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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. 
 
 
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.
 
 
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.
 
 
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