EVIDENCE OF FACILITATION OF SOLEUS-COUPLED RENSHAW CELLS DURING VOLUNTARY COCONTRACTION OF ANTAGONISTIC ANKLE MUSCLES IN MAN

1. The amount of recurrent inhibition onto soleus motoneurones was com pared during plantar flexion and co-contraction of antagonistic ankle plantar and dorsiflexors at matched levels of background activity in t he soleus muscle. 2. During weak plantar flexion and co-contraction (l ess than 10% of maximal voluntary plantar flexion effort) a test refle x discharge (H' reflex), which was conditioned by a previous reflex di scharge, was found to be significantly more depressed in relation to r est than an unconditioned reference H reflex. During strong planter fl exion (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 refl ex was not significantly different from its resting value. 3. At the e nd of the ramp phase of a phasic contraction, large variations of the H' reflex were observed during plantar flexion (large increase in rela tion to rest) and during co-contraction (marked decrease), whereas the reference H reflex was facilitated in the two situations. 4. These ob servations provide evidence that soleus-coupled Renshaw cells are diff erently regulated during co-contraction and plantar flexion. It is sug gested that the Renshaw cells are inhibited during strong plantar flex ion but not during strong co-contraction. The functional significance of the findings is discussed.