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This paper proposes a new optimal control model of the human three-joint arm system (shoulder, elbow, and hand joints) and clarifies its basic performance. The proposed model is characterized by a freezing-like mechanism in its hand joint and optimizes its criterion function composed of three kinds of energy costs and a torque-change cost. The model’s freezing-like mechanism is formulated as a feedback controller to produce feedback torque in the direction opposite to the hand-joint motion. Consequently, it was clarified that the minimization of the moment power of joints or that of the energy consumed by viscous resistance made it possible to simulate human three-joint arm's reaching movements including the hand-joint’s freezing-like characteristics at the constant weight values regardless of target positions, whereas the torque-change minimization failed to do so. This result suggests that the energy minimization model with the freezing-like mechanism in its hand joint can be effective as a general and plausible model of the human three-joint arm control system and that the two kinds of energy can be involved in the trajectory planning for human three-joint arm's reaching movements.

Optimal Control Model of Human Three-Joint Arm System Characterized by Hand-Joint's Freezing-Like Mechanism