Anticipating load torques produced by voluntary movements.

The stability of an object held between the finger and thumb depends on friction developed by grip force, normal to the contact surfaces, to overcome tangential load force. Previous research has shown that in lifting an object, grip force rises with the increase in gravitational load force as the hand takes the weight and that in moving an object, grip force is adjusted to meet movement-induced inertial load force. Those results demonstrated the anticipatory nature of coordination of grip force with load force. Whether grip force anticipates load torque was studied in this research. When participants were constrained to use grasp points where the grasp axis was manifestly distant from object center of mass, it was found that they made grip force adjustments in anticipation of load torques that tended to destabilize an object as a result of lifting or moving it. These adjustments imply use of information about object center of mass in movement planning.