Muscle effort is best minimized by the right-dominant arm in the gravity field.

The central nervous system (CNS) develops motor strategies that minimize various hidden criteria, such as end-point variance or effort. A large body of literature suggests that the dominant (D) arm is specialized for such open-loop optimization-like processes, whereas the nondominant (ND) arm is specialized for closed-loop postural control. Building on recent results suggesting that the brain plans arm movements that take advantage of gravity effects to minimize muscle effort, the present study tests the hypothesized superiority of the dominant arm motor system for effort minimization. Thirty participants (22.5 ± 2.1 yr old; all right-handed) performed vertical arm movements between two targets (40° amplitude), in two directions (upward and downward) with their two arms (dominant and nondominant). We recorded the arm kinematics and electromyographic activities of the anterior and posterior deltoid to compare two motor signatures of the gravity-related optimization process; i.e., directional asymmetries and negative epochs on phasic muscular activity. We found that these motor signatures were still present during movements performed with the nondominant arm, indicating that the effort-minimization process also occurs for the nondominant motor system. However, these markers were reduced compared with movements performed with the dominant arm. This difference was especially prominent during downward movements, where the optimization of gravity effects occurs early in the movement. Assuming that the dominant arm is optimal to minimize muscle effort, as demonstrated by previous studies, the present results support the hypothesized superiority of the dominant arm motor system for effort minimization. The hypothesis of hemispheric specialization for feedforward and feedback control has been developed and is still debated. Here, taking advantage of recent knowledge on optimal planning and control of arm movements in the gravity field, we recorded kinematics and electromyographic activities during vertical arm movements performed with the dominant and nondominant arms. Our results reveal that the nondominant arm takes advantage of gravity effects to minimize muscle effort, but less than the dominant arm.