Effects of vibration-induced fatigue on the H-reflex

Abstract

Vibration exercise (VE) has been suggested as an effective training for improving muscle strength and coordination. However, the underlying physiological adaptation processes are not yet fully understood, limiting the development of safe and effective exercise protocols. To better understand the neuromuscular responses elicited by VE, we aimed at investigating the acute effects of superimposed vibration on the Hoffmann reflex (H-reflex), measured after fatiguing exercise. Twenty-five volunteers performed four isometric contractions of the right Flexor Carpi Radialis (FCR) with baseline load at 80% of their maximal voluntary contraction (MVC), both with no vibration and with superimposed vibration at 15, 30, and 45 Hz. Fatigue was estimated by MVC test and estimation of electromyographic spectral compression. H-reflex suppression was estimated as the relative decrease after exercise. Our results show that fatiguing exercise determined a decrease in H-reflex amplitude compared to rest condition while vibration determined a lower H-reflex suppression as compared to no vibration. The superimposition of 30-Hz vibration determined the largest acute reduction in force generating capacity (36 N, p < 0.05) and the lowest H-reflex suppression (20%, p < 0.05). These results suggest VE to be particularly suitable in rehabilitation programs for rapid restoration of muscle form and function after immobilization periods.

Does vibration superimposed on low-level isometric contraction alter motor unit recruitment strategy?

“…Previous studies reported increased sEMG RMS during VE (Bosco et al 1999, Roelants et al 2006, and others have shown estimates of global sEMG features during vibration (Xu et al 2016b, Sammali et al 2018. However, there are no previous studies that assessed MU recruitment by mCV analysis during vibration.…”

Individualized Whole-Body Vibration: Neuromuscular, Biochemical, Muscle Damage and Inflammatory Acute Responses

“…Conversely, a decrease in the EMG–power ratio after the WBV intervention was observed in our study; this result highlights an improved neuromechanical efficiency at submaximal loads that largely reflects the activity of high-threshold motor units 65 and results from a combination of 2 mechanisms. First, fatigue-induced reflex inhibition of the motoneuron pool excitability after WBV 66 could contribute to a reduction in the neural drive of agonist-antagonist thigh muscles (ie, the “numerator” of the EMG-power ratio); this peripheral inhibition could also explain the fatigue due to central components (central–peripheral interactions) as evidenced by the significant reduction in MIVC in the present study. In fact, when the discharge frequency of motoneurons declines toward low values, following the altered afferents induced by stretch-shorten cycles of WBV, 66 a higher central activation rate, through supraspinal projections of afferents, is delivered to motor units to exert the same relative level of strength; in this way, the central motor commands force the recruitment of new motor units, and central fatigue must appear in the task when the muscles work closest to their maximal levels.…”

“…First, fatigue-induced reflex inhibition of the motoneuron pool excitability after WBV 66 could contribute to a reduction in the neural drive of agonist-antagonist thigh muscles (ie, the “numerator” of the EMG-power ratio); this peripheral inhibition could also explain the fatigue due to central components (central–peripheral interactions) as evidenced by the significant reduction in MIVC in the present study. In fact, when the discharge frequency of motoneurons declines toward low values, following the altered afferents induced by stretch-shorten cycles of WBV, 66 a higher central activation rate, through supraspinal projections of afferents, is delivered to motor units to exert the same relative level of strength; in this way, the central motor commands force the recruitment of new motor units, and central fatigue must appear in the task when the muscles work closest to their maximal levels. 67 In this regard, the MIVCs decreased significantly in both groups, but only in the WBV intervention did EMG activity and the EMG-MIVC ratio show an increasing trend (although not significant).…”

Neuromuscular Electrical Stimulation Does Not Influence Spinal Excitability in Multiple Sclerosis Patients

“…Therefore, our protocol may have been too short to induce acute changes in the spinal circuitry function in MS patients compared to healthy individuals. Nevertheless, with long-term intervention protocols, the effect of muscle fatigue on H-reflex should be taken into account, as muscle fatigue reduces Ia-afferent excitation or increases nerve fiber excitability thresholds [87]. In this regard, we can state that muscle fatigue did not occur as MVIC values did not change between baseline levels and those measured at the end of the entire protocol in both MS and CG [39,49,56].…”