EARLY STAGES OF LOAD COMPENSATION IN HUMAN AIMED ARM MOVEMENTS

We investigated the kinematics of pointing movements in human subjects while varying the weight load carried by their hand. In accordance wi th our previous study 3, we found that movement velocity decreased and duration increased with load size, while skewness of velocity profile s and final pointing position were load-independent. When the load was changed without advance information about the new load size, kinemati cs of the first movement deviated initially (within the first 135 ms) from those of the preceding and following movements. Two types of devi ation were found. Firstly, the initial rise of movement velocity was s lower, independent of the new load size; we attributed this finding to a purposeful motor strategy to 'probe' the new relationship between f orce and acceleration, and thus to determine the new load size. Second ly, the initial portions of movement trajectories deviated downwards a fter a load increase and upwards after a load decrease, depending in a graded way on the change of load size; this finding probably reflects incomplete load compensation. Deviating movement kinematics were foun d only for the first movement after a load change. This suggests that information required for appropriate load compensation is determined d uring the first movement, is stored in memory, and is available for th e execution of subsequent movements. No deviating kinematics were foun d if prior to the first response after a load change, subjects moved t he hand orthogonally to the direction of pointing or suspended the loa d against gravity. This suggests that the stored information is not sp ecific for movements of similar spatial characteristics. Our findings are in line with the hypothesis 3 that load size is represented by a s caling parameter in the motor system; this parameter can be determined before movement onset if appropriate precues are provided. Otherwise, it is calculated during a low-velocity 'probing' portion of the movem ent.