The aim of this study was 1) to test the reproducibility of the H reflex across consecutive days and 2) to investigate the effect on the H reflex immediately after eccentric exercise and during delayed-onset muscle soreness (DOMS) (24h after eccentric exercise).
H reflexes were recorded from subjects' (n=10) dominant middle trapezius muscle by electrical stimulation of the C3/4 cervical nerve. DOMS was induced by eccentric exercise of the shoulder joint in an experimental setting. H reflexes were obtained in four sessions: ``24h before'', ``Pre'', ``Post'', and ``24h after'' eccentric exercise. Ratios of maximal H reflex and M wave responses (Hmax/Mmax) were compared between sessions. Additionally, ratios of H reflex amplitudes obtained from 75% and 50% of stimulus intensity needed to obtain Hmax in the session 24h before eccentric exercise (H75/Mmax, and H50/Mmax) were also compared between sessions.
Results obtained from sessions before eccentric exercise showed no change in ratios. A decrease in H50/Mmax ratios was found immediately after eccentric exercise (P<0.05). The presence of DOMS 24h after eccentric exercise resulted in a decrease in H75/Mmax and H50/Mmax ratios (P<0.05).

This study presents evidence that comparison of baseline measures showed acceptable reproducibility of the H reflex with the study's experimental design. Furthermore, stronger stimulus intensity was needed immediately after and 24h after exercise to reach the same magnitude of the H reflex. This modulation of the stimulus-response relationship may be caused by presynaptic inhibition of Ia afferent fibres' input to the motoneuron by group III and IV afferent fibres.