Acute effect of whole body vibration on jumping performance

INTRODUCTION

In last decades, the application of vibration on human body has been widely documented. This sort of training, applied through vibrating platforms, is know as whole-body vibration (WBV). It has been used to prevent bone loss (Rubin et al., 2004; Verschueren et al., 2004), to improve muscle strength (Verschueren et al., 2004) and balance (Bruyere et al., 2005) in elderly people. It has been also used in parkinsonian subjects (Turbanski et al., 2005), in patient with multiple sclerosis (Schuhfried at el., 2005) and even in persons with spinal cord injury (Gianutsos et al., 2000). However, its most extended application is related to neuromuscular performance in trained and untrained healthy people. WBV has demonstrated an increase in EMG activity (Delecluse et al., 2003), muscle power (Issurin et al., 1999), maximal isometric strength (Torvinen et al., 2002), maximal dynamic strength (Bosco et a., 1999) and counter-movement jump capacity (Bosco et al., 2000; Torvinen et al., 2002). It is still unclear optimum frequency (Hz), amplitud (mm) and duration (s) of the vibration stimulus to originate muscular activation (Luo et al., 2005). The object of this study is to evaluate acute effects of WBV with different frequencies and durations, on jumping performance.

METHODS

Subjects A group of 114 university students (male 37, female n 77) participated in the study (mean (SD) age 19.6 (2.0) years). Body mass was 77.4 (11.0) kg for males and 58.8 (7.5) kg for females. Mean height was 176.6 (0.0) cm and 163.3 (0.0) cm in males and females respectively. Body mass index in males was 24.8 (3.1) kg / m2 and 22.0 (2.2) kg / cm 2. Study setting All the participants was evaluated in two different days, with one week separation between each one. In Day 1, subjects carried out the Counter Movement Jump (CMJ) test. In Day 2, one week after, they carried out the CMJ test again, with a previous vibration stimulus on a vibrating platform (WBV). The vibration stimulus used different frequencies and duration to analyze the influence of these variables over the effect of WBV on jumping ability. It was made 5 experimental groups according to frequency and duration of the WBV intervention applied. Amplitud was equal for all groups. Jumping measurements Vertical CMJ test (hands kept on the pelvis) was used to assess the lower-limb explosive performance capacity (Bosco et al., 1983). CMJ test was performed on a contact platform (Ergojump Bosco System), which gives the time the subject is on air in milliseconds (accuracy 0.001 s). The obtained flight time (t) was used to estimate the height of the rise of body centre of gravity (h) during the vertical jump, i.e. h = g · t2 / 8, where g = 9.81 m/s2. In Day 1, subjects performed three trials of CMJ with 1 minute rest between them. The median value of three measurements was used as a test score. In Day 2, they were exposed to WBV and inmediatly after performed three trials of CMJ: 1) without resting, 2) 1 minute after WBV and 3) 2 minutes after WBV. Vibration loading WBV was carried out in a standing position on the vibration platform (a prototype of Galileo, 900, Novotec Maschinen GmbH, Pforzheim, Germany). Knee angle was kept between 120º-150º. The amplitud of vibrations at the end of the 44.4 cm long tilting platform was 4.2 mm. Vibration amplitud of the stimulus was equal for all five experimental groups, being controlled keeping the feet at 16.3 cm of the centre of the platform for all the subjects. Vibration frequency and duration were different between experimental groups. All groups combined different kinds of position over the platform, using not only static posture but dynamic movements too. That seems to be the most apropiate way to get muscular activation (Luo et al., 2005). Dynamic movements were half squat perfomed with a rithm of 2.4 s to complete a cicle (dow and up movement). Vibration and exercise characteristics of five groups can be consulted in Table 1. Statistical analysis Statistical analysis was carried out using statistic software SPSS 12.0. Mean and standard deviation (SD) are given as descriptive statistics. Kolmogorov-Smirnov test was used to verify the normal distribution of the variables. As the result demonstrated the variabes were not normally distributed, statistically significance differences were analysed using Wilcoxon test for non-paramethric variables.

RESULTS

In Table 2 are shown the results of CMJ test perfomed normally (i.e., without WBV) and perfomed after WBV applied with different frequencies and duration. In all groups, except group 1 (20 Hz – 90 s), CMJ was reduced immediately after the aplication of WBV (p= 0.036, 0.006, 0.001 and 0.009 in groups 2, 3, 4 and 5 respectively). Only the application of WBV at 25 Hz and 90 s produced an enhancement of CMJ perfomed after 1 minute rest (p=0.021). No significance improvements in CMJ were shown in the rest of WBV aplications. However, in group 1 (20 Hz – 90 s) CMJ high was enhanced 0.5 cm 1 minute after WBV. In addition, in group 2 (25 Hz – 90 s) CMJ 2 minutes after WBV was improved more than 1 cm (Fig. 1).

DISCUSSION

CMJ high is decreased immediately after WBV, so it is shown that this kind of training produces acute muscular fatigue. As it is shown by p significance values in 5 Table 1, this decrease immediately after WBV seems to be more pronunciated when frequency and duration are higher. The vibration stimulus of 25 Hz and 90 s has shown as the most appropiate to produce an increase in CMJ high. None other combination of frequency and duration improves CMJ performance, although results show moderate improvements no statistically significant. In all WBV intervention, except 25 Hz -120 s, normal CMJ high is reached two minutes after vibration stimulus. That fact shows a complete recovery after just 2 minutes, so we could say that WBV produces a fast and local fatigue on muscles activated. Torvinen et al. (2002) showed a mean increase of 2.2 % in CMJ high 2 minutes after WBV, with stimulus of 4 minutes and 15-30 Hz. The shorther stimulus used in our estudy could be the reason for the lack of improvements in CMJ high two minutes after WBV. Bosco et al. (2000) showed an increase of more than 1 cm in the CMJ high after WBV applied by ten sets of 1 minute and 26 Hz. This frequency is similar to that used successfully in our study. However, the duration used by Bosco et al. (2000) is quite longer and the subjects used in their estudy were more physically active (engaged in a team sport training program three times a week). Previous studies have shown the positive influence of training level on the effects of WBV (Luo et al., 2005).

CONCLUSIONS

WBV training programs of 25 Hz and (at least) 90 s produces an improvement in CMJ high in healthy young people. Previous studies show that longer stimulus applied to subjects with a higher fitness level, could produces greater benefits of this sort of training on jumping perfomance.

ACKNOWLEDGMENTS

We thank all the participants for taking part in this study. We specially thank the researchers participating in the data collection: J Ruiz Ruiz, M Lang Tapias and MS Cervantes Borunda. Special mention for main researchers of EFFECTS group: A Gutiérrez Sáinz and MJ Castillo Garzón.