Long-lasting inhibition of the soleus muscle H reflex related to realization of voluntary arm movements in humans

In healthy humans, we recorded the H reflex induced by transcutaneous stimulation of the tibial nerve (recording from the soleus muscle). In subjects in the lying position, we studied changes in the H reflex values after preceding voluntary arm movements realized with a maximum velocity after presentation of an acoustic signal. On the 200th to 300th msec after forearm flexion, long-lasting inhibition of the H reflex developed following a period of initial facilitation and reached the maximum, on average, 700 msec from the moment of the movement. Flexion of the contralateral upper limb in the elbow joint induced deeper inhibition than analogous movement of the ipsilateral arm. Long-lasting clear inhibition of the H reflex developed after arm flexion in the elbow joint but was slightly expressed after finger clenching. After inhibition reached the maximum, its time course was satisfactorily approximated by a logarithmic function of the time interval between the beginning of the conditioning voluntary movement and presentation of the test stimulus. Durations of inhibition calculated using a regression equation were equal to 6.6 sec and 8.5 sec after ipsilateral and contralateral elbow-joint flexions, respectively. Inhibition was not eliminated under conditions of tonic excitation of motoneurons of the tested muscle upon voluntary foot flexion. Long-lasting inhibition of the H reflex was also observed after electrical stimulation-induced flexions of the upper limb. The obtained data indicate that movements of the upper limb cause reflex long-lasting presynaptic inhibition of the soleus-muscle H reflex that can play a noticeable role in redistribution of the muscle tone during motor activity. Neirofiziologiya/Neurophysiology, Vol. 40, No. 3, pp. 221–227, May–June, 2008.