Eccentric exercise inhibits the H reflex in the middle part of the trapezius muscle

The objectives of this study were to (1) investigate the modulation of the H reflex immediately after and 24 h after eccentric exercise in the presence of delayed-onset muscle soreness (DOMS) and (2) test the reproducibility of the H reflex in trapezius across days. H reflexes were recorded from the dominant middle trapezius muscle by electrical stimulation of the C3/4 cervical nerve in ten healthy subjects. DOMS was induced by eccentric exercise of the dominant shoulder. H reflexes were obtained in four sessions: “24 h before”, “Pre”, “Post”, and “24 h after” eccentric exercise. Ratios of maximal H reflex and M wave responses (H _max/M _max) were compared between sessions. In addition, a between session comparison was done for the ratios of H reflex amplitudes (H _{i_75}/M _max, and H _{i_50}/M _max) obtained from the stimulus intensity needed to obtain 75 and 50 % of H _max at “24 h before”. No ratio changes were found when comparing “24 h before” and “Pre” recordings. A decrease in H _{i_50}/M _max was found at “Post” (P < 0.05) and decreases in both H _{i_75}/M _max and H _{i_50}/M _max were observed at “24 h after” (P < 0.05). This study presented evidence that an acceptable day-to-day reproducibility of the H reflex could be obtained with the applied experimental setup. Furthermore, immediately after and 24 h after exercise a stronger stimulus intensity was needed to reach the same magnitude of the H reflex reflecting that the recruitment curve was shifted to the right. This modulation of the stimulus–response relationship could be caused by presynaptic inhibition of Ia afferent fibres’ input to the motoneuron by group III and IV afferents.