The origin of force in skeletal muscle.

Since the proposal and rapid acceptance of the sliding-filament theory in 1953-1954, numerous suggestions have been made for the cause of the sliding movement. When the amount of overlap is varied by altering the initial length, the maximum tension is directly proportional to, but the speed of shortening under zero load is independent of, the amount of overlap. This suggests strongly that a relative force between thick and thin filaments is produced by independent force-generators distributed within each overlap zone. These force-generators are identified with projections (cross-bridges) on the thick filament, each consisting of part of a myosin molecule. Measurements of the 'tension transients' when the length of a stimulated muscle fibre is suddenly altered show that the range of action of each cross-bridge is 10-15 nm. The travel within a single contraction may be many times greater, so each cross-bridge must act cyclically by attaching, exerting a force and detaching. Details of the tension transients suggest that each cross-bridge makes its movement in two or three steps, each with a potential energy change a few times kT. Each cross-bridge contains also an elastic element in series. It is sufficient, on present evidence, to postulate that the only action of ATP is to dissociate the cross-bridge from the thin filament after it has completed its stroke.