Both a WBV program and an ER training program improved isokinetic ankle and knee muscle strength and the explosive strength of the subjects after a 6-week training period. These findings are in accordance with those of several authors, affirming that WBV increases the dynamic strength of the lower extremity muscles.
Roelants et al
investigated the effects of 24 weeks of WBV on knee extension strength in 89 postmenopausal women in a randomized controlled study. Isokinetic and dynamic strength of the knee extensors increased in both WBV and traditional resistance training groups, with the training effects not significantly different between the groups. Similarly, Delecluse et al
concluded that a WBV program can induce a strength gain in the knee extensors of previously untrained females to the same extent as a traditional resistance training program. In these studies, the traditional resistance training programs and vibration programs consisted of different exercises. Therefore, whether WBV training had an additional training value remains uncertain.
In our study, the ER training program consisted of exactly the same exercises as in the WBV group in order to evaluate the supplemental value of vibration training. Interestingly, our results reveal that the gains in explosive strength and in plantar-flexor strength at low speed were significantly higher in the WBV group than in the ER group after 6 weeks of training.
Using the same study design, Ronnestad
concluded that the maximal strength of recreationally resistance-trained men increased significantly more after 5 weeks of a vibration program than after an equivalent training program.
have tried to find a plausible explanation for these positive effects of vibration training. Some investigators have suggested that the large strength gain is the result of the tonic vibration reflex. They stated that standing on a vibration plate provokes length changes in the muscle that stimulate the muscle spindles. (In these studies, knee flexor and extensor muscles were tested.) These receptors would elicit the tonic vibration reflex. In addition, it has been proposed
that the recruitment thresholds of the motor units during WBV are expected to be lower than during voluntary contractions, probably resulting in a more rapid activation and training of high-threshold motor units. Therefore, it has been suggested that WBV training specifically trains fast-twitch fibers,
which are responsible for explosive power.
In our study, the WBV group showed a significantly greater gain on the high box test than the ER group after 6 weeks of training. This finding is in agreement with results of previous studies that showed that WBV training has a positive effect on explosive strength. Delecluse et al
reported that jumping height increased significantly over 12 weeks in the WBV group and remained unchanged in the 3 other groups (control, placebo, and traditional resistance training). Also, Ronnestad
found a significant improvement in vertical jumping height after subjects performed squats on a vibration platform for 5 weeks.
In our study, neither WBV nor ER training for 6 weeks resulted in a convincing effect on postural control. This finding is in agreement with that of Torvinen et al,
who showed that 4 months of vibration training produced no effect on the dynamic or static balance of young, healthy subjects. However, stroke patients with unilateral impairment showed an increase in their weight-shifting speed at the balance assessment after 1 session of WBV training.
In geriatric patients, WBV training improved postural control.
After 4 months of training, chair-rising time improved 18% in fit elderly participants, whereas the control group showed no significant differences. Consequently, we can speculate that WBV training only has a positive significant effect when the postural control of the subjects is disturbed.
In order to rate the perceived exertion of both training programs, each subject completed a Borg scale after each training week. No significant differences were noted between the groups except during week 5, when the WBV group rated the exercises lower (easier). We know that the amplitude of the vibrations was reduced from 4 mm to 2 mm just before week 5, whereas the frequency increased from 26 Hz to 28 Hz. It is possible that the amplitude of the vibrations had an important influence on the perceived exertion of the subjects. In previous studies, attention was paid only to the frequency of the vibrations.
The limitations of our study should be noted. Although the WBV training group showed a significantly greater gain in explosive strength and in plantar-flexor strength at low speed, we should take into account the fact that the WBV group was bigger and older than the ER group at baseline. Therefore, we have analyzed the change scores of both groups and not the absolute end values. In our study, no true control group was included. One could suggest that the young subjects might have had strength improvements regardless of training. Another limitation of our study was that we have not studied the length of the training effects. Therefore, future researchers should include a follow-up of the length of the training effects. Finally, not performing a Bonferroni correction in order to take type I errors into account when analyzing several dependent variables is also an important limitation of our study.
In conclusion, neither WBV training nor ER training seemed to have an effect on the postural control of young healthy skiers. However, both training programs improved isokinetic ankle and knee muscle strength and explosive strength after 6 weeks of training. Moreover, WBV training resulted in a significantly greater gain in explosive strength and plantar-flexor strength at low speed compared with ER training after 6 weeks. Therefore, our findings support the hypothesis that WBV training can be a beneficial addition to traditional strength programs.