Stereotypical fingertip trajectories during grasp.

The kinematics of movement of all five digits was analyzed during reach-and-grasp tasks for a variety of objects. Ten healthy subjects performed 20 trials involving the grasp of five objects of distinct size and shape. Joint angles were recorded, and digit trajectories were computed using forward kinematics. For a given subject, fingertip trajectories were consistent across trials. The different-sized objects largely produced movement along different portions of a stereotypical trajectory described by a logarithmic spiral. The spirals fit the actual finger positions with a mean error across all trials of 0.23 +/- 0.25 cm and accounted for over 98% of the variance in finger position. These patterns were consistent independent of initial finger posture. Subjects did not produce straight-line movements, either in Cartesian space or joint space. The direction of the thumb trajectories exhibited a greater dependence on object type than the finger trajectories, but still utilized a small percentage (<5%) of the available workspace. These results suggest that restoration of a small but specific part of the workspace could have significant impact on function following hand impairment.