Acute effects of whole body vibration during passive standing on soleus H-reflex in subjects with and without spinal cord injury

Whole-body vibration (WBV) is being used to enhance neuromuscular performance including muscle strength, power, and endurance in many settings among diverse patient groups including elite athletes. However, the mechanisms underlying the observed neuromuscular effects of WBV have not been established. The extent to which WBV will produce similar neuromuscular effects among patients with neurological impairments unable to voluntarily contract their lower extremity muscles is unknown. We hypothesized that modulation of spinal motorneuronal excitability during WBV may be achieved without voluntary contraction. The purpose of our study was to describe and compare the acute effects of WBV during passive standing in a standing frame on the soleus H-reflex among men with and without spinal cord injury (SCI). In spinal cord intact participants, WBV caused significant inhibition of the H-reflex as early as 6 s after vibration onset (9.0 ± 3.9%) (p < 0.001). The magnitude of the H-reflex gradually recovered after WBV, but remained significantly below initial values until 36 s post-WBV (57.5 ± 22.0%) (p = 0.01). Among participants with SCI, H-reflex inhibition was less pronounced with onset 24 s following WBV (54.2 ± 18.7%) (p = 0.03). The magnitude of the H-reflex fully recovered after 60 s of WBV exposure. These results concur with prior reports of inhibitory effects of local vibration application on the H-reflex. Our results suggest that acute modulation of spinal motoneuronal excitability during WBV can be achieved in the absence of voluntary leg muscle contractions. Nonetheless, WBV has implications for rehabilitation service delivery through modulation of spinal motoneuronal excitability in individuals with SCI.