Coupling of grip force and load force during arm movements with grasped objects.

Numerous studies have investigated the kinematics of arm movements; others have examined grip forces during static holding of objects. However, the coordination of grip force and arm movement when moving grasped objects has not been documented. We show that grip force is finely modulated in phase with load force during movements with grasped objects in which load force varies with acceleration. A tight coupling between grip and load force is seen in point-to-point and cyclic movements of varying rate and direction. We conclude that in transporting an object, the programming of grip force is part and parcel of the process of planning the arm movement.