A neurophysiological analysis of anticipatory postural adjustments (APAs) associated with the early phase of a simultaneous voluntary arm and head movement was carried out in normal human subjects (n=9). Simultaneous arm elevation and head rotation, performed at maximum velocity, was studied in freely standing humans. Electromyographic (EMG) activities of the anterior deltoid (AD), the sternocleidmastoideus (SM), the biceps femoris (BF) and the soleus (Sol) muscles were recorded. To examine how the simultaneous arm and head movement influences spinal reflex depressions induced by APA, Sol H-reflexes of ipsilateral side of arm movement and contralateral side of head rotation were also elicited. Prior to and during simultaneous movement, a sequence of EMG modification and Sol H-reflex depression were observed in BF and Sol muscles as similar as previously reported (Anson and Kasai, 1995; Kasai and Komiyama, 1996). Amount of EMG modification and Sol H-reflex depression, however, was larger in the simulatneous movement than those in the single movement. Those results suggest that descending motor commands related to APA would be flexible and provide for the postural adjustment according to the requirement of stability body equilibrium upon different voluntary movement conditions. That is, the central nervous system may produce the optimal response in a context involving external and internal constraints.