Fig. 11.

A: force mode magnitudes vs. frequency in the index/middle finger alternating normal force task (control task 2). Since in this task production of alternating index and middle fingers was encouraged, not surprisingly, we see a large contribution from the Compensation Mode. There was no motion component in this task, and yet the grasp force contribution near the task frequency of 0.5 Hz (vertical red dashed line) is quite considerable, too. B: measured force mode directions are in agreement with the simulation directions. C: the visual feedback did not represent the 10 N target force but instead represented the sinusoids that mimicked the thumb force Compensation Mode. Hence, greater error on the target force (which is the sum of the individual normal forces) is to be expected, but also a reduced need to correct via the Grasp Mode. D: while the magnitude of force mode variability computed from the unfiltered data is approximately twice of that observed in the original task, the order of force modes in terms of magnitude and their relative magnitudes is preserved.