Trajectories in operating a handheld tool.

The authors studied the trajectories of the hand and of the tip of a handheld sliding first-order lever in aiming movements. With this kind of tool, straight trajectories of the hand are generally associated with curved trajectories of the tip of the lever and vice versa. Trajectories of the tip of the lever exhibited smaller deviations from straight paths than did trajectories of the hand, even though the cursor, which displayed the position of the tip of the lever on a computer monitor, was invisible during movement execution. These observations suggest that movement of the effective part of the tool is the primary kinematic variable in motor planning and control, even in the absence of continuous visual feedback. The presence of continuous visual feedback did not change the basic pattern of results, except that the remaining deviations from straight paths of the tip of the lever became smaller. These deviations most likely result from an inertial anisotropy of the tool, and they are reduced by visually based online corrections.