Objective:
 

To test whether training on a high-frequency (28Hz) vibrating platform improves muscle power and bone characteristics in postmenopausal women.
 

Design:
 

Randomized controlled trial with 6-month followup.
 

Setting:
 

Outpatient clinic in a general hospital in Italy.
 

Participants:
 

Twenty-nine postmenopausal women (intervention group, n=14; matched controls, n=15).
 

Intervention:
 

Participants stood on a ground-based oscillating platform for three 2-minute sessions for a total of 6 minutes per training session, twice weekly for 6 months. The controls did not receive any training. Both groups were evaluated at baseline and after 6 months.
 

Main Outcome Measure:
 

Muscle power, calculated from ground reaction forces produced by landing after jumping as high as possible on a forceplate, cortical bone density, and biomarkers of bone turnover.
 

Results:
 

Over 6 months, muscle power improved by about 5% in women who received the intervention, and it remained unchanged in controls (P
.004). Muscle force remained stable in both the intervention and control groups. No significant changes were observed in bone characteristics.
 

Conclusion:
 

Reflex muscular contractions induced by vibration training improve muscle power in postmenopausal women.
 

Key Words: Bone density; Exercise; Muscles; Postmenopause; Rehabilitation; Vibration; Women.
 

MUSCLE POWER, the capacity of muscles to produce work in the environment, declines significantly over the life span. In women, the rate of decline accelerates after menopause and leads to reduction in physical functioning.1 It has been hypothesized that this process may be responsible for the development of physical frailty and mobility disability^1,2 in old age. Although evidence is overwhelming that physical exercise positively affects muscle strength at all ages, compliance of older persons with traditional exercise programs has generally been low, and only a small percentage of older persons exercise regularly.
 

Vibration exercise on ground-based platforms that oscillate at high frequency has recently been proposed as an intervention for the prevention and the treatment of osteoporosis.^4-6 Highfrequency (28Hz), very-low-magnitude (0.3g) vibration exercise has recently been reported to increase bone mass in experimental animals and in humans.^6-10 However, the mechanism by which vibrations influence the bone tissue remains unclear.¹⁰
 

The high-frequency postural displacements induced by the alternating movements of the platform produce reflex muscle contractions aimed at stabilizing posture.11 Thus, vibration can be viewed as a special form of muscle training that may particularly affect muscle power.12 It has been proposed that the force applied to bone during muscle contraction has a pivotal role in the homeostatic and adaptive regulation of bone strength.^13,14 This hypothesis may explain, in part, the mechanism by which vibration improves bone strength. To test this hypothesis, we conducted a small, randomized controlled trial (RCT) to discover whether training on a high-frequency vibrating board for 6 months would improve muscle power in postmenopausal women and, in turn, positively influence bone characteristics.
 

METHODS
 

Design
 

All the study procedures, including recruitment, measurements, and intervention, were performed in the Nuovo San Giovanni di Dio Hospital in Florence, Italy. The recruitment phase began in spring 1999 and was completed in fall 1999. The intervention began in the winter 1999–2000 and was completed by summer 2000. Among the 67 women belonging to a hospital volunteers association (Associazione Volontari Ospedalieri), 39 women who were at least 1 year postmenopausal and not affected by conditions that contraindicated the vibration training were enrolled in the study population (fig 1). Women on hormone replacement therapy were considered eligible. Women with metabolic bone disorders were excluded from the trial.


 

The screened women entered a 3-month run-in phase during which they received daily 1g of calcium carbonate and .25µg of activated vitamin D (calcitriol). This supplementation was administered to all the participants for the entire study period to avoid any influence of insufficient calcium or vitamin D intake on the effects of vibration exercise on bone apposition and mineralization. Because of the nature of the intervention, no blinding or placebo was considered. Of the 67 screened women, 33 agreed to participate in the study, signed an informed consent, and were randomized to either vibration or control group. A simple randomization procedure was applied using a series of random numbers. Six of the 39 eligible women refused to participate in the trial owing to family problems (n=2) and little interest (n=4).
 

Measurements
 

Blood and urine tests were performed to exclude from the trial subjects affected by metabolic bone disorders like primary hyperparathyroidism or hyperparathyroidism secondary to renal failure. All blood samples were drawn in the morning between 8:00 and 9:00 AM, in the fasting state. Routine biochemical parameters, which included total serum calcium, serum phosphorus, and creatinine, were measured using standard laboratory methods. Serum parathyroid hormone (PTH) was measured by a double-antibody chemoluminescence method^a (interassay cell volume [CV]=2%), and serum bone-specific alkaline phosphatase was measured using an immunoenzymatic method^b(interassay CV=5%). Deoxipiridinoline and N-terminal telopeptide were measured using a 1-step chemoluminescence method^a (interassay CV=3%) and immunoenzyimatic method^c (interassay CV=10%), respectively. To collect the 2-hour morning urine, participants were instructed to get up early in the morning and void. After 2 hours of fasting, during which only ingestion of water was allowed, participants voided again, and all urine samples were collected and used for measurements. To assess muscle power, participants, starting from a standstill, jumped as high as possible and landed on a forceplate^d that measured ground reaction forces.15 The best of 4 attempts was used in the analysis. The acceleration of the center of gravity (COG) was calculated as the ratio of force (N) and body mass (kg). The integration of acceleration by time gives the instantaneous velocity of the COG (m/s). The power (W) is obtained as the product of force and velocity. Tibial bone density, mass, and geometry were assessed by a recent generation, high-resolution, peripheral quantitative computed tomography device (XCT 2000^d). Volumetric total bone density (mg/cm³) was measured as the average density of the whole cross-section of the tibial metaphysis (4% of the tibial length from its distal end); that is, the section mainly composed of trabecular bone surrounded by a thin cortical shell. At the same site we assessed trabecular bone density (mg/cm³) by excluding cortical bone. Measures of cortical bone density (mg/cm3) and cross-sectional area (mm²) were obtained from a cross-sectional image of the tibial diaphysis at 38% of the tibial length from its distal end. In these images, all of the voxels with a density above 710mg/cm3 were considered to belong to cortical bone.
 

Intervention
 

The active intervention consisted of brief training sessions conducted twice weekly for 6 months. In each session, vibration was provided by a commercially available device (Galileo 2000^d). By means of an oscillating board, this device delivers high-frequency vibration through the legs to the whole body. Participants stood with feet side by side on the board, which produced lateral oscillations of the whole body with accelerations in the range of 0.1 to 10g. At the beginning of the training, participants stood on the board with the knees slightly flexed and received three 1-minute bouts of vibration separated by 1-minute resting periods. During the first month of treatment, the frequency of vibration was progressively increased from 12 to 28Hz to allow for gentle adaptation. During the following 5 months of treatment, the frequency was always set at 28Hz, and the bouts of vibration were prolonged to 2 minutes. Participants were invited to separate the feet as far as tolerated to increase the amplitude and speed of the vertical displacement. Previous studies11 have demonstrated that the oscillating movement of the board produces muscle stretching, which elicits alternating reflex contraction of the flexor and extensor leg muscle groups. Participants in the active group attended on average 34 sessions, corresponding to about 200 minutes of treatment, out of 44 sessions potentially available.

 

Statistical Analysis

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

RESULTS

 Women who received the active intervention were similar to controls in age, baseline muscle power, years since menopause, anthropometric measures, routine biochemical measurements, and biomarkers of bone turnover (table 1). Final measurement of the primary outcome (muscle power) was obtained in 29 of the 33 women who had been originally randomized (14 active treatment, 15 controls). Dropouts in the intervention group were caused by family problems (n=2) and knee pain (n=1). In 1 control, a measure of muscle power at the final follow-up could not be obtained because of posttraumatic muscle pain.
 

After 6 months, muscle power improved by about 5% (from 178.9±9.6W to 187.3±9.5W) in women who received the active treatment (table 2), whereas it declined slightly in controls. In a repeated-measure ANOVA, change over time in muscle power differed statistically between the 2 groups (P<.02). Overall, muscle power improved in 80% of the women in the treatment group and in 46% in the controls (P=.06). The velocity increased in the intervention group to a similar extent as the power (from 163.7±6.2m/s to 171.7± 5.3m/s, P<.005), whereas muscle force did not change significantly in either group.

 

Cortical bone density remained stable in the intervention group, whereas it declined significantly in the control group (P<.05). However, in a repeated-measure ANOVA, the decline in cortical bone density over time did not differ statisti- cally between the 2 groups (P=.09). All other bone parameters, including biochemical indices of bone turnover, did not change significantly during the study period in either group.

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

 

DISCUSSION
 

In the present study, 200 minutes of high-frequency wholebody vibration, distributed in biweekly sessions over 6 months, improved muscle power and the velocity of movement in postmenopausal women without significant changes in muscle force. These results suggest that vibration training improves muscle power mainly by enhancing the pattern of recruitment of muscle fibers.
 

This study is the first to show an improvement of muscle power in postmenopausal women using vibration exercise. The decline in muscle power is an early and apparently inexorable occurrence in the life of a woman, perhaps contributing to physical frailty and mobility disability in late life. Studies17 have demonstrated that such a decline may be slowed by strength training exercise. However, the compliance of older persons in traditional exercise programs is poor.
 

High-frequency vibration on a ground-based platform stimulates continuously alternating reflex contractions of flexor and extensor muscle groups of the lower extremities.11 We hypothesized that vibration is a special type of exercise that may be particularly suitable for older persons. It does not require much time or effort, does not cause potentially traumatic vertical displacements of the involved joints, and specifically trains type II muscle fibers, which are selectively lost during the aging process.^16,18 The availability of a simple, safe, and wellaccepted training method that can improve muscle power in postmenopausal women opens a new perspective for the prevention of age-associated loss of muscle function in this group of women.

 

Previous studies have demonstrated that vibration exercise improves bone mineral density in animal and human models. Our findings provide a possible explanation for this effect of vibration exercise. Mechanical stress produced by muscle contraction plays a critical role in the maintenance of bone strength.^19,20 Thus, improvement in muscle force and power may be a strategy for improving bone characteristics and preventing osteoporosis in postmenopausal women. In accordance with this hypothesis, our study showed that the decline in cortical bone density tended to be greater among control women than among women who received the active treatment. Our findings on cortical bone volumetric density are consistent with earlier reports21 and support the hypothesis that vibration exercise may positively affect bone characteristics.10 However, clinical trials that address these issues would require longer follow-up and, probably, a more intensive intervention. Based on earlier reports and on the present findings, our conclusion is that vibration exercise may be a more useful tool for the prevention and treatment of osteoporosis than pharmacologic treatment of osteoporosis,^22,23 a disease that is generally underdiagnosed and undertreated.^24,25
 

The vibration training was safe overall. The only clinically significant side effect was knee pain, which was observed in 2 participants with preexisting osteoarthritis of the knee. This pain caused cessation of treatment in 1 subject. The frequent occurrence of transient lower leg erythema reported16 previously was often observed in the present study, but it was always transient, mild, and not disturbing.
 

The present study has several limitations. First, the small number of participants and the relatively short duration of the intervention might have prevented us from identifying treatment effects on secondary outcomes such as muscle force or bone parameters. However, the effect on the primary outcome, muscle power, was small but clear-cut and therefore unlikely to be due to chance. Likewise, the treatment’s safety clearly needs to be tested in larger studies. Second, the compliance with the treatment sessions was suboptimal; in fact, only 34 of 44 sessions were attended on average. However, an important reason for the low attendance was the restricted choice of days and time offered to the participants for the training sessions (because of our lack of financial resources). The training was perceived as very useful by the participants, who uniformly reported an improved well-being as a consequence of the training. Moreover, it can be considered a striking finding of this study that a substantial improvement in muscle power was obtained with only 200 minutes of training.
 

CONCLUSION
 

The results of this small RCT suggest that high-frequency vibration exercise is a feasible, safe, convenient, and efficacious intervention, which could prevent the decline in muscle and bone strength in postmenopausal women. Such intervention can easily be added as a component of an exercise-based prevention program or even prescribed as the sole intervention when traditional exercise is not feasible.


 

Objective:
The aim of this study was to evaluate the effect on spasticity, muscle strength and motor performance after 8 weeks of whole-body vibration training compared with resistance training in adults with cerebral palsy.

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

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

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

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

INTRODUCTION

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

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

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

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

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

METHODS

Design

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

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

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

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

Measurements

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

Spasticity.

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

Isokinetic muscle strength.

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

Walking ability

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

Balance.

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

Gross motor function.

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

 

Interventions

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

Statistics

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

RESULTS

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

Spasticity

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

Isokinetic muscle strength

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

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

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

Six-Minute Walk Test (6MWT)

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

Timed Up and Go test (TUG)

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

 

DISCUSSION

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

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

The WBV group performed static exercises and therefore it had been appropriate with a measure of isometric strength. It is interesting that the WBV group, however, showed an increase in isokinetic strength. It is possible that this increase had been more marked if the WBV training was performed with dynamic movements similar to movements used in the leg press device.

 

Finally, we wanted to evaluate effects on motor performance, which were walking ability, balance and gross motor function. Walking distance in 6 minutes and balance in walking and turning did not change in any intervention group. The gross motor function increased significantly only in the WBV group. In previous studies on RT in persons with CP (5, 7) the same tests were used (6MWT, TUG and GMFM) and showed increased motor performance. In those studies the RT program consisted of 10 and 20 different exercises compared with only one in this study. It is probably necessary in both WBV training and RT that the training program is more extensive to obtain an effect on motor performance. It should be longer and consist of different types of exercises. Compared with 6 minutes of WBV training in one position in this study, other studies report increasing muscle strength after 20 minutes of WBV training in static and dynamic knee extensor exercises (13-16). In further studies it would be better with a more vigorous training program to get enough overload of the muscles to obtain a more marked effect on muscle strength and motor performance.

Based on the subjective reports of the participants, negative side-effects did seldom occur in the WBV group. One participant was very stiff in her legs in the evening after training. In the RT group negative side-effects were more common. Muscle stiffness was on 3 occasions so bad that the participants chose to cancel the following exercise session. Another participant had problems with back pain at the beginning of the training period caused by uneven loading when performing the leg press exercise. Even if WBV training had few negative side-effects it was not free from risks. For some participants who had difficulties in standing with equal weight on both legs, the feet could slide off the vibration platform. It is therefore important that persons with motor disabilities have someone who is supervising and prepared to stabilize them if this is about to happen.

 

In conclusion, the data in this study suggest that 8 weeks of intervention with WBV can increase muscle strength during rapid movements and increase gross motor performance without negative effects on spasticity. The data also suggest that intervention with progressive RT can increase muscle strength at slow and rapid movements without negative effect on spasticity. Walking distance in 6 minutes and balance in basic mobility manoeuvres did not change significantly in any intervention group. When comparing the groups after the intervention period, there were no significant differences in changes of spasticity, muscle strength or gross motor performance.

ABSTRACT

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

 

KEY WORDS
 

WBV, protocols, countermovement jumpimmediate effects
 

INTRODUCTION
 

Power is an important component of athletic performance. In fact, power training techniques are currently used to prepare athletes for such diverse sports as hockey (15), skiing (27), baseball (16), and football (14). Additionally, power is a major component of general health, fitness, and independence (2,18). Overload techniques such as weight training (24) and plyometrics (29) have traditionally been used to improve power (1,17,35), and specific protocols using thesemethods to optimize power have been studied extensively (1,17,19). Although the benefits of these traditional training methodologies are well established (14–16,21), they are often quite time consuming and require considerable training intensity and volume, making them unattractive to the majority of the population seeking to improve their health and fitness. In addition, time constraints placed on collegiate and professional athletes by controlling agencies and competitive schedules also argue for the development of more effective and efficient training techniques.
 

Within the past 25 years, a relatively new method of neuromuscular overload, whole-body vibration (WBV), has been slowly emerging that may address these concerns. Recent research suggests that mechanical vibrations that incorporate low amplitudes and frequencies are a safe and effective method of exercising the neuromuscular system (6). In fact, mechanical vibrations have been shown to induce nonvoluntary muscle contractions (20), so the application of vibration to a limb or entire body may be an appropriate treatment for sport training (20). Whole-body vibration is starting to be used as an alternative form of strength and power training (6,30,32). It elicits neuromuscular training in a short time period without a great deal of effort (30).
 

Many studies have reported acute (10,11,22,30,31,36) and chronic (6,9,12,26,32,37) increases in power after WBV training; however, a limited number of studies have reported no effect (9) or even a decline (30). The divergent results in these studies may have been attributable to the variations in the frequency, displacement, and duration of the WBV stimulus applied as well as variations in populations tested and WBVdevices. Target populations included men (10,33), women (32), and trained (3–5,10,12,21,22,27,30) and untrained (11,32) individuals. Because of the varied target populations, a specific group most sensitive to the acute effects of WBV has not yet been identified.
 

We have selected healthy untrained adults in a low-risk age group because we feel that this is an important target group, given their propensity to use exercise as a fitness and wellness tool.
 

Among the devices examined are triplanar devices that apply the vibratory stimulus in anterior/posterior, lateral, and vertical directions and uniplanar devices that either tilt on a central axis or move solely in a vertical direction.
 

Potential Mechanisms
 

The exact mechanism that regulates how the body reacts to vibratory stimulus is currently unknown; however, several potential mechanisms have been proposed.

Neuromuscular Facilitation
 

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

Mechanical stimuli are transmitted from the vibration device through the body, where they stimulate sensory receptors, most likely muscle spindles. This activates alpha motoneurons, initiating reflexive muscle contractions—this is referred to as the tonic vibration reflex (6,13,32). Monosynaptic and polysynaptic pathways mediate this response, resulting in an increased activation of motor units (33). The tonic vibration reflex is probably dependent on the frequency of vibration, muscle length, and body position (23). Voluntary muscle contractions may be enhanced by the tonic vibration reflex when used in conjunction with strength-training protocols (23).
 

Increased Gravitational Forces on Muscle
 

Under normal gravitational conditions, muscles can maintain their performance. During conditions of decreased gravitational load, microgravity, a decrease in force capability, occurs. An increase in gravitational load, hypergravity, increases the cross-sectional area and force-generating capability of the muscles (6). Mechanical vibrations applied to the whole body can produce changes in gravitational conditions during the intervention (4,6). The vibrations produce fast and short changes in the length of the muscletendon complex. Sensory receptors, probably muscle spindles, call on a reflex muscular activity in an attempt to dampen the vibratory waves (6). Additionally, conditions of hypergravity have been shown to increase hormone levels, including androgens and growth hormones in the blood (6).
 

Types of Whole-Body Vibration Devices
 

The effects of WBV have been studied using vibrating plates that produce sinusoidal vibrations (7). During WBV training, the subject may stand or move on a platform that generates vibrations with frequencies ranging from 25 to 40 Hz and amplitudes from 2 to 10.5 mm (13,32). This study uses a triple-plane WBV device (Power Plate). The platform simultaneously oscillates up and down, left to right, and front to back. The potential amplitudes are low (2–4 mm) and high (4–6 mm). The frequency options for this device include 30, 30, 40, and 50 Hz.
 

A key reason for inconsistencies in scientific data regarding the effects of WBVmay be that protocols vary from study to study. Different frequencies and amplitudes have been applied to different populations with varying recovery periods. Each of these parameters has the potential to impact biological response to vibration training and, therefore, the effects of vibration training on strength and power performance.
 

In exercises geared toward increasing strength and power, the specific loading parameters must be carefully determined, applied, and controlled to ensure the desired training effect. Application of different training protocols would yield varying results, just as varying vibration training protocols is likely to yield variable physiological responses (23).
 

Whole-body vibration is currently used as an exercise program in many fitness and rehabilitation facilities, but the current knowledge on effective exercise protocols is limited (7). Currently, the sport industry is producing various devices for WBV; therefore, it is important to determine appropriate training protocols (20).
 

Although studies using WBV have been shown to elicit positive acute and chronic responses on strength and power, the exact intensity, duration, and postexercise recovery time that would optimize these benefits are not known. For WBV to be an effective and efficient tool for increasing muscular performance, the appropriate protocols for targeting muscular strength and muscular power must be established (10).
 

Once effective protocols are developed, WBVmay provide a less time-consuming alternative for improving power compared with traditional training methods (6,26). In addition, it has been shown that perceived exertion is lower during WBV compared with standard training techniques eliciting similar results 12,13,20,25). This may encourage sedentary individuals to participate in an exercise program and add to the options currently available to both recreationally active individuals and athletes seeking to improve their health, fitness, and performance.
 

To our knowledge, no work has examined the frequency, displacement, and work-recovery duty cycle that would maximize power development during WBV, and there has been little research examining the duration of the acute effects of WBV (10). Additionally, there is a paucity of information addressing the acute effects of triplanar WBV, such as that provided by the Power Plate vibration platform (Power Plate North America Inc., Northbrook, Ill), on the power of healthy untrained adults. Therefore, the objective of this study was to identify the WBVprotocols that would elicit the greatest improvement in power performance after a single exposure and to determine the duration of these effects.
 

METHODS

Experimental Approach to the Problem
 

Exposure to WBV has been shown to elicit variable power output changes immediately after WBV exposure. The divergent results in these studies may have been caused by the variations in the frequency, displacement, and duration of the WBVstimulus applied as well as variations in populations tested andWBVdevices. The goal of this study was to identify the immediate effects of specific combinations of frequency, displacement, and duration of WBV stimulus on the power output of a healthy, untrained, low-risk population. In accordance with Sayers et al. (34), we felt that a jump pad was appropriate to assess power because we were monitoring pre-post exposure change in power. Each subject served as his or her own control. Additionally, jump pads have been shown to be highly correlated with 3D jump height analysis, with a Pearson r of 0.967 (25).
 

Subjects
 

Twenty-two untrained individuals, 23–39 years of age, volunteered to participate in this study. Two subjects withdrew from the study—one because of time constraints, the other because of discomfort during WBV treatment. Characteristics of the 20 subjects who completed the study are presented in Table 1. Exclusion criteria included any chronic medical condition or medications that could affect skeletal muscle performance, or any contraindications to WBV use. Contraindications included unhealed fresh wounds, serious heart or vascular disease, recent hip or knee replacement, pregnancy, acute hernia, discopathy, spondylolysis, severe diabetes, epilepsy, tumors, acute inflammation, acute migraine, pacemaker or recently placed IUD, or fixation devices such as metal pins, bolts, or plates. This study was approved by the subcommittee for the use and protection of human subjects at the University of Miami.
 

Procedures
 

We developed specific protocols using different combinations of frequency (30, 35, 40, and 50 Hz), displacement (low, 2–4 mm; or high, 4–6 mm), and work duration (30, 45, and 60 seconds). Before testing, we randomized the protocols for each subject to reduce the potential for an order effect. Subjects came to the laboratory for 9 visits. During the first visit, each subject completed a health status questionnaire to confirm study eligibility. If inclusion criteria were met, procedures and risks were thoroughly explained to the subject, and his or her written informed consent was obtained. During this visit, the subject was also familiarized with WBV platform use. For this familiarization session, the subject stood on the plate in a half-squat position with the knees held at a 2.27-rad angle. The exposure time was 30 seconds at a frequency of 30 Hz and low displacement. The subject was also familiarized with the hands-on-hips countermovement jump (CMJ) used to assess power.
 

The treatment protocols used during days 2–9 are presented in Table 2. On each of the testing days, subjects completed the 3 randomized treatment protocols, which were determined during the initial randomization process. Subjects were instructed to stand in a half-squat position with the knees shoulder width apart. The knee angle was set at 2.27 rad using a handheld goniometer. A minimum of 24 hours and a maximum of 1 week separated testing days.
 

Testing
 

Subjects were instructed to refrain from training for 24 hours before testing. The impact of the specific WBV protocols on peak leg power was assessed using the CMJ to determine power (7,16). The starting position for the CMJ was feet shoulder width apart, hands on the hips, and knees at 2.27 rad. Subjects were instructed to keep hand position constant and were encouraged to give a maximal effort for each jump. On hearing the word ‘‘go,’’ the subject performed 3 jumps with a slight delay between jumps to reduce the potential impact of the previous jump on stored elastic energy. All jumps were performed on a pressure-sensitive mat interfaced with a laboratory computer containing an assessment program (Axon Bioingeneria Deportiva, version 2.01, 2005). The Bioingeneria program used time off the mat to compute jump height. We selected the CMJ because it measures the combined effects of contractile, neural, and elastic elements, each of which could have adapted acutely to the WBV stimulus.
 

 

Power for the 3 jumps was computed using the formula presented by Sayers et al. (34):
 

PP (W) = 51:9xCMJ height (cm) +48:9xBM(kg) -2007 where PP = peak power and BM = body mass.
 

This formula was specific to the CMJ and has been successfully used to compute power for the hands-on-hips jump technique employed in our study (8). The highest of the 3 jump heights recorded was used to calculate the power used for statistical analysis. Because each subject served as his or her own control to compare pre- vs. post-WBV jump heights, we determined jump technique consistency through visual observation.
 
 

 

Because the effects of WBV are extremely time sensitive (3,10,36), CMJ data were collected immediately after and at 1, 5, and 10 minutes after each WBV bout. Subjects were instructed to remain seated after each testing session. This procedure allowed comparison of the effects of each protocol over time. All testing sessions were preceded by a 5-minute warm-up on a cycle ergometer at 50 W.

Statistical Analyses
 

The response variable for this study was the peak leg power achieved by the subject. This was calculated usingCMJ height and body mass (34). The peak power score was then normalized as a percentage of the pretest jump score (nPP) according to the method of Cormie et al. (10) to account for intersubject variation. This was necessary because our subject population consisted of men and women of varying athletic abilities.
 

Arepeated-measures analysis of variance was used to assess the impact of frequency, displacement, work duration, and recovery time on nPP. The criterion alpha level was set at p # 0.05. This analysis revealed that the greatest increase in power occurred at 1 minute posttreatment, no significant differences were identified as attributable to duration, and there were no interactions among other independent variables and duration; for these reasons, we performed a second repeated-measures analysis using frequency, displacement, and time (pretest and 1 minute posttreatment) as the independent variables. Bonferroni tests for multiple comparisons were used for all post hoc analyses.
 

RESULTS
 

Repeated-measures analyses revealed a significant time 3 displacement 3 frequency interaction (p <= 0.018). The graphs for the low-displacement (Figure 1) and highdisplacement (Figure 2) conditions illustrate that increases in nPP were greatest for the low-displacement conditions at lower frequencies and at high displacement for highfrequency conditions. Post hoc tests revealed significantly different patterns of change between pretest and immediately posttreatment and between immediately posttreatment and 1 minute posttreatment for low vs. high displacements (p = 0.015 and 0.044, respectively). Trends were also seen for patterns of change from immediately posttreatment to 1 minute posttreatment and from 5 minutes posttreatment to 10 minutes posttreatment for high vs. low displacements (p = 0.062 and 0.056, respectively). Actual PP values are provided in Table 3.
 
 

 

Our analysis also revealed a significant frequency 3 displacement interaction (p <= 0.012). Post hoc analysis identified that the source of this significant difference was the pattern of change from 40 to 50 Hz, with the highdisplacement condition showing an increase in performance and the low-displacement condition showing a decrease (p # 0.038) (see Figure 3).
 

Finally, the analysis showed a significant effect by time (p <= 0.001). Post hoc analyses showed significant differences at various time points. Figure 4 illustrates that significant increases in nPP were seen between pretest values and those recorded at 1 and 5 minutes posttreatment. The figure also shows that there was a significant decline in nPP by 10 minutes posttreatment.
 

 

 
 

The results of our analysis removing duration as an independent variable revealed a time 3 frequency 3 displacement interaction (p <= 0.002). Post hoc analysis demonstrated that, at low displacement, there was a decline in nPP when the 30-Hz condition was compared with 40 Hz, whereas high displacement showed the opposite pattern.
 

This analysis also showed a timexdisplacement interaction, with high displacements producing greater nPP values than low displacements (p <= 0.05).
 

DISCUSSION
 

Three significant findings resulted from this investigation. First, an interaction between time, frequency, and displacement was detected, where higher displacements elicited greater power increases at higher frequencies, whereas lower displacements elicited greater power increases at lower frequencies. Our second finding was that an acute bout of WBV led to a transient increase in power that peaked at 1 minute posttreatment, remained significantly elevated at 5 minutes posttreatment, and declined below significant levels by 10 minutes posttreatment. Finally, we found that varying the duration of exposure within the range of 30–60 seconds had no impact on subsequent power measurements.
 

To our knowledge, these are the first data that have shown a frequency-displacement relationship; however, the results from previous studies that examined Power Plate and other devices support our results to some degree. It should be recognized that these comparisons are confounded by the use of a vast array of frequencies, displacements, training protocols, and WBV devices. The Power Plate device used in this study vibrated in 3 planes and provided low (2–4 mm) and high (4–6 mm) displacement. In contrast, studies using the Galileo platform applied displacements as high as 10 mm (4,8,10,29), but this platform oscillates by pivoting laterally about a central axis. Additionally, 2 studies examined vertical vibration devices using frequencies from 24 to 40 Hz and displacements of 2–4 mm (5,38).
 
 

 

In the only study evaluating the impact of a single bout of WBV on explosive power using a Power Plate, Cormie et al. (10) found that applying low-frequency (30 Hz) and low-amplitude (2.5 mm) WBV for 30 seconds significantly increased normalized CMJ height immediately after treatment. Roelants et al. (32) has reported a significant increase in muscle activity and force production after a 30-second bout at 2.5 mm and 35 Hz on the Power Plate. The results of these studies support our finding that matching low frequencies with low displacement can positively affect neuromuscular performance. However, they provide no comparisons with other frequency and displacement interactions.
 

The results from studies using horizontal displacement WBV devices can also be compared with our results; however, it should be recognized that these comparisons are affected by the differences in the patterns of displacement. In a study using a Kuntatory vibration platform at a displacement of 2 mm and frequencies of 25, 30, 35, and 40 Hz for each minute of a 4- minute protocol, Torvinen et al. (38) found no significant increase in vertical jump measured at 2 and 60 minutes after exposure. Because the final minute of the protocol employed minute protocol, Torvinen et al. (38) found no significant increase in vertical jump measured at 2 and 60 minutes after exposure. Because the final minute of the protocol employed
 

The results of studies examining the acute impact of WBV on the Galileo platform are more difficult to compare with our results. The Galileo works by tilting on a central axis to produce a vibratory stimulus. Because this is quite different from both the Power Plate and the vertical displacement devices reviewed above, the frequencies and displacements used also differ considerably. The frequencies range from 5 to 30 Hz, and the displacements range from 0 to 13 mm. In one study, Rittweger et al. (31) examined the effect of loaded squatting exercises on a Galileo WBVplatform at a frequency of 26 Hz and a displacement of 6 mm. The stimulus was applied until exhaustion as assessed by each subject’s rating of perceived exertion. Durations ranged from 150 to 500 seconds. This protocol produced no changes in serial jump heights or isometric knee extension. In a second study, Rittweger et al. (30) examined the impact of WBV at a frequency of 26 Hz and a displacement of 11 mm. Once again, the subjects remained on the platform until selfreported fatigue. The durations of these exposures ranged from 200 to 475 seconds. This protocol produced a significant decline in CMJ performance. Finally, de Ruiter et al. (11) examined the impact of WBV at 30 Hz and 8-mm displacement for five 60-second repetitions with a 120-second recovery interval. This protocol produced no change in CMJ performance. The lack of acute improvements using the Galileo may be specific to this mode of vibration, or it may reflect the exhaustive nature of these protocols. Regardless of the underlying causes, comparing frequency/displacement patterns between WBV on the Galileo and the Power Plate seems inappropriate.
 
 

 

As indicated in the summary above, our second finding was that an acute bout of WBV led to a transient increase in power that peaked at 1 minute after WBVand declined to baseline levels by 5 minutes posttreatment. This finding is in agreement with the results of a number of studies that examined acute responses to WBV (5,10,21). For example, Cormie et al. (10) found a significant increase in CMJ height immediately after WBV (30 Hz, low displacement, 30 seconds), but this increase fell below baseline by 5 minutes posttreatment (10). Bosco et al. (4) also found that a single vibration bout (26 Hz, 10 mm) showed temporary increases (lasting 10 minutes) in muscle average force, average velocity, and average power with all loads used on the treated leg. This resulted in an increase in both velocity and power post WBV for the treated leg at all external loads tested during the leg press test (4). The control legs showed no changes from pre to post WBV tests (4). These results support our findings that there is a limited window during which power performance will be maximized after an acute bout of WBV. Unfortunately, they do not allow comparison with our findings that this window seems to be maximized 1 minute after the stimulus ends.
 
 

 

Finally, we found that varying the duration of exposure within the range of 30–60 seconds had no impact on power. Once again, comparisons with other studies are difficult, because acute responses have not been previously compared within the 30- to 60-second range; however, results of previous studies do lend support to our findings. Bosco et al. (4) found that 10 WBV exposures lasting 60 seconds with 60 seconds of recovery between exposures on a Galileo increased average power as well as peak power in women volleyball players. This WBV treatment was applied at a frequency of 26 Hz and displacement of 10 mm. Cormie et al. (10) found significant increases in CMJ height immediately after a 30-second exposure to the triple-plane WBV device, Power Plate, at a frequency of 30 Hz and 2.5-mm displacement. The fact that both studies reported significant improvements when the duration of the WBV varied by 30 seconds supports our findings.
 

In contrast, prolonged exposures toWBVhave been shown to elicit power decrements (11,30). Rittweger et al. (30) found decreased power after exhaustive exposures (200–475 seconds) on a Galileo WBV platform at a frequency of 26 Hz and a displacement of 11 mm with additional external loads between 35 and 40% of body weight. De Ruiter et al. (11) reported a significant decrease in both maximum voluntary contraction and electrically stimulated maximum force-generating capacity of the knee extensors after five 1-minute bouts on a Galileo 200 at 30 Hz and 8-mm displacement. These declines in performance may have been the result of fatigue resulting from the application of WBV until exhaustion or an ineffective work/recovery duty cycle. We suggest that our data indicating similar responses with WBVbouts of 30, 45, and 60 seconds, and our data indicating transient power increases lasting between 1 and 5 minutes, may help when designing WBV programs to maximize power gains.

PRACTICAL APPLICATIONS
 

Short bouts ofWBVused for warm-up before explosive efforts have been shown to improve neuromuscular performance (3,22). Therefore, WBV may prove effective as a precompetition or pretraining warm-up activity for both individual and team sports where power is a dominant factor. However, our results indicate that if WBV is to be used acutely as either a precompetition or pretraining warm-up, coaches should consider combining high displacements with high frequencies or low displacements with low frequencies. Additionally, preparatory bouts should be temporally positioned so that they precede the competitive performance by 1–5 minutes. The fact that we found no differences between 30, 45, and 60 seconds of stimulation indicates that the time required to generate a positive impact on performance is relatively short, but that durations of up to 1 minute will still generate positive results.
 

The greater time efficiency and lower training volume and intensity associated with WBV compared with traditional training methodologies may make it an effective training tool for athletes (6,27), especially during tapers when reduced volume and intensity are used to prepare athletes for competition (24). Whole-body vibration may be an effective tool for providing the benefits of a taper while further stimulating neuromuscular adaptations for power. Our data also suggest that WBV may be an effective warm-up preceding plyometric training.
 

We suggest that these results be considered when designing the proper work-recovery duty cycles when WBV is used as a training tool to maximize power; however, further studies must be conducted to determine the optimal duration of recovery, the optimal number of cycles, and the frequency and duration interactions necessary to maximize power gains in both competitive athletes and other special populations, whose responses may vary because of age, gender, body composition, and training status.