In contrast to robots, in humans internal and manipulation forces are coupled

Internal force is defined as a set of contact forces which does not perturb object equilibrium. The internal forces cancel each other and therefore do not contribute to the resultant (manipulation) force acting upon the object. Mathematically, the internal and manipulation forces are independent. Hence they can be controlled independently and corresponding controllers have been implemented in robotic manipulators. The purposes of this study are to examine whether in humans internal force is coupled with the manipulation force and what kind of grasping strategy the performers utilize. The subjects (n=6) were instructed to make cyclic arm movements with a customized manipulandum and the orientation and the movement direction of the manipulandum were varied. Two major grasping patterns were demonstrated: symmetric grasping synergy when the manipulation force is parallel to finger-object interface; and reciprocal changes of forces when the manipulation force is orthogonal to digit-object interface. In contrast to robotic gripper where controls of internal force and manipulation force are decoupled, in humans the internal and manipulation forces are coupled.