Evidence of facilitation of soleus-coupled Renshaw cells during voluntary co-contraction of antagonistic ankle muscles in man.

1. The amount of recurrent inhibition onto soleus motoneurones was compared during plantar flexion and co-contraction of antagonistic ankle plantar and dorsiflexors at matched levels of background activity in the soleus muscle. 2. During weak plantar flexion and co-contraction (less than 10% of maximal voluntary plantar flexion effort) a test reflex discharge (H' reflex), which was conditioned by a previous reflex discharge, was found to be significantly more depressed in relation to rest than an unconditioned reference H reflex. During strong plantar flexion (more than 50% of maximal voluntary plantar flexion effort) the H' reflex either increased more or to the same extent as the reference H reflex in relation to rest. In contrast to this, the H' reflex was strongly depressed during co-contraction, whereas the reference H reflex was not significantly different from its resting value. 3. At the end of the ramp phase of a phasic contraction, large variations of the H' reflex were observed during plantar flexion (large increase in relation to rest) and during co-contraction (marked decrease), whereas the reference H reflex was facilitated in the two situations. 4. These observations provide evidence that soleus-coupled Renshaw cells are differently regulated during co-contraction and plantar flexion. It is suggested that the Renshaw cells are inhibited during strong plantar flexion but not during strong co-contraction. The functional significance of the findings is discussed.