Optimal whole-body vibration settings for muscle strength and power enhancement in human knee extensors Pierre-David Petit a , Manuela Pensini a , Joël Tessaro a , Claude Desnuelle b , Patrick Legros a,b , Serge S. Colson a,⇑ a University of Nice-Sophia Antipolis, Laboratory of Human Motricity, Education and Health, Faculty of Sport Sciences, B.P. 3259, F 06205 Nice Cedex 3, France b Physical Medicine and Rehabilitation Department, Archet 1 University Hospital (CHU), 151 Route Saint-Antoine de Ginestière, B.P. 79, F 06202 Nice Cedex 3, France article info Article history: Received 30 June 2010 Received in revised form 2 August 2010 Accepted 2 August 2010 Keywords: Strength gains Muscle activation M wave Twitch contractile properties Isokinetic Jump performance abstract This study compared the effects of 6-week whole-body vibration (WBV) training programs with different frequency and peak-to-peak displacement settings on knee extensor muscle strength and power. The underlying mechanisms of the expected gains were also investigated. Thirty-two physically active male subjects were randomly assigned to a high-frequency/high peak-to-peak displacement group (HH; n = 12), a low-frequency/low peak-to-peak displacement group (LL; n = 10) or a sham training group (SHAM; n = 10). Maximal voluntary isometric, concentric and eccentric torque of the knee extensors, maximal voluntary isometric torque of the knee flexors, jump performance, voluntary muscle activation, and contractile properties of the knee extensors were assessed before and after the training period. Sig- nificant improvement in knee extensor eccentric voluntary torque (P < 0.01), knee flexor isometric volun- tary torque (P < 0.05), and jump performance (P < 0.05) was observed only for HH group. Regardless of the group, knee extensor muscle contractile properties (P < 0.05) were enhanced. No modification was observed for voluntary muscle activation or electrical activity of agonist and antagonist muscles. We con- cluded that high-frequency/high peak-to-peak displacement was the most effective vibration setting to enhance knee extensor muscle strength and jump performance during a 6-week WBV training program and that these improvements were not mediated by central neural adaptations. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Whole-body vibration (WBV) is a fairly new method to increase muscle strength and power in healthy humans (Bosco et al., 1998; Mahieu et al., 2006; Paradisis and Zacharogiannis, 2007; Savelberg et al., 2007; Colson et al., 2010). After short-term training programs lasting 6 weeks or less, maximal voluntary isometric (Savelberg et al., 2007; Colson et al., 2010) and concentric (Mahieu et al., 2006) muscle strength was reported to be improved, particularly for knee extensor muscles, which are most often analyzed. In addi- tion, substantial and significant gains in muscle power, generally assessed by means of vertical jump performance, were achieved within this training duration (Bosco et al., 1998; Rønnestad, 2004; Paradisis and Zacharogiannis, 2007; Wyon et al., 2010). In contrast, some studies found no benefit from WBV training pro- grams of the same duration (de Ruiter et al., 2003a; Delecluse et al., 2005) or even longer (de Ruiter et al., 2003b). The heteroge- neity in WBV protocols and settings, devices, subjects’ pretraining status, gender and inter-individual differences can account for these discrepant results. Also, not all the studies systematically included a sham intervention group in their experimental design. For all these reasons, the optimal prescription of WBV training pro- grams to improve muscle strength and power in physically active subjects continues to be debated. A WBV training session is characterized by the interaction between the vibration frequency and the peak-to-peak displacement of the platform that determines the WBV acceleration, and thus the load that WBV imposes on the neuromuscular system during train- ing (Mester et al., 2002; Luo et al., 2005). Due to the diverse com- binations of WBV frequency and peak-to-peak displacement, it is extremely challenging to establish optimal recommendations for effective WBV training programs. Moreover, although the body of literature on WBV is steadily increasing, information is sparse regarding the effects of both WBV frequency and peak-to-peak dis- placement post-training. Surprisingly, only one study has at- tempted to determine the optimal WBV settings for a 4-week training program (Savelberg et al., 2007). Although these authors did not include a sham training group in their experimental design, they concluded that the WBV frequency did not affect muscle strength gains. Moreover, to the best of our knowledge, no study has investigated the isolated effect of peak-to-peak displacement after WBV training. Therefore, it remains unclear whether the 1050-6411/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2010.08.002 ⇑ Corresponding author. Tel.: +33 0 492 296 535; fax: +33 0 492 296 549. E-mail address: colson@unice.fr (S.S. Colson). Journal of Electromyography and Kinesiology 20 (2010) 1186–1195 Contents lists available at ScienceDirect Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin