ACCURACY OF PLANAR REACHING MOVEMENTS .2. SYSTEMATIC EXTENT ERRORS RESULTING FROM INERTIAL ANISOTROPY

This study examines the source of direction-dependent errors in moveme nt extent made by human subjects in a reaching task. As in the precedi ng study, subjects were to move a cursor on a digitizing tablet to tar gets displayed on a computer monitor. Movements were made without conc urrent visual feedback of cursor position, but movement paths were dis played 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 degrees and 150 degrees) from one of two initia l positions. Trajectory shapes were stereotyped, and movements to more distant targets had larger accelerations and velocities. Comparison o f movements in the two directions showed that in the 30 degrees direct ion responses were hypermetric, accelerations and velocities were larg er, and movement times were shorter. Since movements in the 30 degrees direction required less motion of the upper arm than movements in the 150 degrees direction, we hypothesized that the differences in accura cy and acceleration reflected a failure to take into account the diffe rence in total limb inertia in the two directions. To test this hypoth esis we simulated the initial accelerations of a two-segment limb movi ng in the horizontal plane with the hand at shoulder level when a cons tant force was applied at the hand in each of 24 directions. We compar ed 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 greates t in the two directions perpendicular to the forearm, where inertial r esistance is least, and lowest for movements directed along the axis o f the forearm. In all subjects, the directional variation in peak acce leration was similar to that predicted by the model and shifted in the same way when the initial position of the hand was displaced. The pat tern 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 indicat e that, in programming the magnitude of the initial force that will ac celerate the hand, subjects do not fully compensate for direction-depe ndent differences in inertial resistance. The direction-dependent diff erences in peak acceleration were associated with systematic variation s in movement extent in all subjects, but the variations in extent wer e proportionately smaller than those in acceleration. This compensatio n for inertial anisotropy, which differed in degree among subjects, wa s associated with changes in movement duration. The possible contribut ions of elastic properties of the musculoskeletal system and proprioce ptive feedback to the compensatory variations in movement time are dis cussed. The finding that the magnitude of the initial force that accel erates the hand is planned without regard to movement direction adds s upport for the hypothesis that extent and direction of an intended mov ement are planned independently. Furthermore, the lack of compensation for inertia in the acceleration of the simple reaching movements stud ied here suggests that they are planned by the central nervous system without explicit inverse kinematic and dynamic computations.