Influence of voluntary movement and posture on recurrent inhibition in human subjects.

Recurrent inhibition elicited by a conditioning H reflex is measured in normal man by the amount of inhibition of a test H reflex produced by motoneurones that have already given rise to the conditioning reflex. The recurrent inhibition of soleus motoneurones is lowered during a voluntary contraction of this muscle while it is enhanced during a voluntary contraction of its antagonist, the tibialis anterior, and not modified during a contraction of the quadriceps. This recurrent inhibition is also enhanced when the subject is standing without support. These changes in recurrent inhibition are thought to imply supraspinal mechanisms acting on Renshaw cells. The functional significance of these changes is discussed in relation to the recent finding in animals of recurrent depression of the transmission in the Ia inhibitory pathway.