Whole body vibration (WBV) is an oscillatory training protocol widely promoted in commercial gyms as a means of improving strength. The commercial WBV machine generates a deterministic sinusoidal wave form that is different from the vibration wave form generated by motorized vehicles or industrial machines (21). An oscillatory movement is generated by motors underneath a platform, which are subsequently transmitted to the human body, enhancing the tonic vibration reflex that stimulates reflex muscle contraction (6). The regular training effect of WBV probably produces neuromuscular adaptation to enhance function of neuromuscular systems (5,35).
A number of studies have been conducted with a variety of methods, vibration protocols, and measurements with the aim to improve muscle power and strength. However, the long-term effects of regular WBV training on muscle power and strength are unclear with contradictory results reported. Luo et al. (22) reviewed the chronic effect of 5 randomized controlled studies and concluded that WBV training may provide beneficial effects on strength and power. Similarly, Rehn et al. (30) found moderate-to-strong evidence for improvement of muscle strength or jump height after long-term WBV training. Nordlund and Thorstensson (26) reviewed 12 long-term studies and reported that WBV had greater improvement on strength and/or jump performance compared with a passive control group and had little or no effect compared with control group performing the same exercise. Wilcock et al. (38) analyzed the effect size of WBV on countermovement jump (CMJ) height and reported a small beneficial effect on explosive power in trained athletes (effect size = 0.24–0.39) in 4 studies of 5–8 weeks of training.
To date, there has been no meta-analytical study completed which provides an overall statistical assessment of the effects of regular WBV training on jumping performance. The lack of such a study is possibly because of the large variations in study design and measures of effects and/or outcomes. To overcome such limitations, the current study has limited the study criteria to those studies using CMJ and squat jump (SJ) as their performance measure. The aim of this study, therefore, was to analyze the current evidence for a beneficial long-term effect of WBV training on jump height (either via CMJ or SJ) by meta-analyzing all appropriate studies to date. It also aims to identify the vibration parameters that are more effective at increasing jump height.
Experimental Approach to the Problem
To assess the effectiveness of vibration training on jump height performance, a meta-analytic review was conducted. A comprehensive literature search was undertaken which identified individual pertinent studies, which were combined and analyzed using a random effects model to provide an indication of the overall effects. Results were interpreted and conclusions made based on the available literature. Finally, suggestions for practical applications for the strength and conditioning professional were given.
Whole body vibration articles were searched from MEDLINE, Web of Knowledge, Sciencedirect, Proquest, Scopus, Google Scholar, and SPORTDiscus databases using the keywords: whole body vibration, vibration platform, randomized controlled trials, muscle strength, muscle performance, and jump height. Studies published in foreign language journals were not included. Abstracts and citations from scientific conferences were also not included.
All randomized controlled trials or matched design studies in humans were included in this study. The participants in the studies were between 17 and 80 years of age. The study design had to be long-term WBV training (regular training) compared with having no additional exercise (the control) or additional exercise (cardiovascular or cardiovascular with resistance exercise). The outcomes of interest were CMJ height or SJ height. We excluded studies on the acute effect of WBV training (effects immediately after WBV) and the effect of WBV training combined with extra exercises. Studies found but not included in the meta-analysis were for the following reasons: non-English article, nontypical vertical jump test, and vibration combined with exercise.
The methodological quality of assessment was evaluated by the criteria list, which was used for rating studies on effects of WBV on muscular performance (30). The criteria list in Table 1 was modified from the original scale for evaluating control trials in treatment of low back pain (37). The score classified the methodological quality of studies into high (score of 11–16), moderate (score of 7–10), and low (score of 6 and lower). The quality assessment was performed by 2 independent researchers who were masked to the title, author, and journal name. The score of the included studies in Table 2 ranged from 9 to 13 points (moderate to high). The characteristics of each study are described in Table 3.
Data Extraction and Analyses
Since jump height is a continuous variable, the standardized mean difference (SMD) was used to estimate the effect size of each study. To calculate the SMD, the mean difference and SD of difference were extracted from the intervention and the control groups of the included articles. In the event of any missing data, the original authors were contacted and the data requested. If there was no response from the authors, the Cochrane Handbook for systematic review interventions was used as the guideline for analysis. If the SD was missing, the SD was calculated from the standard error, confidence interval or p-value, whichever was available. If the statistical values were not provided, the SD was calculated from the difference of the SD of the raw pre- and post-baseline data. If the mean and SD difference were presented by line or bar graphs, the mean and SD were estimated by measuring values from the graphs directly. The SMD was then calculated from the formula: (the mean difference of WBV group − the mean difference of control group)/pooled SD of the difference of the WBV and control groups. The SMD of all studies were combined with a random effects model using the RevMan5 statistical program. According to Cohen's effect size, an SMD value of less than 0.25 is considered trivial, 0.25–0.5 small, 0.5–0.8 moderate, and greater than 0.8 large (10). The included studies were subdivided into studies that compared WBV with control groups that did or did not allow additional exercise. A factor analysis was conducted on the effect of WBV compared with no exercise group on CMJ height. Other WBV comparisons were not subjected to a factor analysis because of the insufficient number of studies. The possible publication bias was evaluated by funnel plots and the trim and fill procedure to assess the potential missing studies.
Whole Body Vibration Vs. Control (No Additional Exercise)
A total of 15 studies were used in the analysis of the effects of WBV on CMJ height (2–4,9,11,14–18,24,27,32,35,36). A full description of the study characteristics is shown in Tables 3 and 4. Apart from Delecluse et al. (16), who had control subjects train on a stationary (placebo) vibration machine, all control groups were asked to maintain their conventional lifestyle. The WBV training protocol for most studies was multi-positional (e.g., subjects were asked to move to different positions on the vibration platform during WBV) and progressively loaded (i.e., duration of WBV increased during the study). Only 1 study asked subjects to train on the platform in a stationary position for the duration of the study (17). All vibration machines used in the studies are available commercially. Ten of the 15 studies stated the manufacturers' acceleration details (2,4,14–17,27,32,35,36), 1 calculated the acceleration from first principles (35), and 3 validated the acceleration with additional equipment (15–17). Regarding adverse effects of WBV, 1 study reported shin pain from 1 subject (15) and another study reported some erythema, edema, and itching of legs, which rapidly improved after training during the first session (32). The overall effect of WBV training without any additional exercise on CMJ height was a positive SMD of 0.77 (95% confidence interval, 0.55–0.99, p < 0.001) compared with controls (Table 5). There was no heterogeneity between studies (τ2 = 0.03, χ2 = 16.96, df = 14 [p = 0.26]; I2 = 17%). Examination of the funnel plot of SMD and standard error showed that no publication bias was detected. The trim and fill analysis showed no evidence of any potential missing studies.
Factors affecting CMJ performance are shown in Table 6. Whole body vibration produced greater results in nonathletes (0.96, 0.63–1.30; SMD, 95% CI) compared with athletes (0.59, 0.31–0.88). In addition, longer WBV training periods produced greater effect sizes (>12 weeks, 0.87, 0.56–1.19) compared with intermediate (4–12 weeks, 0.72, 0.35–1.09) or shorter training periods (<4 weeks, 0.58, −0.08–1.23). Whole body vibration training, consisting of higher frequency (>30 Hz, 0.86, 0.62–1.10), higher amplitude (>3 mm, 0.84, 0.52–1.17), and longer durations (>10 minutes per session, 0.92, 0.48–1.36), had a greater benefit for CMJ height than lower frequency (≤30 Hz, 0.56, 0.13–0.99), lower amplitude (≤3 mm, 0.66, 0.35–0.98), or shorter durations (≤10 minutes per sessions, 0.68, 0.45–0.92). The analysis of combined parameters showed the beneficial effect of WBV with high frequency and high amplitude, high frequency and low amplitude, and low frequency and high amplitude, whereas no beneficial effect of WBV with low frequency and low amplitude was detected. There was no statistically significant heterogeneity in the analysis of each contributing factor (p > 0.05).
As with CMJ performance, the overall effect of WBV training on SJ performance compared with controls without any additional exercise was a positive SMD of 0.68 (0.08–1.11, p = 0.02) (Table 7). There was no significant heterogeneity in this analysis (τ2 = 0.00; χ2 = 0.45, df = 2 [p = 0.80]; I2 = 0%).
Whole Body Vibration Vs. Exercise
A total of 4 studies compared WBV training with normal exercise on CMJ performance (Table 8). Three of the 4 studies, from the 15 previously analyzed (on WBV vs. non-exercise), had 3 comparison groups: WBV, exercise, and controls (no exercise) (3,16,32). The fourth study had 2 groups: WBV and exercise (29). Exercise protocols comprised of cardiovascular and resistive training in 3 studies (3,16,32) and a walking program in another (29). Using WBV training compared with a normal exercise program produced a moderate beneficial effect (SMD = 0.63, 95% CI = 0.10–1.15, p = 0.02). There was no significant heterogeneity detected in the analysis (τ2 = 0.17; χ2 = 7.58, df = 3 [p = 0.06]; I2 = 60%).
This study found that WBV training compared with non-exercise controls produced a moderate positive effect on CMJ (effect size, 0.77) and SJ height performance (effect size, 0.68). Similarly, WBV training compared to a normal exercise program of similar duration had a moderate effect on CMJ height (effect size, 0.63). In the same way, both the review study of Issurin (20) and the systematic review study of Rehn et al. (30) suggested a beneficial effect of WBV on jump height (20,30). However, others have found little or no effect, probably because of insensitive subjects (i.e., having a high levels of fitness) or inadequate study design (e.g., poor compliance of WBV training or high variation of routine exercise during the intervention period) (26,38).
The mechanisms behind the improvements found in muscle power (CMJ and SJ) are probably related to neural adaptations (20). Whole body vibration activates muscle spindles, thereby stimulating alpha motor neurons and enhancing stretch reflexes (6). An increase in stretch-reflex activation should increase muscle contraction, thereby producing greater muscle tone during acute WBV. However, it has been proposed that over the long term (i.e., during training), neural adaptation following WBV is similar to resistance exercise training producing enhancement of motor unit firing, motor unit synchronization, synergist muscle contraction, antagonist muscle inhibition, and adaptation of the reflex response (5,35). Enhanced synchronization of motor units and increased ability of motor units firing together produce a more effective muscle contraction and greater force production. In addition, muscular adaptation or muscle hypertrophy after vibration has been reported from experimental studies in mice (39).
It is also proposed that increases in strength after WBV may be because of hormonal changes, such as increases in growth hormone, insulin-like growth factor 1 (IGF-1), and testosterone (31). Whole body vibration may stimulate hormonal secretion via neural-hormonal regulation (higher brain or cerebral motor cortex stimulates the hypothalamic-anterior pituitary axis and autonomic system) (6). Serum growth hormone and testosterone levels were increased by 360 and 7%, respectively, immediately after 10 minutes of WBV (26 Hz, 4 mm) in young adults (5), and IGF-1 levels increased by 30% and remained for up to 2 hours after an acute bout of 5 minutes of WBV exercise (30 Hz, 4 mm) (7). Such hormonal changes may induce muscle hypertrophy, leading to improved muscle strength and power. However, the effect of regular WBV training on hormonal changes has not been investigated fully and requires additional research.
Whole body vibration training seems to improve both static and dynamic muscle strength similar to traditional exercise; however, WBV training seems to improve muscle explosive power to a greater extent than conventional exercise. Previous studies suggested that WBV training and resistance training improved CMJ height and isometric and dynamic knee extensor strengths (3,32). Muscle mass also increased in both WBV and resistive exercise groups (3). However, there was no significant difference in muscle strength and muscle mass between WBV training and resistance exercise (3,32). In contrast, Delecluse et al. (16) found that WBV and resistive exercise improved both isometric and dynamic knee extensor strength but only WBV significantly improved CMJ height by 7.6% compared with control group. Our meta-analysis study also supported the beneficial effect of WBV training on CMJ height over and above a normal exercise program. Vibration training may elicit biological adaptations similar to resistive exercise (16,32). Whole body vibration stimulates the sensory receptors and Ia afferent input, leading to a greater enhancement of stretch reflex, that generates the greater force production during a stretch-shortening contraction in CMJ (16). The neurological adaptation of resistance training (performance of voluntary contractions) has been found in the supraspinal center, descending corticospinal tracts, and spinal circuitry (8). However, resistance training has limited effect on Ia afferents, which may result in a smaller motor response in a CMJ (16). In addition, Raimundo et al. (29) also found that WBV training improved power produced during a CMJ compared with walking training in postmenopausal women. However, walking training improved time to complete the 4-m walk compared with WBV. Taking these data together, it seems that WBV training is better at improving explosive power than traditional resistance or aerobic exercise but not as good as traditional exercise when it comes to improving strength or aerobic endurance.
The results of our study suggest that a higher frequency vibration is more beneficial than lower frequency at improving jump performance. Da Silva et al. (12) showed that a frequency of 30 Hz produced an increased CMJ height (4.6%) compared with 20 Hz (0.8%) and 40 Hz (−2.7%). Similarly, Ronnestad (33) showed that a frequency of 50 Hz increased peak average power for CMJ (4.4%) of untrained subjects, whereas there was no significant change at 20 or 35 Hz. It has been found that the frequency of vibration is positively correlated with the tonic vibration reflex, which acts to enhance motor unit synchronization (25). However, a frequency that is very high (>150 Hz) has been found to reduce motor unit synchronization (25). Our results are in accord with previous research (20), which suggests that the optimal WBV frequency is between 30 and 50 Hz. It is assumed that this frequency produces muscle spindle firing rates that correspond to discharge rates of motor units during maximal muscle contraction.
Regular vibration training with a high amplitude (>3 mm) had a greater effect on jump height compared with lower amplitude (≤3 mm). Likewise, Luo et al. (22) suggested that higher amplitude (4 mm) induced larger effects than lower amplitudes. It is known that vibrations of a higher amplitude (4 mm) induce greater electromyographic activity than lower amplitudes (2 mm) (23). Increased electromyographic activity is correlated with enhanced muscle activation and therefore a greater training stimulus (13). In contrast, Gerodimos et al. (19) found that there was no significant effect of amplitudes of 4, 6, and 8 mm on SJ during acute WBV training. The inconsistency in findings is probably because of the difference in training methods, loading parameters, body positions, and types of platforms (19). Adams et al. (1) suggested that the effect of amplitude may be related to frequency; that is, to generate the greatest effect, a high amplitude (4–6 mm) should be applied with a high frequency (50 Hz), whereas a low amplitude (2–4 mm) should be applied with a low frequency (30 Hz). Petit et al. (28) found the WBV training with high frequency (50 Hz) and high amplitude (4 mm) was the most effective to significantly increase knee extensor strength and CMJ performance compared with low frequency (30 Hz) and low amplitude (2 mm). Our study suggested, in the same way, that high frequency and high amplitude was the most effective parameters to improve jump height, whereas low frequency and low amplitude may not be as effective at improving muscle explosive power.
The current study revealed that longer duration exposure (>10 minutes per session) in regular vibration training produced greater benefits than shorter duration exposures (≤10 minutes). Alternatively, it has been found that in acute studies, short duration vibration facilitated neuromuscular function, whereas long duration exposure caused muscle fatigue and decreased neuromuscular function (22). The beneficial effect of long duration exposures found in this study may be explained by the overload training principle. Most of the studies reviewed (11 of 15 studies in Table 3) used progressive loading of vibration training programs that gradually increased vibration stress on muscles. Overload training improves both neural adaptation and muscle hypertrophy, and WBV training over a longer period probably provides sufficient stimulus/overload for such adaptations to be manifested.
We found an almost linear increase in the effect of the intensity of vibration (frequency, amplitude, and exposure duration) with the duration of the training period; training of more than 12 weeks was almost twice as effective as a training period of less than 4 weeks. Presumably, regular WBV training requires a certain amount of stimulation time for adaptation and physiological changes, including possible hormonal and biochemical changes to occur. The time required, particularly for hormonal and biochemical changes, with vibration training may be similar to resistance training, which tends to be more profound after 4 weeks of training (34).
Factor analysis in the current study showed the nonathlete group had a size effect greater than the athlete group similar to what was reported by Rehn et al. (30). It is likely that WBV enhances muscle strength adaptation proportionally to the baseline strength level, so that athletes who regularly exercise, keeping the muscle trained to some degree, improved less, whereas nonathletes, who are less active and therefore less adapted, improved more.
The vulnerability of this study lies in the quality of the separate studies used in the analysis. We found that moderate-to-high methodological quality studies (9–13 points) were included in this meta-analysis study. Additionally, there was no significant heterogeneity among the included studies, which suggests that the effect sizes of the individual studies had similar treatment effects. Importantly, the trim and fill analysis and the symmetrical funnel shape graph indicated that there was no identification and selection bias detected. Other factors, such as the type of vibration machine, technique, and position of training, should be analyzed in the future for possible variances in effects.
Meta-analysis allows researchers to estimate the overall magnitude of an effect, which is useful information for practitioners in the field. Of most importance from this meta-analysis was the fact that vibration exercise was shown to improve explosive muscle power (jump performance) compared with a normal exercise program (including cardiovascular and traditional resistance training) of similar duration. The use of vibration training, particularly protocols using higher frequencies, higher amplitudes and of longer durations per session, can result in beneficial improvements in explosive power generation. We therefore suggest that vibration training should be considered along with other well-established training regimes for improvement in explosive power generation.
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