Abstract
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.
Highlights
Muscle contraction can be characterized by the velocity of shortening and the magnitude of isometric force
We studied the effect of inorganic phosphate Pi and low pH on the Vmax and Po to give information about mechanisms underlying muscle fatigue, all without complications arising from deteriorating cut ends of skinned fibers
The P–V relation was fitted to part of rectangular hyperbola as (P + a)V = b(Po − P) and was taken to indicate that the rate of energy flux (PV + aV) is dependent on the load
Summary
Muscle contraction can be characterized by the velocity of shortening and the magnitude of isometric force (tension). When the magnitude of isometric force reaches the value equal to the amount of load (P) imposed on the muscle by the weight (W), the muscle starts shortening with a constant velocity (V). Force and length changes in the muscle during a twitch are recorded by a force transducer (F) and a photoelectric device (PE), respectively. Peak twitch but force is still than peak tetanus force This finding has been on intact single fiberssingle [3] and on skinned fibersmuscle [4,5,6,7].
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