Anticipatory postural adjustments in arm muscles associated with movements of the contralateral limb and their possible role in interlimb coordination.

While sitting on a turnable stool, with both shoulders flexed at 90 degrees or, alternatively, with arms parallel to the trunk and the elbows flexed at 90 degrees--the hands being semisupine--subjects performed unidirectional and cyclic movements on the horizontal plane of the right arm (adduction-abduction) or hand (flexion-extension). The left arm was still, in a position symmetrical to that of the right limb and with the hand contacting a fixed support by the palmar or dorsal surface. During both unidirectional and cyclic arm or hand movements, activation of the prime mover muscles (right Pectoralis Major for arm adduction and Infraspinatus for abduction; right Flexor Carpi Radialis and Extensor Carpi Radialis for the hand movements) was accompanied by activation of the homologous muscles of the contralateral arm and inhibition of antagonists. The contralateral activities (1) regularly preceded the burst in the movement prime movers and (2) were organised in fixation chains that, exerting forces on the hand fixed support, will counterbalance the rotatory action exerted on the trunk by the primary movement. Based on these features, these activities may be classified as anticipatory postural adjustments (APAs). The observed APAs distribution is such as to favour the preferential (mirror symmetrical) coupling of upper limb movements on the horizontal plane. The possible role of these APAs in determining the different constraints experienced when performing mirror symmetrical versus isodirectional coupling is discussed.