When fibres go slack and cross bridges are free to run: a brilliant method to study kinetic properties of acto-myosin interaction

Muscle contraction is brought about by independent force or movement generators, which can be identified with cross bridges, i.e. with myosin heads interacting with actin. This implies that isometric tension is dependent on the degree of filament overlap, i.e. on sarcomere length, and on activation level, whereas shortening velocity at zero load is not (Huxley, 1957, 1974). Whereas the dependence of isometric tension on the degree of filament overlap was demonstrated by the length clamp experiments published in 1966 (Gordon et al. 1966; see companion Classical Perspective by Allen &Westerblad, 2007), the definitive demonstration that shortening velocity at zero load is independent of sarcomere length was only given in 1979 (Edman, 1979).