Is the Reflex Pathway Activated by Whole-body Vibration Change in Case of Voluntary Contraction?

Whole-body vibration (WBV) has beneficial neuromuscular effects on muscle strength increase. Supraspinal, spinal, and peripheral mechanisms have been proposed to explain these beneficial effects. The most commonly proposed explanatory mechanism is spinal segmental reflexes. However, the neuronal circuit and receptors of the reflex response have not been defined precisely. A group of researchers found that the reflex system is the Tonic vibration reflex (TVR) under the neuromuscular effects of WBV; Other researchers claim that WBV activates a different spinal reflex than TVR.

Tonic vibration reflex is a polysynaptic reflex that occurs as a result of muscle spindle activation, in which more than 100 Hz vibrations are applied to the belly or tendon of the muscle.

A group of researchers argues that WBV activates the spinal reflex response, but this reflex response is different from TVR. According to them, WBV-induced reflex (WBV-IR) response latency is longer than TVR latency. WBV activates TVR at very attenuated amplitude; WBV activates a different spinal reflex with longer latency at medium and high amplitude vibration. They reported that although the H-reflex, T-reflex, and TVR latency was longer in the spastic soleus muscle than normotonic soleus muscle, where the muscle spindle and Ia afferent pathway were hyperactive. However, the WBV-IR latency was similar in both spastic and normotonic soleus muscle.

According to hypothesis of the present study , the reflex system activated by WBV changes depending on whether there is voluntary contraction or not: if the vibration is applied during voluntary contraction, the tonic vibration reflex is activated; In the absence of voluntary contraction (when the muscle is at rest), the bone myoregulation reflex is activated. The purpose of this research is to test this hypothesis.