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The relation between the force (load) and the velocity of shortening ( V ) in contracting skeletal muscle is part of a rectangular hyperbola: ( P + a) V = b( Po - P ); where Po is the maximum isometric force and a and b are constants. The force-velocity ( P-V ) relation suggests that muscle can regulate its energy output depending on the load imposed on it (Hill, 1938). After the establishment of the sliding filament mechanism (H.E. Huxley and Hanson, 1954), the P - V relation has been regarded to reflect the cyclic interaction between myosin heads in myosin filaments and the corresponding myosin-binding sites in actin filaments, coupled with ATP hydrolysis (A.F. Huxley, 1957). In single skeletal muscle fibers, however, the P - V relation deviates from the hyperbola at the high force region, indicating complicated characteristics of the cyclic actin-myosin interaction. To correlate the P - V relation with kinetics of actin-myosin interaction, skinned muscle fibers have been developed, in which the surface membrane is removed to control chemical and ionic conditions around the 3D lattice of actin and myosin filaments. This article also deals with experimental methods with which the structural instability of skinned fibers can be overcome by applying parabolic decreases in fiber length.

Physiological Significance of the Force-Velocity Relation in Skeletal Muscle and Muscle Fibers