Multi-digit manipulation of a circular object

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Multi-digit prehension tasks are commonly encountered in our daily activities. Previous studies investigated behavioral characteristics and neuromuscular mechanisms during manipulation actions. The purpose of this dissertation is to investigate hand-digit control and coordination during multi-digit circular object manipulation. In particular, the dissertation focuses on peak torque, force distribution, safety margin, force regularity, and multi-digit synergy in static and dynamic manipulation. In a series of experiments, subjects grasped a customized circular handle with a precision grip, i.e. without palm contact, and performed isometric maximum/submaximal, or repetitive torque production tasks under visual feedback. The factors studied include wrist position, torque direction, and initial grasping force level in the static tasks, movement frequency and moment of inertia in the dynamic task. The findings are: (1) in the maximum voluntary contraction task, it was found that peak torque in the counterclockwise direction was greater than the clockwise direction; (2) in submaximal tasks, a large initial grasping force slowed down the subsequent torque producing process and resulted in a large safety margin; the thumb and ulnar fingers (ring and little finger) generated more torque in the clockwise direction, while radial fingers (index and middle finger) produced more torque in the counterclockwise direction; the modulation gain between normal force and tangential force was larger in the torque increase direction than in the torque decrease direction; (3) in the repetitive dynamic task, the modulation gain increased with movement frequency and moment of inertia; within-cycle and between-cycle force regularity increased with moment of inertia, but was not affected by movement frequency; multi-digit synergy was found in the isometric task, but not in the repetitive dynamic task. In summary, this dissertation provides experiment evidence that in manipulation of circular objects, various task constraints have characteristic influence on motor output in task-specific manner, which can be understood from the perspectives of biomechanics and anatomy.