Facilitation of soleus‐coupled Renshaw cells during voluntary contraction of pretibial flexor muscles in man.

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Abstract
Recurrent inhibition to soleus motoneurones, brought about by a conditioning H‐reflex discharge, was estimated in human subjects by a subsequent test H reflex. Changes in recurrent inhibition during voluntary ankle dorsiflexion were evaluated by comparing the amplitude of the test H reflex to a reference H reflex: both reflexes were subjected to the same type of influences which modified soleus monosynaptic reflex excitability during pretibial flexor contraction, but only the test H reflex was subject to the recurrent inhibition evoked by the conditioning H‐reflex discharge. During tonic or phasic ramp contractions of the pretibial flexors the inhibition of the test H reflex, as compared to rest, was more marked than that of the reference H reflex. Evidence is presented that this may indicate a facilitation of soleus‐coupled Renshaw cells. Since this facilitation of soleus‐coupled Renshaw cells was also observed before ramp contraction, it is, at least in part, supraspinal in origin. Within the range of forces studied (8‐45% of maximum force) there was no evidence that the facilitation of soleus‐coupled Renshaw cells increased along with increased force of the pretibial flexor voluntary contraction. During voluntary phasic ankle dorsiflexion, facilitation of soleus‐coupled Renshaw cells was maximum at the moment when soleus motoneurones were most facilitated by the stretch‐induced soleus I a discharge. There was no evidence for changes in Renshaw cell excitability during ballistic contractions. It is suggested that this facilitation of soleus‐coupled Renshaw cells may be one of the mechanisms preventing the occurrence of a soleus stretch reflex during a voluntary ankle dorsiflexion. Such a mechanism could become important if reciprocal inhibition, via I a inhibitory interneurones, were not strong enough, e.g. because of a weak gamma‐drive to the contracting muscles.