Studies of bimanual movements typically report interference between motions of the two arms and preference to perform mirror-symmetrical patterns. However, recent studies have demonstrated that the two arms differ in the ability to control interaction torque (INT). This predicts limitations in the capability to perform mirror-symmetrical movements. Here, two experiments were performed to test this prediction. The first experiment included bimanual symmetrical and asymmetrical circle drawing at two frequency levels. Unimanual circle drawing was also recorded. The increases in cycling frequency caused differences between the two arms in movement trajectories in both bimanual modes, although the differences were more pronounced in the asymmetrical compared with the symmetrical mode. Based on torque analysis, the differences were attributed to the nondominant arm's decreased capability to control INT. The intraarm differences during the symmetrical pattern of bimanual movements were similar (although more pronounced) to those during unimanual movements. This finding was verified in the second experiment for symmetrical bimanual oval drawing. Four oval orientations were used to provide variations in INT. Similar to the first experiment, increases in cycling frequency caused spontaneous deviations from perfect bimanual symmetry associated with inefficient INT control in the nondominant arm. This finding supports the limitations in performing mirror-symmetrical bimanual movements due to differences in joint control between the arms. Based on our results and previous research, we argue that bimanual interference occurs during specification of characteristics of required motion, whereas lowerlevel generation of muscle forces is independent between the arms. A hierarchical model of bimanual control is proposed.
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