SOLEUS H-REFLEX DEPRESSION INDUCED BY BALLISTIC VOLUNTARY ARM MOVEMENT IN HUMAN

A biomechanical and neurophysiological analysis of anticipatory postur al adjustments associated with the early phase of a voluntary arm move ment was carried out in normal human subjects. Arm elevation, performe d at maximal velocity, was studied with unilateral arm movement in fre ely and counterbalanced (suspended in a safety harness) standing human s. The ground reaction force of both legs, tangential acceleration of the shank and electromyographic activity (EMG) of the anterior deltoid (AD), biceps femoris (BF) and soleus (Sol) muscles were recorded. Sol H-reflexes of both legs were also elicited. To examine how the Ia inh ibitory and presynaptic inhibitory pathways are related to anticipator y postural adjustments, additional Sol H-reflexes were elicited using the classical conditioning-test technique (time interval 2 and 15 ms, respectively). In this study, we systematically described biomechanica l and EMG phenomena that precede and follow the onset of voluntary arm movements. Prior to and during the arm movement (i.e. AD activation), a sequence of EMG modifications occurred in ipsilateral BF and Sol mu scles. Those modifications preceding BF activation included silent pha ses in Sol EMG and depression of Sol H-reflexes. By comparing EMG modi fications with depressions of the Sol H-reflex, we conclude that Ia an d presynaptic inhibitory pathways do not play a specific role in those Sol H-reflex depressions. Moreover, because Sol H-reflex depression o ccurs in both freely standing and counterbalanced postures, anticipato ry postural adjustment appear to be preprogrammed in the central nervo us systems unrelated to peripheral neural mechanisms. Since changes of EMG activities of ipsilateral BF and Sol appeared simultaneously and Sol H-reflex depressions were dependent on EMG activities of ipsilater al BF, control commands to anticipatory postural adjustment would exci te BF and inhibit Sol.