Soleus H-reflex dynamics during fast plantarflexion in humans.

The relationship between the size of the soleus (Sol) Hoffmann (H-) reflex and the level of background (BG) electromyographic (EMG) activity was examined during plantarflexing at different force levels. The experiments were carried out on seven healthy male subjects aged 20-37 years. The subjects were asked to perform fast plantarflexion under a reaction-time condition. The amounts of contraction force were 10, 20, 50 and 80% of maximum voluntary contraction (MVC). Since the maximum size of the M-wave (Mmax) changed systematically during the plantarflexion, we tried to maintain the size of the reference M-wave, an indicator of the efficiency of the electrical stimulation, at a constant value (20% of Mmax) throughout the experiment. The size of the H-reflex was rapidly increased at the very beginning of the movement, and then it tended to decrease in the later phase of the movement. Consequently, even with the same level of BG EMG, the size of the H-reflex was always larger in the early rising phase of the EMG activity than in the later falling phase. The maximum size of the H-reflex was poorly correlated with the force exerted. In contrast, the size of the F-response was proportional to the force exerted. The non-linear relationship between the size of the H-reflex and the BG EMG suggests that the level of the presynaptic inhibition onto Ia terminals was modified depending on the required force level and during the course of the movement.