Dynamic force responses of muscle involving eccentric contraction.

Normal movements commonly involve dynamic conditions where active muscles operate against other muscle forces, or against forces arising from 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 mathematical descriptions of muscle action are usually constrained to small operating ranges (requiring very restricted inputs), or do not apply to conditions involving lengthening contractions. Although Hill's hyperbolic 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) contractions in isolated muscle, and to document muscle response to complex length inputs that combine concentric and eccentric phases as might occur 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 simulated model responses were compared to experimental records exhibiting complex, dynamic force responses involving both eccentric and concentric contractions, and reproduced these forces with average errors ranging from 2.3 to 13.4%.