Abstract
Increased muscle activation during whole-body vibration (WBV) is mainly ascribed to a complex spinal and supraspinal neurophysiological mechanism termed the tonic vibration reflex (TVR). However, TVR has not been experimentally demonstrated during low-frequency WBV, therefore this investigation aimed to determine the expression of TVR during WBV. Whilst seated, eight healthy males were exposed to either vertical WBV applied to the leg via the plantar-surface of the foot, or Achilles tendon vibration (ATV) at 25Hz and 50Hzfor 70s. Ankle plantar-flexion force, tri-axial accelerations at the shank and vibration source, and surface EMG activity of m. soleus (SOL) and m. tibialis anterior (TA) were recorded from the unloaded and passively loaded leg to simulate body mass supported during standing. Plantar flexion force was similarly augmented by WBV and ATV and increased over time in a load- and frequency dependent fashion. SOL and TA EMG amplitudes increased over time in all conditions independently of vibration mode. 50Hz WBV and ATV resulted in greater muscle activation than 25Hz in SOL when the shank was loaded and in TA when the shank was unloaded despite the greater transmission of vertical acceleration from source to shank with 25Hz and WBV, especially during loading. Low-amplitude WBV of the unloaded and passively loaded leg produced slow tonic muscle contraction and plantar-flexion force increase of similar magnitudes to those induced by Achilles tendon vibration at the same frequencies. This study provides the first experimental evidence supporting the TVR as a plausible mechanism underlying the neuromuscular response to whole-body vibration.
Highlights
Whole-body vibration (WBV) exercise has been shown in some cases to acutely increase muscle activation during exposure [1,2], lead to post-activation potentiation [3,4], and improve muscular performance [5,6]
Whilst the tonic vibration reflex (TVR) could account for increases in muscle activity seen during WBV, other mechanisms such as voluntary muscle conditioning contractions [13] and increased muscle temperature may contribute to any performance effects seen after WBV, with muscle-tuning and neuromuscular factors of both peripheral and central origin proposed as candidate mechanisms [3,8]
plantar-flexion force (PFF) increased over time during 50Hz vibration of both loaded Achilles tendon vibration (ATV) (p
Summary
Whole-body vibration (WBV) exercise has been shown in some cases to acutely increase muscle activation during exposure [1,2], lead to post-activation potentiation [3,4], and improve muscular performance [5,6]. Various neural mechanisms have been implicated in WBVinduced increased muscle activity. Despite the lack of direct evidence, the most frequently cited mechanism underpinning the WBV response is a reflex muscular contraction termed the tonic vibration reflex (TVR) that occurs during direct vibratory musculo-tendinous stimulation [7,8]. Whilst the TVR could account for increases in muscle activity seen during WBV, other mechanisms such as voluntary muscle conditioning contractions [13] and increased muscle temperature may contribute to any performance effects seen after WBV, with muscle-tuning and neuromuscular factors of both peripheral and central origin proposed as candidate mechanisms [3,8]
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