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It is not well understood how posture (nonfocal joints) is stabilized during rapid joint movements. Interaction torque arising at the joints due to the movement of the adjacent joints will disturb the nonfocal joint unless it is compensated by muscle torque. The aim of this study was to clarify the mechanism underlying the incomplete stabilization of nonfocal posture during two types of fast single-joint flexion tasks, i.e., elbow flexion and shoulder flexion, with and without a hand-held weight. Participants were 5 men and 4 women (M age = 22.2 yr., SD = 4.8). The relationship between interaction torque and muscle torque and between angular displacement and torque components of the nonfocal joints were examined. The anticipatory muscle activity at the nonfocal joint produced compensatory muscle torque during the early phase of the rapid movement. However, nonfocal joints displayed a consistent displacement pattern, i.e., an initial extension followed by a flexion during the movement. The initial extension was in the direction of the interaction torque and its magnitude was related to the onset delay of the muscle torque against the interaction torque. The flexion movement of the nonfocal joints during the task was in the direction of the overcompensating muscle torque against interaction torque. This mechanism may have certain implications for anticipatory postural adjustment problems.

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