Torques do not influence proprioceptive localization of the hand

Because muscle torques counteracting gravity vary systematically during a movement of the arm, it has been suggested that torque differences that occur during a movement provide important information for judging the distance moved away from the body. To test this suggestion, we examined whether external vertical forces applied to the hand (and the torque differences due to these forces) influence proprioception. In a first experiment, the added vertical forces were constant, resulting in a change in torque that was proportional to the gravitational torque, as when holding an object in your hand. This did not affect proprioception. In a second experiment, gradient force fields were used to dramatically change the torque differences. Again, no effect on proprioception was found. Thus, vertical forces caused by hand-held objects do not play an important role in judging the position or movement of the hand.