The time course for kinetic versus kinematic planning of goal-directed human motor behavior

The present psychophysical study compares motor planning during goal-directed reaching movements and isometric spatial force generation. Our objective is to characterize the extent to which the motor system accounts for the biomechanical details of an impending reach. One issue that the nervous system must take into account when transforming a spatial sensory signal into an intrinsic pattern of joint torques is that of limb dynamics, including intersegmental dynamics and inertial anisotropy of the arm. These will act to displace the hand away from a straight path to an object. In theory, if the nervous system accounts for movement-related limb dynamics prior to its initial motor output, early force direction for a movement will differ from an isometric force to the same spatial target. Alternatively, biomechanical details of motor behavior may be implemented into the motor act following its initiation. Limb position and force output at the wrist were recorded while subjects displaced a cursor to targets viewed on a computer monitor. To generate isometric forces, a magnetic brake held a mechanical linkage supporting the arm in place. Subjects were cued to displace the cursor by using either isometric force or limb movement. On random trials, a movement was cued but an isometric force was unexpectedly required. Results show that there is not a significant directional difference in the initial force trajectory when planning a movement versus planning an isometric force. These findings suggest that the motor system may initially use a coarse approximation of movement-related limb dynamics, allowing for the refinement of the motor plan as the movement unfolds.