Changes in presynaptic inhibition of Ia fibres to soleus motoneurones during voluntary dorsiflexion of the foot.

Variations of presynaptic inhibition in heteronymous Ia fibres projecting to soleus motoneurones were studied during the first 250 ms of phasic voluntary isometric contractions of the antagonist tibialis anterior muscle in human subjects. During the first 60-80 ms of TA e.m.g activity, presynaptic inhibition was often more marked than at rest, but not in all experimental sessions. After 60-80 ms, presynaptic inhibition was always increased compared to rest and to the onset of TA e.m.g. activity. A "rebound" in femoral nerve induced Ia facilitation was often observed between 90-150 ms. The early increase in presynaptic inhibition was widespread and non specific since it was observed at the onset of extensor carpi radialis contractions of maximal strength. The "rebound" in heteronymous Ia facilitation was interpreted as a relative decrease in presynaptic inhibition to which nonspecific suprasegmental and cutaneous effects contributed. The late increase in presynaptic inhibition in Ia fibers to soleus motoneurones was considered as reciprocally inhibiting the Sol H-reflex, thus counteracting the phasic stretch of the antagonist muscle during TA contraction.