Task-relevance of grasping-related degrees of freedom in reach-to-grasp movements.

The kinematic redundancy of the human arm enables the rotation of the arm plane about the shoulder-wrist axis, represented by a swivel angle, which is affected by hand orientation when grasping. The coordination of grasping-related degrees of freedom (GR-DOFs), including swivel angle, forearm supination, wrist flexion and radial deviation, depends on their task-relevance, which can be quantified by the ratio of a joint's active motion range to its total motion range (R-AMR). The R-AMR values are computed across the target position and orientation to compare the task-relevance of the GR-DOFs. Statistical analysis of R-AMR values at the end of reach-to-grasp movements shows that among the GR-DOFs, radial deviation is most sensitive to changes in target position, while forearm supination is most sensitive to changes in target orientation. The forearm supination and swivel angle coordinate for energy-efficiency such that the swivel angle, which adjusts the posture of the whole arm, is largely unused until the forearm supination approaches its joint limit. The results further the understanding of the human motor control system in arm motion control and may benefit the design of the control algorithm for the upper limb exoskeleton.