Figure 4.

Simulations reveal that just muscle activation-contraction dynamics impose a physical limit on directional accuracy of the initial fingertip force vector on contact. The labeled contour lines show the misdirection of the initial fingertip force vector with respect to the surface normal. The abscissa shows the duration of the transition between the joint torque pattern for motion and that for force; and the ordinate shows inaccuracies in the timing of this transition, represented by its termination time with respect to contact time. For physiologically tenable values of transition duration (> 35 ms), the initial fingertip force vector is misdirected by at least 7°, and the misdirection increases by over 60% for every 10 ms increase in timing inaccuracy (contour values along vertical dotted line at 35 ms abscissa).