The Coordination Between Trunk and Arm Motion During Pointing Movements

The coordination between the trunk and arm of six subjects was examined during unrestrained pointing movements to five target locations. Two targets were within arm's length, three were beyond. The trunk participated in reaching primarily when the target could not be attained by arm and scapular motion. When the trunk did contribute to hand transport, its motion started simultaneously with arm movement and continued until target contact. Redundancy in the degrees of freedom used to execute the movement had no effect on the configuration of joints and segments used to attain a specified target; no difference in variability was noted regardless of whether redundancy existed. However, different configurations were used to achieve the same wrist coordinates along a common endpoint path, depending on the final position of the hand. The addition of trunk flexion, rotation and scapular motion did not alter the coupling between the elbow and shoulder joints and had no effect on the path of the hand or the smoothness of its velocity profile. Thus, trunk motion was integrated smoothly into the transport phase of the hand. As the trunk's contribution to hand transport increased, it played a progressively greater role in positioning the hand close to the target during the terminal stage of the reach. Of the movement components measured, trunk flexion was the last component to complete its motion when target reaches were made beyond arm's length. Hence, the trunk not only acts as a postural stabilizer during reaching, but becomes an integral component in positioning the hand close to the target.