Force-Velocity Relation: Its Implications about Molecular Mechanism of Muscle Contraction

In 1938, Hill described the relation between force (P=load) and initial velocity of shortening (V) in contracting whole frog muscles as part of rectangular hyperbola: (P + a) V=b (Po -P), where Po is the maximum isometric force and a, b are constants. This equation indicates that muscle can regulate its energy output depending to the amount of load imposed on it. In 1954, H.E. Huxley and Hanson made a monumental discovery that muscle contraction results from relative sliding between actin and myosin filaments, providing structural basis to study and discuss mechanisms underlying the P-V relation. In 1957, A.F. Huxley has constructed a contraction model, in which muscle contraction characteristics including the P-V relation are explained in terms of attachment-detachment cycle between myosin heads extending from myosin filaments and the site in actin filaments. The Huxley contraction model stimulated the interest of muscle investigators to study the P-V relation using intact single muscle fibers with the following results: (1) the shape of P-V relation deviates from hyperbola at loads >~0.8 Po; (2) at the beginning of muscle shortening under lads Po is imposed on muscle fibers, they show remarkable length oscillations before being stretched. To eliminate the gap between muscle contraction characteristics and biochemical studies on actomyosin ATPase reaction, experiments are now performed using skinned muscle fibers, from which surface membrane is removed. In this article, we not only describe contraction models other than the Huxley contraction model to explain the results obtained, but also experimental techniques to obtain P-V relations especially from skinned muscle fibers having structural instability.