EXPERIMENTAL OBSERVATIONS ON THE CENTRAL NERVOUS SYSTEM'S GOVERNING STRATEGIES ON THE ARM REACHING WITH REDUCED MOBILITY
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abstract
For the motion planning of a pointtopoint reaching task with a healthy arm, the CNS tends to plan the shortest hand path between two task points with a bellshaped velocity profile. If any kinematic or dynamic constraints are imposed on the arm, the CNS adapts to the changes by incorporating learning mechanism into the motion planning. This paper seeks to identify the modified motion planning strategies of the CNS when the elbow joint is constrained to move. We present an experimental protocol, where subjects perform pointtopoint reaching tasks with a lightweight elbow brace to restrict the joint kinematics with a minimal effect on the arm dynamics. From the experimental observations, the human strategies on each aspect of motion planning (i.e. hand path geometry, speed of the motion, and the arm posture selection) are hypothesized. The hypothesized strategies are developed as models and compared with the experimental data. As a result, we found that the hand path follows the rhumb line on the constraint workspace while the speed profile preserves a bellshape, which can be roughly approximated by the minimum jerk model. In addition, by comparing the joint contributions data with and without the elbow constraint, it is hypothesized that the CNS resolves the redundancy of the inverse kinematics problem by reducing the kinetic energy of the limbs.
