Ia afferents of the antagonist are inhibited presynaptically before the onset of a ballistic muscle contraction in man.

The aim of the present investigation was to study whether the pre-movement inhibition of the H reflex in the antagonist of a ballistic voluntary contraction was due to a reduced activity of the motoneuronal pool of the antagonist, or to a reduced excitatory effect of the afferent volley reaching these motoneurons. Human subjects performed visually conditioned ballistic dorsal flexions of the ankle. The inhibition of the H reflex in the antagonist (soleus muscle) was similar if the muscle was initially relaxed or if there was a preexisting level of motor discharge. Since the soleus muscle was inhibited before movement onset in parallel with the H reflex inhibition, the relation between the level of a background activity and the size of superimposed H reflexes was studied. The finding that H reflexes were only slightly reduced in size with decreasing steady EMG levels could not explain the pre-movement inhibition, and it was concluded than an increased presynaptic inhibition of Ia terminals was the source of the H reflex inhibition.