Accuracy of planar reaching movements - II. Systematic extent errors resulting from inertial anisotropy

This study examines the source of directiondependent errors in movement extent made by human subjects in a reaching task. As in the preceding study, subjects were to move a cursor on a digitizing tablet to targets displayed on a computer monitor. Movements were made without concurrent visual feedback of cursor position, but movement paths were displayed on the monitor after the completion of each movement. We first examined horizontal hand movements made at waist level with the upper arm in a vertical orientation. Targets were located at five distances and two directions (30° and 150°) from one of two initial positions. Trajectory shapes were stereotyped, and movements to more distant targets had larger accelerations and velocities. Comparison of movements in the two directions showed that in the 30° direction responses were hypermetric, accelerations and velocities were larger, and movement times were shorter. Since movements in the 30° direction required less motion of the upper arm than movements in the 150° direction, we hypothesized that the differences in accuracy and acceleration reflected a failure to take into account the difference in total limb inertia in the two directions. To test this hypothesis we simulated the initial accelerations of a two-segment limb moving in the horizontal plane with the hand at shoulder level when a constant force was applied at the hand in each of 24 directions. We compared these simulated accelerations to ones produced by our subjects with their arms in the same position when they aimed movements to targets in the 24 directions and at equal distances from an initial position. The magnitudes of both simulated and actual accelerations were greatest in the two directions perpendicular to the forearm, where inertial resistance is least, and lowest for movements directed along the axis of the forearm. In all subjects, the directional variation in peak acceleration was similar to that predicted by the model and shifted in the same way when the initial position of the hand was displaced. The pattern of direction-dependent variations in initial acceleration did not depend on the speed of movement. It was also unchanged when subjects aimed their movements toward targets presented within the workspace on the tablet instead of on the computer monitor. These findings indicate that, in programming the magnitude of the initial force that will accelerate the hand, subjects do not fully compensate for direction-dependent differences in inertial resistance. The direction-dependent differences in peak acceleration were associated with systematic variations in movement extent in all subjects, but the variations in extent were proportionately smaller than those in acceleration. This compensation for inertial anisotropy, which differed in degree among subjects, was associated with changes in movement duration. The possible contributions of elastic properties of the musculoskeletal system and proprioceptive feed-back to the compensatory variations in movement time are discussed. The finding that the magnitude of the initial force that accelerates the hand is planned without regard to movement direction adds support for the hypothesis that extent and direction of an intended movement are planned independently. Furthermore, the lack of compensation for inertia in the acceleration of the simple reaching movements studied here suggests that they are planned by the central nervous system without explicit inverse kinematic and dynamic computations.