Effects of initial grasping forces, axes, and directions on torque production during circular object manipulation

Manipulation of circular objects is a common task in our everyday activity. Human tends to have a stable grasp before they start to maneuver the object. However, the effect of initial grasping forces on the subsequent dynamic performance is not yet clear. In this experiment, subjects were asked to produces different levels of grasping force, followed by a ramp and hold torque production task. The safety margin, an extra amount of grasping force to prevent objects from slippery, showed quicker response with smaller initial grasping force. The resultant force was found always point from the thumb to other fingers, although the vertical component showed distinct behaviors under different wrist postures. It was found that the thumb contribute largest grasping force while the index finger contribute similar or larger torque in counterclockwise direction.