle groups and are characterised by many coordinative factors. However, boxers, handball and volleyball players and shot putters undergo strength training sessions with the aim of improving their level of performance. Any ideas injected into the deve1opment of a training plan for such sporting disciplines must therefore be related to the specificity of each of the movement patterns involved.

An optimal training plan should be developed with some general exercises to improve muscle strength and some specific exercises to improve muscle power and speed. The mechanical basis of strength training is thus very simple: overload the biological system in order to determine specific adaptations. Since the environment of our biological sys- tem is characterised by the fact that we are all subject to the action of gravity which pro- vides the
major portion of the mechanical stimulus responsible for muscle structure in everyday life and training, we need to alter the biological system by increasing the gravitational load in order to enhance strength. It should be remembered that specific programmes for strength and explosive power training employ exercises performed with fast, abrupt variations of gravitational acceleration (Bosco, 1992).

To give an example, the simulation of hypergravity (wearing vests with extra Ioads) has been utilised for improving explosive muscle power (Bosco et al. 1984; Bosco 1985). The overload or simulation of hyper- gravity are not the only means for changing the gravitational conditions. In fact, mechanical vibrations applied to the body can produce changes in the gravitational conditions and determine specific responses. The studies conducted from our group were aimed to investigate the effects of vibrations applied to the whole body or to part of it in terms of hormonal responses, explosive power, neuromuscular performance and strength. This article aims to present the latest findings on vibrations and some considerations for their use in the athletic setting.


The effects of vibrations on human performance:

The first study carried out by our group was conducted to study the effects of whole body vibrations on the mechanical power of the lower limbs. For this aim, fourteen active subjects involved in team sports training voluntarily participated in the experiment. After being randomly assigned to either an experimental or a control group, they were tested on an Ergojump (MAGICA, Rome, Italy) for assessing vertical jumping ability. The treatment group underwent whole body vibrations at a frequency of 26 Hz (displacement = 10mm, acceleration = 54 mos-1 ) for 5 repetitions lasting 90 sec. each and separated by an interval of 40 sec.. This procedure was continued for 10 days, the duration of vibration series being extended by 5 sec. every consecutive day up to a total of 2 min. per set.
 
Continuous jumping graph

Considering the fact that the 5sCJ test is a testing protocol characterised by a stretch-shortening cycle (SSC), a small angular displacement and fast stretching speed, it can be considered that since leg extensors muscles experience fast stretching this may elicit a concurrent gamma-dynamic fusimotor input that would enhance primary afferent discharge. Taking this into account, it was argued that the biological mechanism pro- duced by vibrations was similar to the effect produced by explosive power training (Bosco etal.1998).

After the latter experiment, another study was conducted to observe the behaviour of human skeletal muscle following one session of 10 minutes application of whole body vibration treatment. In this case the subjects were six female elite volleyball players. They
were tested before and after the treatment while performing a maximal dynamic leg press exercise with increasing loads (70, 90, 110 and 130 kg respectively) with a sensor machine (Muscle Lab. Ergotest, Langesund Norway) able to calculate average velocity, average power and average force corresponding to load displacements (for details see Bosco et al. 1995).

After the control test, one leg was randomly assigned to the experimental treatment consisting of vibrations and the other was considered as a control.

Results showed an alteration of the V-F and P-F relationships after VT lasting only 10 min (see Figure 3; Bosco et al. 1999a). In fact, both relationships were shifted to the right indicating a clear enhancement of performance which was previously observed only after several weeks of resistance training (i.e. Coyle et al. 1981; Hakkinen and Komi, 1985).
The above mentioned findings are all related to the effectiveness of vibrations in enhancing performance in the lower limb muscles.Force vs Average Velocity
Vibrations applied to the upper limbs have also been found to produce an enhancement of neuromuscular performance. In fact, in a study conducted on 12 national level boxers, vibrations applied to the arm determined an increase in average mechanical power (See Figure 4) during a maximal arm curl with an extra load of 5% body mass. In this study, in which the treatment consisted of five repe- titions of vibrations lasting 1 min. each with 1 min. rest interval at a frequency of 30 Hz, EMGrms was collected during the arm curl test and during the treatments. The results showed a significant decrease in the EMGjPower ratio following the treatment, suggesting an improvement in the neural efficiency of the neuromuscular system (see Figure 5). Moreover, the EMGrms recorded during each set of the vibration treatment reached values higher than 200% of the baseline values.
Mechanical power measured during arm curl
 
Treatment of Chronic Back Pain
Power Plate Studies
A Randomized Controlled Trial
Jo¨rn Rittweger, MD,* Karsten Just, MD,† Katja Kautzsch, MsPsych,‡ Peter Reeg, MD,§ and Dieter Felsenberg, PhD
Study Design. A randomized controlled trial with a 6-month follow-up period was conducted.

Objective. To compare lumbar extension exercise and whole-body vibration exercise for chronic lower back pain.

Summary of Background Data.
Chronic lower back pain involves muscular as well as connective and neural systems. Different types of physiotherapy are applied for its treatment. Industrial vibration is regarded as a risk factor. Recently, vibration exercise has been developed as a new type of physiotherapy. It is thought to activate muscles via reflexes.

Methods. In this study, 60 patients with chronic lower back pain devoid of “specific” spine diseases, who had a mean age of 51.7 years and a pain history of 13.1 years, practiced either isodynamic lumbar extension or vibration exercise for 3 months. Outcome measures were lumbar extension torque, pain sensation (visual analog scale), and pain-related disability (pain disability index).

Results. A significant and comparable reduction in pain sensation and pain-related disability was observed in both groups. Lumbar extension torque increased significantly in the vibration exercise group (30.1 Nm/kg), but significantly more in the lumbar extension group (+59.2 Nm/kg; SEM 10.2; P < 0.05). No correlation was found between gain in lumbar torque and pain relief or painrelated disability (P > 0.2).

Conclusions. The current data indicate that poor lumbar muscle force probably is not the exclusive cause of chronic lower back pain. Different types of exercise therapy tend to yield comparable results. Interestingly, wellcontrolled vibration may be the cure rather than the cause of lower back pain. [Key words: back pain, physiotherapy, resistance training, treatment] Spine 2002;27:1829–1834
 
In Western countries, chronic lower back pain (CLBP) constitutes a major health care problem. Moreover, it challenges the social insurance systems. In Germany, for example, CLBP is one of the most important reasons for early retirement.12

It is thought that CLBP emerges from acute pain of muscle and connective tissues, which persists in approximately 30% of acute cases and becomes chronic.30 This generally occurs without specific damage or symptoms that could be shown through imaging or neurophysiological techniques. Besides somatic factors, psychological and social factors play an important role in chronification. 20 Therefore, CLBP often is referred to as “unspecific.”

The pathophysiologic emergence of CLBP is as unclear as its diagnostic criteria. Besides pain sensation, findings generally encountered in patients with CLBP are reduced lumbar flexibility, reduced flexion–relaxation observed in healthy subjects, and static balance. 2,7,14,18,28 Hence, it is mostly accepted that muscular systems as well as connective tissues and neural systems are involved in the pathophysiology of CLBP.

An often-repeated view is that different initial damages may lead to a muscular hypertonus, and hence to an inadequate circulation, which promotes and enhances pain. In the long run, this leads to immobilization,20 followed by muscular atrophy17 and pathophysiological loading patterns, which further establish pain chronification.1,19

Although exercise therapy appears to be without benefit in the acute state, some types of exercise seem to be effective once the pain has become chronic.33 Among these types are conventional physiotherapy, medical resistance training, stretching, or freely chosen exercise. 10,13,15,31 In particular, lumbar extension has turned out to be effective.23

Whole-body vibration exercise (VbX) is a new type of exercise currently being tested in sports, geriatrics, and rehabilitation.3,8,24 It is thought to elicit muscular activity via stretch reflexes. Recently, we have shown that metabolic power increases during whole-body vibration exercise,26 and that this VbX-related metabolic power is augmented by the application of additional loads to the shoulders,25 suggesting an enhanced activity of the trunk muscles.

In experimental acute lower back pain, stretch reflexes are unchanged, whereas EMG modulation during voluntary lumbar flexion–extension clearly is affected.34 Hence, we hypothesized that VbX could elicit trunk muscle stretch reflexes, and thus be a means of activating and strengthening these muscles. It has been shown that  vertical platform vibration of 3 to 10Hz evokes electrical activity of the erector spinae muscle, indicating an increased muscular torque caused by vibration.27 The researchers, however, discussed their results mainly with respect to the emergence and chronification of lower back pain. Generally, industrial and nonindustrial exposure to vibration is viewed as a risk factor for CLBP rather than its cure.29

Personal experience, however, and single case observations have shown that controlled VbX may indeed be beneficial for lower back pain. Therefore, the current study was designed to test the applicability of VbX for patients with CLBP in a randomized therapy control study. Isodynamic lumbar extension exercise (LEX), an established intervention for patients with CLBP, was used as a reference therapy.
 
Material and Methods

Study Survey. The minimum sample size was computed according to Dixon and Massey.6 Improvement in pain and the standard deviations on LEX were taken from Leggett et al.16 Given an alpha of 0.05 and a beta of 0.1, the sample size required was 23. With an expected dropout of approximately 20%, 60 female and male patients with CLBP were recruited by a local newspaper announcement. The inclusion criteria required lower back pain without any specific underlying disease, either continuously for more than 6 months or intermittently for more than 2 years, and an age of 40 to 60 years.

An orthopedic and, if required, radiologic examination was performed to exclude specific lesions or dysfunctions. The medical exclusion criteria specified vertebral osteoporosis, spinal tumors or metastases, acute vertebral disc herniation, recent fractures of the axial skeleton, inflammatory diseases of the spine, cauda equina syndrome or progressive neurologic deficits, rheumatoid arthritis, osteogenesis imperfecta or other generalized bone diseases, a poor state of health because of tumors or inflammatory diseases, heart failure (NYHA III or IV), recent abdominal surgery, hip or knee endoprothesis or other metal implants, aortic aneurism, recent venous thrombosis, arterial occlusive disease (II or higher), and pregnancy.

Patients also were excluded if they were currently applying for early retirement or taking pain medication regularly (once per day or more often). During the study, patients were asked not to engage in any other fitness or training program, or any other therapy (including pain medication) for their back pain.

After the patients had given their written informed consent, they were randomly assigned to a group that practiced VbX or a group that practiced LEX. The training for both groups was free of charge. Among the 60 participants recruited, 19 were smokers and 41 were nonsmokers. In terms of work, 38 of the participants were employed, 10 self-employed, 5 occupied as housewives, 5 early retired, and 2 unemployed. They had a mean age of 51.7 ± 5.8 years, and mean CLBP history of 13.1 ±10.0 years. During the study, nine subjects dropped out for nonspecific reasons. One subject who did not pass the honesty criteria11 in the psychological test for depression (ADS) was discarded from further analysis.
 
Training:  In both groups, 18 exercise units were performed within 12 weeks: 2 units per week during the first 6 weeks and 1 unit per week thereafter. This schedule was maintained very strictly. On the average, the participants spent 12 weeks and 4 days in the training phase.

The participants performed LEX on an LE Mark1 (MedX, Gainesville, FL). After 1 minute of warming up with lumbar extension (61Nmfor the women and 102Nmfor the men), the participants rested for 1 minute. Then they exercised, performing repetitive contraction cycles at a constant speed with a torque corresponding to 50% of the baseline maximum isometric values. As soon as the patient was capable of performing the LEX longer than 105 seconds (11 cycles), the load was increased in steps of 2.5 kg. After completion of the LEX units, an additional resistance exercise of the abdominal and thigh muscles was performed (sit-ups and leg presses).

The performance of VbX was on a Galileo2000 (Novotec, Pforzheim, www.galileo2000.de). This exercise device has been described elsewhere.26 In brief, it consists of a platform that oscillates around a resting axis between the subject’s feet. Hence, the amplitude can be controlled by adjusting the foot distance. As applied in the current study, the device had a maximum amplitude of 6 mm, a vibration frequency set at 18 Hz, and 4 minutes of duration for each exercise unit at the beginning, with 2 minutes of warm-up on the vibration platform (mere standing or squatting with small amplitude). The exercise duration was increased in steps to 7 minutes. During the exercise units, the subject performed slow movements of the hips and waist, with bending in the sagittal and frontal planes, and rotation in the horizontal plane. After three sessions, all the participants exercised at the maximum amplitude of 6 mm. In a further progression of the program, increasing weights up to 5 kg were applied to the shoulders in subsequent sessions. The complete exercise instructions are given in the Appendix.
 
Measures of Outcome: The primary measures of outcome were pain sensation and pain relief. Pain sensation was assessed on a visual analog scale (P-VAS) ranging from 0 (pain free) to 10 (maximum pain). At the beginning of each visit, P-VAS was assessed using a slide with the numerical scale hidden from the patient. The patients were asked to visualize their worst back pain in the preceding 24 hours. Pain relief was assessed as the P-VAS difference (dVAS) between the last and the first visit. The secondary measures of outcome were the pain-related limitation, the maximum isometric lumbar extension torque, the range of motion (ROM) in lumbar flexion and extension, and the general depressivity. Pain-related limitations in everyday life were assessed by the pain disability index (PDI). This questionnaire of seven questions was answered on a visual analog scale with a range from 0 to 10.5 To quantify the improvement in pain-related limitation, dPDI-0 was computed as the difference between the PDI values immediately after the training phase and the baseline values. The same computation 6 months after completion of the training program yielded the variable dPDI-6. The maximum isometric lumbar extension torque was measured using the LE Mark1 lumbar extension machine on which the LEX was performed. Maximum voluntary isometric lumbar extension torque was assessed in several positions, starting with 72° of flexion, and then moving in 12° steps to full extension. Integration of the values in all positions and division of the sum by the patient’s body weight yielded the lumbar extension torque (LET). Two measurements were obtained before the intervention, with at least a 2-day interval between. The greatest LET value was taken. The same procedure was performed after the intervention, yielding the post values. Gain in torque (i.e., increases in LET after the training phase as compared with the baseline values) was computed as the difference in the values (dLET). The range of motion was assessed in 3° steps on the lumbar extension machine according to the painrelated tolerance of the patient. The tendency to depression was assessed by a general depression scale, the Allgemeine Depressions Skala (ADS), which is based on 20 items that cover emotional, motivational, cognitive, somatic, and motor symptoms.11 The ADS is a validated German equivalent to the CES-CD scale. The normal range lies between 40 and 60. Changes in ADS from baseline to completion of the training phase were computed as for PDI, yielding the variables dADS-0 and dADS-6.
 
Statistical Analyses. Statistical analyses were conducted with SPSS for Windows, version 10.0. Differences between groups in interval-scaled and normally distributed data were checked with Student’s t test or ANOVA. Otherwise, the Wilcoxon or Mann–Whitney tests were applied. Differences in the P-VAS over treatment weeks within groups were tested with Friedman’s test. Spearman’s rank correlation coefficient was computed to test correlations between LET on inclusion, dLET, dVAS, and dPDI-0.

Results :

Baseline Data: 
Dropout and exclusion based on honesty criteria in psychological testing yielded 25 patients in each group (for survey see Table 1). There was no significant group difference in baseline data in weight, P-VAS, PDI, ADS, or LET. The VbX group, however, was significantly older and taller on the average.
BaseLine Data
Torque and Range of Motion After completion of the training program, isometric lumbar torque, as measured by LET, increased significantly both in the LEX (59.2 Nm/kg; SEM, 10.2) and VbX (30.1 Nm/kg; SEM 5.7) groups. This increase in LET (dLET) was significantly more pronounced in the LEX group (P < 0.05). In the LEX group, seven participants had an increased lumbar ROM after completion of the program, whereas only three participants in the VbX group had a gain in ROM. This difference, however, was not significant.
 
Pain Sensation: In both groups, there was a significant decrease in pain sensation, as measured by P-VAS (P<0.001, Friedman’s test). In the LEX group, P-VAS decreased from 4.52 ± 2.21 on the first visit to 1.20 ±1.76 on the last visit (Figure 1). In the VbX group, P-VAS decreased from 4.16 ±1.86 to 1.40 ±1.83. No significant difference in P-VAS values was observed between the groups in any of the weeks (P >0.2 in all cases, Mann–Whitney test).
Pain on VAS
 
The influence of whole body vibration on the mechanical behaviour of skeletal muscle
Power Plate Studies
Bosco C. 1,2,3, M. Cardinale, 4, R. Colli 5, J. Tihanyi 3, S.P. von

Duvillard 6, A. Viru 7

1 University of Rome -Tor Vergata, Fondazione “Don Gnocchi”, Rome, Italy
2 Department of Biology of Physical. Activity, University of Jyvaskyla, Finland
3 Department of Biomechanics, Hungarian University of Physical Education, Budapest, Hungary
4 Research and Study Center, Italian Handball Federation
5 Research and Study Center Italian Cycling Federation
6 Department of HPER, University of North Dakota, Grant Forks, USA
7 Institute of Exercise Biology, University of Tartu, Estonia

Key Words: Vibration, muscle mechanics, muscle power

Correspondence:
Carmelo Bosco Ph.D.
C /o Societa Stampa Sportiva
Via G.Guinizzelli,56
00152 Roma,Italy
Phone: +39335283024;+393482680265
Fax : + 39 6 5806526
E-mail : C.Bosco@ Quipo.it

ABSTRACT

The aim of this study was to investigate the effects of whole body vibrations on the
mechanical behaviour of human skeletal muscles. For this purpose, fourteen
physically active subjects were recruited and randomly assigned to an experimental
(EG) and a control group (CG). The EG was treated fur ten days with 5 sets of
vertical sinusoidal vibrations lasting up to two minutes each, for a total volume of
ten minutes per day. The subjects of CG were asked to maintain their normal
activity and avoid strength or jumping training. Subjects were tested at the
beginning and at the end of the treatment with specific jumping tests performed on
a resistive platform. Results showed remarkable and statistically significant
enhancement in the EG of the height of the best jump (1.6 %, P<O.5), the
mechanical power of the best jump (3.3 %, P<O.5) and the average jumping
height during 5s Cj (12 %, P<0.01). In contrast, no statistically significant
variations were noted in the CG. Consequently, it was suggested that the effect of
WBV treatment elicit fast biological adaptation connected to neural potentiation.

INTRODUCTION:

The adaptation to the training stimulus is related to the modification induced by
the repetition of the daily exercise, which are specific for the movement executed
[12]. Strength training response has been shown to be mediated by both
neurogenic and myogenic factors [22]. The first phase of adaptation is
characterised by an improvement of neural factors, while the myogenic factors
becomes more important as the adaptations continues over several months (e.g.
[20]. Enhancement of explosive power performance (e.g. jumping abilities) and the
corresponding biological adaptations to a specific training stimulus are still not
understood. Gravity normally provides the major portion of the mechanical
stimulus responsible for the development of the muscle structure during everyday
life and during training. It should be remind, that strength and explosive power
training specific programs are based on exercises performed with rapid and
violent variation of the gravitational acceleration [8 ]_ In this connection, simulation
of hypergravity (wearing vests with extra loads) conditions has been utilised for
enhancement of human explosive muscle power [5,6], On the other hand, changes
of the gravitational conditions can be produced also by mechanical vibrations
applied to the whole body. Thus, in light of the above observations, it was
assumed that application of whole body vibration to physical active subjects could
influence the mechanical behaviour of the leg extensor muscles .

METHODS:

Fourteen subjects voluntarily participated to the study, they were physically active
and were engaged in team sport training program 3 times a week. The subjects
were not engaged in strength and explosive power training but participated
regularly for tactical and technical training program according to the discipline
practised (handball and water polo). They were equally divided into two groups:
an experimental group (EG) and a control group (CG). Each subject was
instructed on the protocol and signed an informed consent, approved by the ethical
committee of the Italian Society of Sport Science, to participate to the experiment.
Subjects with previous history of fractures or bone injuries were excluded from the
study together with the ones under the adult age. Table 1 presents physical
characteristics of the subjects.
Procedures: Anthropometric measures (height and weight) were recorded
together with the age of the subjects. Following this phase a ten minutes warm up
was performed consisting of 5 minutes of bicycling at 25 kmh-1 on a cycle
ergometer (Newform s.p.a., Ascoli Piceno, Italy) and five minutes of static
stretching for the quadriceps and triceps surae muscles. After the warm up, the
subjects peformed the followings jumping exercises: counter movement jump
(CMJ) and 5s of continuous jumping (5s CJ).The flight time (tf) and contact time
(tJ of each single jump were recorded on a resistive (capacitative) platform [4]
connected to a digital timer (accuracy f 0.001s) (Ergojump, Psion XP,
MA.GI.CA.Rome, Italy). To avoid unmeasurable work, horizontal and lateral
displacements were minimised, and the hands were kept on the hips through the
gravity above the ground (h in meters) in were measured from flight time (tf in
seconds) applying ballistic laws:
h=t&g&3-‘(m)
where g is the acceleration of gravity (9.81 m . sm2) During CJ exercises the subject
were required to perform the maximal jumping effort minimising knee angular
displacement during contact. From the recordings of tf and b the average
mechanical power (AP), average rise of center of gravity (AH) were calculated for
the total 5s continues jumping. From 5s CJ the best jumping performance was
selected and maximal mechanical power (PBJ) as well as the highest rise of center
of gravity (HBJ) were obtained using the equation introduced by Bosco et al [4] :
AP = Tf + T * 24.06 e ( T, )-’ (W * kg brn-‘) (2)
where P is the mechanical power per kilogram of body mass, Tf the sum of the
total flight time, Tt the total working time (5s), and T, the sum of the total contact
time. The average height during 5s CJ and the HBJ were computed using formula
1.
&~~o&cibibi&v of rne~~~~~rneni,~: The reproducibility of the mechanical power
test (5s CJ) and CMJ performances were high with respectively r =.95 and r =,90
[4,27]
S&II~~~Y~~CG~ meGzu& Conventional statistical methods used included mean, standard
deviation and paired Student’s t-test. The level of significance was set at p<.O5.
T’~~a&zet?f ~R.xxx.&~~s:Subjects were exposed to vertical sinusoidal whole body
vibration (WBV) using the device called GALiLEO 2000 (Novotec, Pforzheim,
tiermany) . The frequency of the vibrations used in this study was set at 26 Hz
(displacement = 1 Omm; acceleration = 27 m - s-‘). The subjects were exposed five
5
times for a duration of 90s with 40s of rest between the treatment each. This
procedure was repeated for ten days, each day five seconds were added for each
treatment up to a total of 2 minutes per position. Following the ten days the
subjects of both groups were again tested and data were statistically analysed.
Type of treatment employed: The first applicaticm was performed in the standing
position with the toes on the vibrations platform. The second bout was performed
with the subject in the half squat position. The third application was realised with
the feet rotated externally on the vibration platform. The knee angle was pre-set at
900 flexion. The fourth treatment was performed with the subjects standing on the
leg on the right side of the vibration platform with the knee at 90” flexion. Finally
the fifth application was given while the subjects standing on another leg on the
left side of the vibration platform with the knee at 90” flexion. During the 4th and
5th treatment subjects were allowed to keep themselves in balance with the aid of
a bar mounted on the platform. During all the treatments the subjects wear
gymnastic-type shoes to avoid bruises. The E group was treated with WBV for ten
days, the C group was not treated during the project and was asked to maintain
their typical activities. Testing procedures were administered at the beginning and
at the end of the experiments for both E and C groups.

RESULTS
After almost two weeks of regular technical and tactical training program, the
subjects of the C group, as expected, failed to showed changes in any of the
mechanical or anthropometric parameters studied (P>0.05). The jumping height in
CMJ remained the same in E group after 10 days of WBV (Table 2). This
treatment, in contrast, produced remarkable and statistical significant (P< 0.05)
enhancement of the HBJ ( Fig. 1) and the PBJ ( Fig. 2). In addition, the average
height during 5s CJ was also improved in E group, demonstrating a statistical
significant difference of P< 0.01 (Table 2). On the other hand, the average power
developed during 5s CJ failed to demonstrate statistically significant change after
the treatment (Table 2).

DISCUSSION:
Less than two weeks of regular tactical and technical training programme, as
expected, did not induce any modification in the mechanical properties and
anthropometric profile of the control subjects_ This is not a surprising findings,
since no changes, in jumping performances, was noted after four weeks either in
physical active subjects [ 14], or in volleyball players [2]. In contrast, a remarkable
improvement of the neuromuscular characteristics studied was observed after the
WBV period in the E subjects. Significant enhancement was noted for the HBJ
(Fig. 1), PBJ (Fig. 2) and the average jumping height during 5s CJ (Table 2). On
the other hand, no changes were noted for the AP during 5s CJ. It should be
remind that, during the continues jumping test [4], the average jumping height
possessed higher significance and sensitivity than AP in differentiating athletes
[28] or in revealing the effect of creatine supplementation [9]. In addition no
changes in CMJ were noted after the vibration treatment in E group. Apparently
these are contradictory results. However, a reasonable explanation can be found
analysing the mechanical behaviour of the leg muscles during CMJ and 5s CJ. In
fact, both exercises are characterised by the so called stretch- shortening cycle
(SSC). This means that, before the concentric work (pushing phase), leg extensor
muscles are actively stretched (eccentric phase) in both exercises. Nevertheless,
the neuromuscular activation in CMJ is different than that found in 5s CJ. The
CMJ is characterised by large angular displacement and slow stretching speed (3-
6 rad l s-l) [3], while 5s CJ are performed with fast stretching speed (1 O-12 rad
l s-l) and small angular variation [7]. This means that, only in 5s CJ the leg
extensor muscles experience fast stretching which may elicit a concurrent gamma
dynamic fusimotor input that would enhance primary afferent discharge. This
notion is supported by the studies of Bosco, et al. [3], who showed that during
eccentric phase of drop jumping exercises (similar to 5s CJ), EMG activity was
high and comparable to maximal concentric ballistic movements. Thus there is a
possibility of enhanced neural potentiation either via spinal or cortical reflex. On
the other hand, it is likely that CMJ is not a suitable activity to elicit stretch reflex,
since high EMG activity has not been recorded during the stretching phase (e.g.
On the background of these considerations it is likely that the effect of WBV
treatment elicits a biological adaptation connected with neural potentiation. Thus,
it can be argued that, the biological mechanism produced by vibration treatment is
similar to the effect produced by explosive power training (jumping and bouncing
exercises). In fact, this suggestions is consistent with knowledge that mainly the
specific neuronal components and its proprioceptive feedback mechanism are the
first structure to be influenced by specific training [2,14].
Training with high stretching loads may improve stretch-reflex potentiation
and increase the threshold of firing for the Golgi tendon organs (GTO). The latter
one, would then improve the possibility to recruit greater amount of motor units
during eccentric phase [2]. Furthermore, there are several ways in which the
explosive power training can infIuence neural activation, for example by
increasing the synchronisation activity of the motor units [21]. I t cannot be
excluded also an improvement of co-contraction of synergist and increased
inhibition of antagonist muscles. In any case, what ever it is the intrinsic
mechanism which enhance neuromuscular activation after specific explosive power
training, it is likely that, the vibration treatment have to improve the
proprioceptors’ feedback mechanism, since it is filly operating and elicited
during 5s GJ performance, which was enhanced after WBW. On the other hand,
the lack of modifications observed in GMJ test after the VBV treatment suggests
that the proprioceptors’ feedback mechanism is not strongly operating in CMJ . In
fact, this exercise is strongly influenced by the voluntary recruitment capacity and
by the fiber type composition of leg extensor muscles [ 1]. However, there is no
doubt that stretch reflex play an important role in stiffness regulation [IS], and that
muscle spindles and GTO operate actively in the control of muscle length and
tension [ 16]. Consequently, it can be suggested that WBV treatment may affect
dramatically the neuromuscular functions and properties which are regulating
muscle stiffiess through the control of length and tension.
During vibration the body and the skeletal muscle undergo to small changes
in muscle length. Facilitation of the excitability of spinal reflex has been elicited
through vibration to quadriceps muscle [ 11). The idea that vibration may elicit
excitatory flow through short spindle - motoneurons connections in the overall
motoneuron inflow has been suggested also by Lebedev and Peliaksv [IX]
pointed on the possibility. It has been shown also that vibration drives alphamotoneurons
via la loop, producing force without descending motor drive [25].
Burke et al. [ 101, suggested that vibration reflex operates predominantly or
exclusively on alpha motoneurons and that it does not utilise the same cortically
originating efferent pathways as are in the performance of voluntary contractions.
In addition, the results of Kasai et al. [ 17] are consistent with vibration induced
activation of muscle spindle receptors not only in the muscle where vibration is
applied, but also to the nearest muscles. Mechanical vibration (10 - 200 Hz)
applied to the muscle belly or the tendon can elicit a reflex muscle contraction (e.g.
[ 13]). This response has been named tonic vibration reflex (TVR). It is not
known wheather it can be elicited by low WBV frequency (l-30 Hz), even if it
has been suggested to occur [26].
Finally, it should be remind that not only nervous tissue, but also muscle
tissue can be affected by vibration [ 23]. In fact, 5 hours daily for 2 days of
vibration exposure at two different frequencies were sufficient to induce
enlargement of slow and fast fibers in rats [24].
In the present study, no neurogenic potentiatian or modification in the
morphological structure of the muscles was demonstrated since neither EMG
recordings nor muscle biopsy sampling were performed. However enhanced
mechanical behaviour during 5 s CJ, strongly suggests that a neurogenic
adaptation have occurred in response to the vibration treatments. Even if the
intrinsic mechanism of the adaptive response of neuromuscular functions to WBV
could not be explained,
importance. Adaptive
hypergravity conditions
the effectiveness of the stimulus seems to have relevant
response of human skeletal muscle, to simulated
(1 . 1g), applied for only three weeks, caused a drastic
enhancement of the neuromuscular functions of the leg extensor muscles [6].
Chronic centrifugal force (2 g) for 3 months [ 19] has initiated conversion of fiber
type. In the present experiment, the total length of the WBV application period
was not very long (only 100 minutes), the perturbation of the gravitational filed
was rather consistent (2.7 g )_ An equivalent length and intensity of training
stimulus can be reached only by performing 200 drop jumps from 60 cm, twice a
week for 12 months. In fact, the time spent for each drop jump is less than 200 ms,
and the acceleration developed can hardly reach 2.7 g [8]. This means to
11
stimulate the muscles for 2 min / week for the total amount in one year of 108
minutes, which is almost the total time of vibration applied to the E subjects.
 
Good vibrations and strong bones?
Power Plate Studies
THE HUMAN PHYSIOLOGY of bone perfusion has been neglected.

The issue may be explained in part by technical difficulties in assessing bone blood flow in vivo. Currently available techniques may be rather expensive, and the access of interested scientists to these techniques may be limited. Another possible explanation for the neglect is the fact that the integration
between cardiovascular and bone research fails because each research area is narrowly focused on its own organ or tissue system. This state of affairs is unfortunate given the potentially important interactions between the cardiovascular system and bone. Indeed, bone and vascular disease frequently coexist in the same patients. Osteoporosis risk is increased in patients with atherosclerosis and vice versa. The correlation is probably explained in part by a common underlying mechanism rather than a spurious association. Bone perfusion may be such a common mechanism.

Perfusion appears to be matched to the metabolic demands of the bone. For example, increased bone turnover and inflammation are associated with an increased blood flow. Blood flow decreases as bone turnover normalizes or the inflammation has resolved. Failure of the vasculature to respond to metabolic needs of the bone might predispose to bone disease. Alterations in vascular function and in intraosseous angiogenesis may be contributory. Several studies suggest a correlation between bone perfusion and bone density. Studies used different methodologies and are, therefore, difficult to compare. In one study (8), magnetic resonance imaging was used to obtain an indirect measure of bone marrow perfusion at the level of the lumbar
spine. Bone marrow perfusion was correlated with bone mineral density in postmenopausal but not in premenopausal women. In another study (2), decreased bone marrow perfusion was associated with progression of collapse of fractured vertebra in patients with osteoporosis.

Perhaps “bone vascular disease” contributes to osteoporosis. One might further speculate that interventions that improve bone vascular function may have a beneficial effect on bone structure. The anatomic structure of blood small blood vessels within the bone is similar to the structure of blood vessels in other tissues. These vessels may be susceptible to the same genetic and environmental risk factors. If bone vascular disease and, thus, alterations in perfusion were a cause of excessive bone loss, atherosclerosis risk factors should also increase the risk for osteoporosis. Indeed, smoking, diabetes mellitus, elevated low-density lipoprotein cholesterol, reduced high-density lipoprotein cholesterol, and hyperhomocystinemia are associated with increased cardiovascular risk and reduced bone mineral density(4). Both the risk for cardiovascular disease and the risk for osteoporosis increase sharply after menopause. A study in rabbits suggests that experimental “postmenopause” through oophorectomy leads to changes in bone vascular function. In this study (1), oophorectomy increased the responsiveness of isolated vascular rings from small bone arteries to norepinephrine and to endothelin. However, whether vascular damage to the bone vasculature explains the association between osteoporosis and cardiovascular risk factors in humans is unknown.

Interestingly, treatment of some cardiovascular risk factors appears to have a beneficial effect on osteoporosis. For example, smoking cessation leads to an improvement in markers of bone turnover within a 6-wk period (5). Lipid-lowering therapy increases bone mineral density (3). Thiazide diuretics appear to lower the bone fracture rate (7). Moreover, beta blockers appear to do the same (6). Finally, estrogen replacement therapy improves bone density and endothelial function in humans.

If bone perfusion has an important effect on bone density, could an increase in bone perfusion also increase bone density? How can bone blood flow be increased? In this issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, Stewart et al. (9) reviewed the literature on bone perfusion and bone mass. The few available publications seem to suggest that increased venous pressure and increased perfusion
tend to increase bone mass. They reasoned that an increase in leg and, perhaps, bone perfusion may contribute to the recently described increase in bone mass with whole body vibration (10).

To address this issue, they assessed changes in leg hemodynamics and fluid shifts using strain-gauge and impedance plethysmography
before and during whole body vibration. The vibration was elicited by placing the subjects on a vibrating platform. Whole body vibration increased blood flow to the lower body while subjects were in the supine position. Furthermore, the intervention reversed the decrease in leg blood flow in the upright position.

Finally, leg vibration shifted the microvascular filtration relation to higher pressures, both in the supine and in the upright position. The shift is probably explained by improved lymphatic drainage. Thus whole body vibration substantially altered leg hemodynamics. The study by Stewart et al. (9) necessarily has some limitations. The authors did not measure bone perfusion directly. It is difficult to know whether the change in leg blood flow is associated with a change in bone perfusion. I would suggest comparing “cheap” leg blood flow measurements with “costly” more direct measurements of bone blood flow in future studies. It would be tremendously helpful to have inexpensive methods that could be used to obtain hemodynamic measurements that
are relevant for bone hemodynamics. Furthermore, the authors did not provide data linking changes in hemodynamics and bone turnover. Perhaps more questions were raised than answered. Nevertheless, the study is of importance because it may generate interest in studying the interaction between bone and the cardiovascular system. Promising clinical and epidemiological data linking vascular disease and osteoporosis ought to be supported by solid physiological work. Equally important, the study suggests that even in the era of molecular medicine, a simple and “old-fashioned” physiological method is still useful to raise new scientific hypotheses. A final question for those who will not be interested in bones: Do good vibrations add to angiogenesis elsewhere?

 
Good vibrations and strong bones?
Power Plate Studies

Jens Jordan
Franz-Volhard Clinical Research Center, Charite´, Campus Buch and HELIOS Klinikum, Berlin, Germany

 
 
THE HUMAN PHYSIOLOGY of bone perfusion has been neglected. The issue may be explained in part by technical difficulties in assessing bone blood flow in vivo. Currently available techniques may be rather expensive, and the access of interested scientists to these techniques may be limited. Another possible explanation for the neglect is the fact that the integration between cardiovascular and bone research fails because each research area is narrowly focused on its own organ or tissue system. This state of affairs is unfortunate given the potentially important interactions between the cardiovascular system and bone. Indeed, bone and vascular disease frequently coexist in the same patients. Osteoporosis risk is increased in patients with atherosclerosis and vice versa. The correlation is probably explained in part by a common underlying mechanism rather than a spurious association. Bone perfusion may be such a common mechanism. Perfusion appears to be matched to the metabolic demands of the bone. For example, increased bone turnover and inflammation are associated with an increased blood flow. Blood flow decreases as bone turnover normalizes or the inflammation has resolved. Failure of the vasculature to respond to metabolic needs of the bone might predispose to bone disease. Alterations in vascular function and in intraosseous angiogenesis may be contributory. Several studies suggest a correlation between bone perfusion and bone density. Studies used different methodologies and are, therefore, difficult to compare. In one study (8), magnetic resonance imaging was used to obtain an indirect measure of bone marrow perfusion at the level of the lumbar spine. Bone marrow perfusion was correlated with bone mineral density in postmenopausal but not in premenopausal women. In another study (2), decreased bone marrow perfusion was associated with progression of collapse of fractured vertebra in patients with osteoporosis. Perhaps “bone vascular disease” contributes to osteoporosis. One might further speculate that interventions that improve bone vascular function may have a beneficial effect on bone structure. The anatomic structure of blood small blood vessels within the bone is similar to the structure of blood vessels in other tissues. These vessels may be susceptible to the same genetic and environmental risk factors. If bone vascular disease and, thus, alterations in perfusion were a cause of excessive bone loss, atherosclerosis risk factors should also increase the risk for osteoporosis. Indeed, smoking, diabetes mellitus, elevated low-density lipoprotein cholesterol, reduced high-density lipoprotein cholesterol, and hyperhomocystinemia are associated with increased cardiovascular risk and reduced bone mineral density(4). Both the risk for cardiovascular disease and the risk for osteoporosis increase sharply after menopause. A study in rabbits suggests that experimental “postmenopause” through oophorectomy leads to changes in         bone vascular function. In this study (1), oophorectomy increased the responsiveness of isolated vascular rings from small bone arteries to norepinephrine and to endothelin. However, whether vascular damage to the bone vasculature explains the association between osteoporosis and cardiovascular risk factors in humans is unknown. Interestingly, treatment of some cardiovascular risk factors appears to have a beneficial effect on osteoporosis. For example, smoking cessation leads to an improvement in markers of bone turnover within a 6-wk period (5). Lipid-lowering therapy increases bone mineral density (3). Thiazide diuretics appear to lower the bone fracture rate (7). Moreover, beta blockers appear to do the same (6). Finally, estrogen replacement therapy improves bone density and endothelial function in humans. If bone perfusion has an important effect on bone density, could an increase in bone perfusion also increase bone density? How can bone blood flow be increased? In this issue of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, Stewart et al. (9) reviewed the literature on bone perfusion and bone mass. The few available publications seem to suggest that increased venous pressure and increased perfusion tend to increase bone mass. They reasoned that an increase in leg and, perhaps, bone perfusion may contribute to the recently described increase in bone mass with whole body vibration (10). To address this issue, they assessed changes in leg hemodynamics and fluid shifts using strain-gauge and impedance plethysmography before and during whole body vibration. The vibration was elicited by placing the subjects on a vibrating platform. Whole body vibration increased blood flow to the lower body while subjects were in the supine position. Furthermore, the intervention reversed the decrease in leg blood flow in the upright position. Finally, leg vibration shifted the microvascular filtration relation to higher pressures, both in the supine and in the upright position. The shift is probably explained by improved lymphatic drainage. Thus whole body vibration substantially altered leg hemodynamics. The study by Stewart et al. (9) necessarily has some limitations. The authors did not measure bone perfusion directly. It is difficult to know whether the change in leg blood flow is associated with a change in bone perfusion. I would suggest comparing “cheap” leg blood flow measurements with “costly” more direct measurements of bone blood flow in future studies. It would be tremendously helpful to have inexpensive methods that could be used to obtain hemodynamic measurements that are relevant for bone hemodynamics. Furthermore, the authors did not provide data linking changes in hemodynamics and bone turnover. Perhaps more questions were raised than answered. Nevertheless, the study is of importance because it may generate interest in studying the interaction between bone and the cardiovascular system. Promising clinical and epidemiological data linking vascular disease and osteoporosis ought to be supported by solid physiological work. Equally important, the study suggests that even in the era of molecular medicine, a simple and “old-fashioned” physiological method is still useful to raise new scientific hypotheses. A final question for those who will not be interested in bones: Do good vibrations add to angiogenesis elsewhere?
 
 
Whole body vibration training IMPROVES SPRINT PERFORMANCE
Power Plate Studies
Effects of whole body vibration training on sprint running kinematics and explosive strength performance.

This is a summary of a study published in the international scientific journal “Journal of Sports Science and Medicine” (2007) 6, 44 – 49.
By Giorgos Paradisis and Elias Zacharogiannis. Track and Field Unit, Department of Sport and Exercise science, University of Athens, Greece


Study Conclusions:Figure 1. Squat and Wide Squat positions exercised

Performance on the 10 m, 20 m, 40 m, 50 m and 60 m sprint improved significantly after 6 weeks of whole body vibration training, with an overall
improvement of 2.7%.

Step length and running speed improved by
5.1% and 3.6% respectively.

Countermovement jump height increased by 3.3%, and explosive strength endurance improved by 7.8% overall.

The whole body vibration training period of 6 weeks
- performed on the “classic” Power Plate® machine
- produced significant changes in sprint running kinematics and explosive strength performance.


Introduction

Sprint performance is determined by the ability to attain maximum running speed as fast as possible, achieving the highest running speed and by maintaining this speed for the required time or distance. By improving specific kinematics such as step length, step rate and running speed, as well as increasing explosive strength, sprint performance can be improved. These kinematics can be trained by improving optimal motor neuron excitability and fast twitch fiber recruitment.



Previous studies suggest that whole body vibration trainingFig. 2 A whole body vibration training period of 6 weeks produced significant positive changes in kinematical characteristics of sprint running. The results of the present study indicate that the gain of the step length was greater than the decrease of step rate (5.6% vs. - 3.9 %), so the net effect was an improvement of running speed, resulting in enhanced sprint performance.
causes length changes in the muscle which stimulates receptors,
most likely muscle spindles, eliciting the ‘tonic vibration reflex’.

This reflex plays a part in making movements more efficient.
Additionally, there are indications that the recruitment
thresholds of motor units of muscles during vibration are
lower, compared to voluntary contractions.

This means your
muscles will contract with a smaller stimulus, resulting in
faster reactions. As whole body vibration training is also
reported to improve fast twitch recruitment, it was hypothesized that whole body vibration training would result in a significant increase in sprint running kinematics and explosive strength/jumping performance in non-experienced
athletes.






Method

Twenty-four volunteers were randomized into two groups.
One group performed a 6-week training program on the Power Plate® machine; the control group did not participate in any training. The training group performed a warming up followed by a session on the Power Plate® machine for 16 to 36 minutes, three times per week. They performed 4 static exercises (squat, wide squat, one-legged squat for both legs, see fig. 1). For the first weeks, all of the exercises were performed at 30 Hz low and an acceleration of 2.28 g. During the course of their training, the program was intensified according to the overload principle:


Week Exercises Time
(sec)
Repetitions Rest
between
exercises
(min)
Sets Rest
between
sets
(min)
1-3 4 40 2 1 3 2
4-6 4 60 3 1 3 2





Results and conclusion

It can be argued that increasing step length could produce more velocity. However, if step length increases and muscle force remains the same, step rate should decrease.

Accordingly producing a slower step rate should lose the gain from a greater step length.

The results of the present study indicate that the gain of step length was greater than the decrease of step rate (5.6% vs. - 3.9 %), so the net effect was an improvement of running velocity (see fig. 2).


The whole body vibration training period of 6 weeks produced significant positive changes in kinematical characteristics of sprint running and explosive strength characteristics in non experienced sprinters, most likely due to the improved muscle contractions it provokes. The whole body vibration group showed improvement in all of the parameters that were tested: running time, running speed, step length, step rate and counter movement jump. The explosive strength endurance improved by 7.8 % (see fig. 2).


Sprint performance was enhanced, with a net effect of improvement of running velocity and decreased time interval over 60 meters. Jump height and explosive strength endurance also improved in the group that used the Power Plate® machine.



Overall, the conclusion of the researchers is that whole body vibration stimulates the sensory receptors and the afferent pathways, leading to a more efficient use of the stretch reflex. It allows for specific training of the fast-twitch fibers, contributing significantly to high-speed movements. In everyday life, improving these qualities will allow people to increase efficiency of movement and to prevent injuries.

 
5 Week Pre-Christmas Health and Fitness Challenge
Power Plate Studies

 5 Week Pre-Christmas Health and Fitness Challenge

Lose Weight. Feel Great.

 

Includes:

  • Initial naturopathic consultation
    • 5 week diet plan
    • Protein Powder specially designed with a unique combination of herbs and nutrients to support healthy weight management
    • Cellular Heath Assessment* to determine other requirements to optimise weight loss.
  • 2 x weekly Personal training sessions  
  • Weekly Use of state of the art gym equipment 
  • Weekly measure and weigh
  • Weekly information sessions 
    • Topics include 
      • Weight loss stoppers      
      • GI foods
      • Importance of protein
      • Good fats vs Bad fats
 
Oxygen Uptake During Whole Body Vibration in Overweight Women
Power Plate Studies
OXYGEN UPTAKE DURING WHOLE BODY VIBRATION IN OVERWEIGHT WOMEN

DIRK VISSERS, KRIS IDES, CARL VERCRUYSSE,
STEVEN TRUIJEN and LUC VAN GAAL.
university Antwerp logo

1University College of Antwerp , Dept. Of Health Sciences, Belgium
2 University of Antwerp and Antwerp University Hospital, Belgium

INTRODUCTION
  • Acceleration training or whole body vibration training has been described as an effective method for strength training.
  • To the best of our knowledge there are no studies on oxygen ventilation and energy expenditure during whole body vibration in overweight women.

AIM OF THE STUDY
To assess the effect of additional whole body vibration on the ventilation of oxygen which can be regarded as a measure for energy expenditure

METHODS
A controlled randomized trial.
  • Anthropometric measurements were taken in twenty adult overweight premenopausal women.
  • Ventilation of oxygen (VO2)and carbodioxide (VCO2) and heart rate were measured using a portable gas-analysis system (Cortex Metamax 3B) and a Polar heart rate monitor.
  • After each exercise a Borg scale score was assessed.
  • Exercises were performed on a vibration platform (Power-plate, Next Generation) with a frequency of 35 Hz and the intensity set on ‘high’ (amplitude of 4 mm).
  • Two dynamic exercises (standing on toes and squatting) and 1static exercise (standing) were performed during 3 minutes with and without vibration in a randomized order with 10 minutes rest between exercises. Mean values of the third minute of exercise were compared.
Overweight Women exercising on power plateStatistics of the resultsPower Plate Results Studying on Women

RESULTS
Ventilation of oxygen, carbon dioxide and heart rate were consistently significantly higher in the exercises with vibration
compared to the exercises without vibration.


CONCLUSIONS
The addition of whole body vibration to both static and dynamic exercises seems to increase the oxygen ventilation significantly in overweight women.

More research is needed to find out what is causing this increase and whether this increase is clinically relevant.