Arm Movement-Related Forward Facilitation of the Soleus H Reflex in Humans

In healthy humans, we recorded the H reflex (EMG discharge from the soleus muscle) evoked by transcutaneous stimulation of the tibial nerve. Changes in the magnitude of the reflex discharge within the premotor period preceding the movement of the ipsilateral upper limb (elbow joint flexion performed after a light signal; the respective EMG was recorded from the m. biceps brachii) were examined. The H reflex magnitude began to increase 80-90 msec prior to initiation of forearm flexion; such facilitation reached the maximum synchronously with the development of EMG discharges of the biceps. Tonic excitation of motoneurons of the m. soleus related to moderate voluntary plantar flexion of the foot exerted no significant influence on facilitation of the H reflex within the premotor period. The intensity of background soleus EMG activity did not undergo significant changes during the above time interval. The obtained data indicate that forward facilitation of the H reflex within the premotor period of the voluntary movement of the ipsilateral upper limb is due to the processes developing in the presynaptic part of the arc of this reflex. We hypothesize that weakening of background presynaptic inhibition of terminals of Ia afferents of the m. soleus under the action of central motor commands is the reason for such modifications.