Assessment of the H-reflex at two contraction levels before and after fatigue

The aim of the present study was to compare the H-reflex evoked at rest and at 20% maximal voluntary contraction (MVC) prior to and after fatiguing the lateral gastrocnemius (LG). The maximal H-reflex and M-wave were recorded in the LG, and soleus (SOL). Electrical evoked potentials were delivered to the posterior tibial nerve when muscles were inactivated and at 20% MVC. After fatigue, the H_max /M_max ratio of the fatigued LG was increased for both contraction levels (rest and 20% MVC) and remained unaltered for non-fatigued SOL. Before fatigue, the H_max /M_max ratio of SOL was enhanced at rest compared with the H_max /M_max ratio at 20% MVC. No differences were observed for LG. Fatigue of a single muscle leads to increased spinal reflex activity of the homonymous muscle. Contrary to previous recommendations in the literature, there appears to be no benefit with regard to the H-reflex amplitude in evoking electrical potentials during constant voluntary contractions at 20% MVC compared with inactivated muscles. The observed difference in SOL prior to fatigue was most likely due to hyperpolarization of the muscle fiber membrane.