Modulation of soleus H-reflex amplitude and variance during pretibial contraction - Effects of joint position and effort level

To investigate the effects of joint position and effort level of antagonist contraction on reciprocal inhibition, experiments with 15 healthy volunteers were performed to compare the changes of standardized soleus H reflex during dorsiflexion contraction (0%, 25%, 50%, 75% maximal voluntary contraction [MVC] associated with different ankle positions (plantarflexion 20°, neutral, and dorsi-flexion 20°). The results indicated that a decrease of the soleus H response was dependent on the level of prefibial contraction, but a significant interaction of ankle position and pretibial contraction was noted (p ≤ .05). Post hoc analysis suggested that progressive H-amplitude decrement as the pretibial contraction increased was especially significant in the plantar-flexed position, but not in the dorsiflexed position. In contrast to H-reflex amplitude, standardized intrasubject variances of the H amplitude (H_var/H_mean) significantly increased with pretibial dorsiflexion (p < .05), but neither with the ankle position nor interaction effect (p > .05). These findings suggest that progressive reciprocal inhibition was position relevant pertaining to joint stability as well as force variability.