Whole-body vibration (WBV) has been widely used as an adjunct to traditional resistance training among sports and rehabilitation professionals to improve muscular performance. WBV differs from traditional weight training program in which the loading is a passive sinusoidal vibration induced to the musculoskeletal system while for weight training program, the targeted muscles are actively recruited and loaded at specific intensity. The sinusoidal vibration has been suggested causing rapid lengthening and shortening of the muscle tendon unit (Crochrane et al., 2009). This is based on the earlier work by Eklund and Hagbarth (1966); vibration evokes muscle contractions via tonic vibration reflex (TVR) produced by tendon vibration. The change in muscle length during vibration is detected by muscle spindles, innervated by the Ia-afferents resulting in facilitating homonymous alpha-motorneurons and induces a non-voluntary muscular contraction. Therefore, it is reasonably hypothesized that vibration of the musculoskeletal system leads to enhancement of the stretch-reflex loop and subsequently improves neuron excitability and motor recruitment of the muscle (Bosco et al., 1999). However, this proposed mechanism has seldom been investigated in either acute or chronic exposure to whole body vibration. If immediate muscle facilitation is the consequence of facilitation of the homonymous alpha-motorneurons via the enhancement of the muscle spindles activities, the stretched induced reflex should be the most direct measurement of this response. Moreover, the enhanced motoneuron excitability should facilitate the peak torque performance. However, the stretch reflex amplitude had been shown increase (Rittweger et al., 2003), remain static (Cochrane et al., 2010; Hopkins et al., 2009), and even decrease (Ritzmann et al., 2013) following acute exposure of WBV. In the same light, research on the effects of acute WBV on muscular performance has not provided unequivocal results. An acute bout of WBV has been reported to improve power (e.g., Cochrane and Stannard 2005; Ronnestad 2009), but its effects on strength seems to be less clear (e.g., Torvinen et al., 2002a; 2002b). However, there are also studies that showed WBV did not enhance muscle performance (e.g., Bagheri et al., 2012 and Gerodimos et al., 2010). Thus, the possible underlying neuromuscular mechanisms and muscular performance in response to acute exposure to WBV are not well understood.
The neurophysiological effects of isolated vibration applied to muscle belly or the tendon has been well established. Earlier studies by Burke and others (1976) have shown that the TVR is primarily attributable to muscle spindle Ia fibers, which are able to respond one-to-one to vibration frequencies of up to 200 Hz. In humans, it has been reported that tendon vibration of the pretibial muscles with a peak-to-peak amplitude of 0.2-0.5 mm, selectively activating the muscle spindle Ia afferents (Roll et al., 1989), and larger amplitude vibration (1-5 mm) causes muscle spindle primary and secondary endings in the same muscles to respond in a 1:1 manner with vibration frequency of up to 100 Hz (Burke et al., 1976). While these phenomenons were primarily shown in vibration applied directly to the muscle belly and tendon, Pollock et al. (2012) recorded the effects of WBV on the motor units (MU) discharge of the vastus lateralis muscle. The results demonstrated that the MU firing is phase locked to the vibration cycle, indicating the presence of reflex muscle activity similar to the TVR. If the TVR were observed during WBV, then a reduction in MU recruitments threshold were likely occur, and this might result in the enhancement of power and strength performance.
The aims of this study were to determine whether stretch-induced patellar tendon reflex will be enhanced after acute exposure to WBV, and if stretch reflex is enhanced, whether isokinetic knee extensor peak torque performance will be improved. More specifically, the reflex latency and electromechanical delay (EMD) of the patellar tendon reflex subjected to a WBV protocol of 45 Hz, amplitude of 0.69 mm, and duration of 3 minutes were investigated. The reflex latency is the time interval from the impact to the onset of muscle activation and is a good indicator of the muscle spindle sensitivity. EMD refers to the lag time between the stimulation of a muscle and the development of muscle tension; it is an indicator of spindle sensitivity through alpha-gamma co-activation. We opted for WBV device that produced vertical oscillation with a higher frequency (ie 30 Hz and low amplitude
Experimental approach to the problem
A prospective, double-blinded, randomized study was designed to compare the neurophysiological changes and muscular performance in the intervention group and the control group. Both the assessor and the subjects were blinded to the group assignment. The intervention session was supervised by another assessor who was not involved in the measurements. The subjects participated in a single testing session. The neural effect (the latency of reflex and the EMD) and the muscular performance (i.e., the peak torque) were performed before (pre) and immediately after (post) the intervention.
A total of 27 subjects, 11 male (age 25.7 [+ or -] 3.1 yrs, body height 1.72 [+ or -] 0.10 m, body mass 64.5 [+ or -] 13.4 kg) and 16 female (age 25.8 [+ or -] 3.7 yrs, body height 1.61 [+ or -] 0.11 m, body mass 50.2 [+ or -] 10.4kg), volunteered to participate in the study. Participants were between 20 and 35 years old. Ten of the subjects (37.03%) did not regularly exercise, 11 (40.74%) exercised 1-3 hours per week, 3 (11.11%) exercised 3-6 hours per week, 2 (7.41%) exercised 6-10 hours per week, and 1 (3.7%) exercised for more than 10 hours per week. None of the subjects trained regularly with sports teams or clubs. Inclusion criteria required that subjects did not have any of the following: history of lower limb injury in the previous 2 years, neurological or circulatory disorders, lower limb prosthesis, or current pregnancy. Subjects were randomly assigned to the intervention group (with WBV) or the control group (without WBV) by drawing...