DYNAMIC FORCE RESPONSES OF MUSCLE INVOLVING ECCENTRIC CONTRACTION

Normal movements commonly involve dynamic conditions where active musc les operate against other muscle forces, or against forces arising fro m decelerating limb inertia. In these situations, some active muscles spanning the joint are lengthened. Presently, our understanding of the muscle mechanics which operate in lengthening contractions, or during large muscle length changes is incomplete. Consequently, existing mat hematical descriptions of muscle action are usually constrained to sma ll operating ranges (requiring very restricted inputs), or do not appl y to conditions involving Iengthening contractions. Although Hill's hy perbolic relation between muscle force and shortening velocity is well established, the force-velocity relation during lengthening is poorly defined. Experiments were performed to measure the steady-state force -velocity curve for both concentric and eccentric (lengthening) contra ctions in isolated muscle, and to document muscle response to complex length inputs that combine concentric and eccentric phases as might oc cur in natural movements. A Hill-type muscle model applicable to these motions was synthesized to determine how well a description based on steady-state parameters captures dynamic muscle behavior. The simulate d model responses were compared to experimental records exhibiting com plex, dynamic force responses involving both eccentric and concentric contractions, and reproduced these forces with average errors ranging from 2.3 to 13.4%. Copyright (C) 1996 Elsevier Science Ltd.