Whole body vibration (WBV) has been suggested as an attractive and time efficient complement to traditional forms of exercise for an athlete prior to performance (Cochrane, 2011, 2013; Ronnestad and Ellefsen, 2011). WBV evokes muscle contractions via tonic vibration reflex (TVR) through tendon vibration (Rittweger, 2010). The change in muscle length during vibration is detected by muscle spindles and induces a non-voluntary muscular contraction (Rittweger, 2010). An enhancement of the stretch-reflex, proposed improvements in neuron excitability and motor unit recruitment of the muscle are cited as reasons for improvements in strength and power output (Bosco et al., 1999). However, other mechanisms such as muscle temperature (Cochrane et al., 2008), blood flow (Kerschan-Schindl et al., 2001) and post activation potentiation (PAP) (Cochrane et al., 2010) have also been suggested as contributing factors. Yeung et al. (2014) suggest that these mechanisms are seldom investigated in acute WBV settings and suggest that if immediate muscle facilitation is the result of homonymous [alpha]--motoneurons activation, the effect should be seen in force output and motor unit recruitment. An observation not detected by Yeung et al. (2014) who reported no change in quadriceps stretch-induced reflex or peak force, findings consistent with other researchers who have also questioned the effectiveness of acute WBV (Hannah et al., 2013).
Acute WBV has been investigated as a potential ergogenic aid amongst coaches to induce immediate performance benefits prior to performance (Bullock et al., 2008) or during half-time rest periods in soccer to help prepare for the second half performance (Lovell et al., 2013). Towlson et al. (2013) discuss how 58% of Premier League/Championship football team practitioners incorporated half-time re-warm up strategies and Russell et al. (2015) suggest a PAP activity should be incorporated during the final 5 minutes of a half-time scenario in team sports. Any additions to a warm up routine should be carefully selected as these could contribute to an increased risk of fatigue leading to a decrease in performance or increased injury risk (Impellizzeri et al., 2013). Level of conditioning of the athlete should also be considered when implementing an acute conditioning exercise with the aim of PAP due to initial strength levels being dependent on the success of the intervention (Chiu et al., 2003; Seitz et al., 2014; Seitz and Haff, 2015). Depending on the extent of the pre-conditioning activity, the muscle's impending activation can either be impaired by fatigue or enhanced by a phenomenon known as PAP (Sale, 2002), through a combination of neurogenic and non-neurogenic responses (Sale, 2002). This is seen by coaches as a particularly positive quality when preparing athletes for competition. Rittweger et al. (2003) identified increases in mean power frequency during sustained isometric contraction of the vastus lateralis after acute WBV. The authors suggest a central nervous recruitment of predominantly large motor units to maintain force output following acute WBV (Rittweger et al., 2003). Torvinen et al. (2002) identified that an acute bout of WBV improvements in strength and power of the lower extremities suggesting neural adaptations may have occurred, the authors note that the acute WBV was long enough (4 minutes) to stimulate without fatiguing the muscle.
The idea of non-neurogenic factors such as potentiation of muscle twitch force has been suggested with electromyography (EMG) (Bosco et al., 2000), and Cochrane et al. (2010) identified that acute WBV induces PAP via non-neurogenic twitch potentiation and not neurogenic twitch potentiation. Jordan et al. (2010) however questions the influence of PAP following bouts of acute WBV, indicating an attenuation in knee extensor peak force values, not an improvement. These findings suggest further investigation is warranted into twitch potentiation and non-neurogenic factors following acute WBV. Knee extensor twitch potentiation has been correlated with improved performance in sprint and counter movement jump in elite soccer players (Requena et al., 2011).
A large amount of the current WBV research has used non-elite/moderately trained or sedentary populations as participants, with a few notable exceptions (Bullock et al., 2008; Cochrane and Stannard, 2005; Despina et al., 2013; Issurin and Tenenbaum, 1999; Lovell et al., 2013; Ronnestad, 2009). This has highlighted the need to investigate the different responses amongst different groups and possibly sub-groups and the underpinning neuromuscular effects (Ronnestad and Ellefsen, 2011). One specific group of interest is the difference between amateur and elite athletes. Evidence demonstrates that amateur and elite athletes differ in responses to WBV, with elite athletes showing greater increases in force output, muscle sensitivity to stimulus and balance (Cloak et al., 2014; Ronnestad, 2009). Further, Cloak et al. (2014) suggested acute WBV may impair balance and landing stability amongst amateur soccer players due to fatigue, in comparison to elite soccer players' balance and landing stability. Cloak et al. (2014) speculated differences between groups were attributed to differences in strength levels and neuromuscular responses to WBV. Isometric peak force output of the knee extensors has been identified as a distinguishing strength characteristic between professional and amateur soccer players, with professional players producing significantly higher values then amateurs (Gissis et al., 2006).
Few studies have compared perceptions of benefits of acute WBV between trained and untrained individuals. Belief effects are typically studied under the rubric of examining a placebo effect, defined as positive outcome arising from the belief that a beneficial treatment has been received (Beedie and Foad, 2009). Ronnestad et al. (2013) identified improved sprint performance related to perceived improvement in feeling of well-being in the legs following acute WBV in elite ice-hockey players. Marin et al. (2015) reported that untrained participants indicated a higher RPE when exposed to acute WBV at 50Hz compared to 30Hz and recommend 30Hz for untrained individuals. Beliefs in the likely effectiveness of an intervention or ergogenic aid have found to have an incremental effect on performance (Beedie and Foad, 2009). Individuals who positively believed that an intervention will be effective appear to gain greater benefits than participants who do not. Results of Beedie and Foad (2009) suggest that a belief effect could shape the efficacy of an intervention. Therefore, it seems prudent to assess beliefs in the effectiveness of an intervention during the evaluation. Assessing beliefs is becoming used more regularly in applied research. For example, Finch (2011) proposed monitoring athletes via self-report measures to try to identify perceived benefits on training interventions for injured athletes.
The aim of the present investigation, therefore, was to compare the acute effects of WBV amongst professional and amateur soccer players on muscle activation, PAP and peak isometric force of the knee extensors during a maximal voluntary contraction (MVC) and the perceptions of benefits of the intervention between groups.
Forty-four male soccer players (age 23.1 [+ or -] 3.7 yrs., body mass 75.6 [+ or -] 8.8 kg and height 1.77 [+ or -] 0.05 m) volunteered to take part in this study. The 22 professional players (English Football League 1) (age 24.1 [+ or -] 3.8 yrs., body mass 77.1 [+ or -] 7.4 kg and height 1.78 m [+ or -] 0.07 m) on average trained 12-14 hours per week and played one or two games a week for the previous 3-5 years. The 22 amateur players (age 22.1 [+ or -] 3.4 yrs., body mass 74.1 [+ or -] 9.9 kg and height 1.76 [+ or -] 0.06 m) on average trained between 3-6 hours per week and played up to one game per week over the previous season. All players reported to be free from injury, including concussion or mild head injury within the last year and no reported musculoskeletal injury within the last three months prior to the study. All participants had a minimum of 1 year's regular strength and power training and two days before familiarisation and testing days participants were instructed to minimize strength and power training involving the lower body and avoid adrenergic-enhancing substances, such as caffeine (Chiu et al., 2003). This research protocol was approved by the University Ethics Institutional Review Board of the University of...