Comparison of soleus H reflex facilitation at the onset of soleus contractions produced voluntarily and during the stance phase of human gait.

Variations in the soleus monosynaptic reflex have been studied in normal human subjects during soleus contractions while walking, and compared with those observed during voluntary soleus activation. The monosynaptic reflex was always more facilitated during voluntary contractions than during walking. The EMG activity was identical in both situations. Under the assumption that this reflects an identical excitability of the motoneuronal pool, this different soleus H-reflex facilitation can be attributed to a difference in the excitatory efficiency of the group I volley evoked by the H-reflex stimulation. It is suggested that the level of presynaptic inhibition exerted on Ia fibers could be lower during walking than during voluntary contraction.