Evoked H-reflex and V-wave responses during maximal isometric, concentric, and eccentric muscle contraction

This study was designed to investigate the modulations of H-reflex and V-wave responses during passive and maximal active dynamic actions. Experiments were performed on 16 healthy males [ age: 24 +/- 4 ( SD) yr]. Maximal H-reflexes (H-max) and M-waves (M-maxR) were evoked at the same muscle length during passive isometric, shortening and lengthening actions and during maximal voluntary isometric, concentric, and eccentric plantar-flexion. In all contraction types, supra-maximal stimulus intensity was used to evoke the superimposed maximal M wave (M-maxA) and V wave ( V) of the soleus muscle. At rest, the H-max/ M-maxR ratio was significantly reduced during lengthening with respect to isometric and shortening actions ( P < 0.05). For each action type, the ratio between H reflex superimposed to the contraction (H-sup) and M-maxA was not different from H-max/ M-maxR ratio. When plantar flexors were maximally voluntary activated, the H-sup/ M-maxA ratio was still lower during eccentric contraction as compared with isometric and concentric efforts (0.33 +/- 0.03 vs. 0.47 +/- 0.02 and 0.50 +/- 0.03, P < 0.001), whereas V/M-maxA ratios were similar for all contraction types ( isometric 0.26 +/- 0.02; concentric 0.23 +/- 0.03, and eccentric 0.24 +/- 0.02; P > 0.05). The V/M-maxA ratio was significantly lower than H-sup/ M-maxA during isometric and concentric MVC ( P < 0.001). No difference was observed between V/M-maxA and H-sup/ M-maxA ratios during eccentric efforts. The H-reflex modulations, present during lengthening actions, were mainly attributed to presynaptic inhibition of Ia afferents and to homosynaptic postactivation depression. Results on V wave and H reflex suggest that during eccentric MVC, the spinal loop is specifically modulated by the supra-spinal centers and/or neural mechanisms at spinal level.