Pinch force matching errors predicted by an equilibrium-point model.

Varying the size or stiffness of an external load often results in systematic grasp force or finger span matching errors, typically judged via contralateral matching. In the present study, subjects squeezed compliant or rigid manipulanda in each hand using three-finger pinch in order to generate, and simultaneously match, a reference force indicated by a visual cue. Subjects were not informed which hand was the reference and were given only 1 s to make a match. Under these conditions, subjects appeared to match central commands rather than perceived force. Forces were always exaggerated when squeezing an isometric load to match a compliant load and were too small when matching a compliant load to an isometric load. The matching errors were largest for the most compliant spring and increased with the reference force. When the loads were symmetric (e.g., both compliant), errors were small, but subjects persistently squeezed slightly harder with the left (non-dominant) hand. Apart from the left-hand bias, the matching results agree quantitatively and in detail with the predictions of an equilibrium point model (the lambda model), in which central commands shift the rest length of spring-like muscle. The matching data are fit best assuming that the muscle's compliance characteristic is an accelerating function, in agreement with previous direct measurements of pinch stiffness.