Early stages of load compensation in human aimed arm movements

Abstract

We investigated the kinematics of pointing movements in human subjects while varying the weight load carried by their hand. In accordance with our previous study3, we found that movement velocity decreased and duration increased with load size, while skewness of velocity profiles and final pointing position were load-independent. When the load was changed without advance information about the new load size, kinematics of the first movement deviated initially (within the first 135 ms) from those of the preceding and following movements. Two types of deviation were found. Firstly, the initial rise of movement velocity was slower, independent of the new load size; we attributed this finding to a purposeful motor strategy to ‘probe’ the new relationship between force and acceleration, and thus to determine the new load size. Secondly, the initial portions of movement trajectories deviated downwards after 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 found only for the first movement after a load change. This suggests that information required for appropriate load compensation is determined during the first movement, is stored in memory, and is available for the execution of subsequent movements. No deviating kinematics were found if prior to the first response after a load change, subjects moved the hand orthogonally to the direction of pointing or suspended the load against gravity. This suggests that the stored information is not specific for movements of similar spatial characteristics. Our findings are in line with the hypothesis3 that load size is represented by a scaling 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 movement.