Evaluating the Differential Electrophysiological Effects of the Focal Vibrator on the Tendon and Muscle Belly in Healthy People

OBJECTIVE: To investigate the electrophysiological effects of focal vibration on the tendon and muscle belly in healthy people. METHODS: The miniaturized focal vibrator consisted of an unbalanced mass rotating offset and wireless controller. The parameters of vibratory stimulation were adjusted on a flat rigid surface as 65 µm at 70 Hz. Two consecutive tests on the different vibration sites were conducted in 10 healthy volunteers (test 1, the Achilles tendon; test 2, the muscle belly on the medial head of the gastrocnemius). The Hoffman (H)-reflex was measured 7 times during each test. The minimal H-reflex latency, maximal amplitude of H-reflex (H(max)), and maximal amplitude of the M-response (M(max)) were acquired. The ratio of H(max) and M(max) (HMR) and the vibratory inhibition index (VII: the ratio of the H(max) after vibration and H(max) before vibration) were calculated. The changes in parameters according to the time and site of stimulation were analyzed using the generalized estimating equation methods. RESULTS: All subjects completed the two tests without serious adverse effects. The minimal H-reflex latency did not show significant changes over time (Wald test: χ(2)=11.62, p=0.07), and between the two sites (χ(2)=0.42, p=0.52). The changes in H(max) (χ(2)=53.74, p<0.01), HMR (χ(2)=20.49, p<0.01), and VII (χ(2)=13.16, p=0.02) were significant over time with the adjustment of sites. These parameters were reduced at all time points compared to the baseline, but the decrements reverted instantly after the cessation of stimulation. When adjusted over time, a 1.99-mV decrease in the H(max) (χ(2)=4.02, p=0.04) and a 9.02% decrease in the VII (χ(2)=4.54, p=0.03) were observed when the muscle belly was vibrated compared to the tendon. CONCLUSION: The differential electrophysiological effects of focal vibration were verified. The muscle belly may be the more effective site for reducing the H-reflex compared to the tendon. This study provides the neurophysiological basis for a selective and safe rehabilitation program for spasticity management with focal vibration.