PINCH FORCE MATCHING ERRORS PREDICTED BY AN EQUILIBRIUM-POINT MODEL

Varying the size or stiffness of an external load often results in sys tematic grasp force or finger span matching errors, typically judged v ia contralateral matching. In the present study, subjects squeezed com pliant or rigid manipulanda in each hand using three-finger pinch in o rder to generate, and simultaneously match, a reference force indicate d by a visual cue. Subjects were not informed which hand was the refer ence and were given only 1 s to make a match. Under these conditions, subjects appeared to match central commands rather than perceived forc e. Forces were always exaggerated when squeezing an isometric load to match a compliant load and were too small when matching a compliant lo ad to an isometric load. The matching errors were largest for the most compliant spring and increased with the reference force. When the loa ds were symmetric (e.g., both compliant), errors were small, but subje cts persistently squeezed slightly harder with the left (nondominant) hand. Apart from the left-hand bias, the matching results agree quanti tatively and in detail with the predictions of an equilibrium point mo del (the a model), in which central commands shift the rest length of spring-like muscle. The matching data are fit best assuming that the m uscle's compliance characteristic is an accelerating function, in agre ement with previous direct measurements of pinch stiffness.