Kinetic analysis of arm reaching movements during voluntary and passive rotation of the torso.

Reaching movements made to targets during exposure to passive constant velocity rotation show significant endpoint errors. By contrast, reaching movements made during voluntary rotation of the torso are accurate. In both cases, as a consequence of the simultaneous motion of the arm and the torso, Coriolis forces are generated on the arm tending to deflect its path. Our goal in the present paper was to determine whether during voluntary torso rotations arm movement accuracy is preserved by feed forward compensations for self-generated Coriolis forces. To test this hypothesis we analyzed and quantified the contribution of torso rotation and translation to arm dynamics and compared the kinematics and kinetics of pointing movements during voluntary and passive torso rotation. Coriolis torques at the shoulder increase nearly sixfold in voluntary turn and reach movements relative to reaches made without torso rotation, yet are equally accurate. Coriolis torques during voluntary turn and reach movements are more than double those produced by reaching movements during passive body rotation at 60 degrees /s. Nevertheless, the endpoints of the reaches made during voluntary rotation are not deviated, but those of reaches made during passive rotation are deviated in the direction of the Coriolis forces generated during the movements. We conclude that there is anticipatory pre-programmed compensation for self-generated Coriolis forces during voluntary torso rotation contingent on intended torso motion and arm trajectory.