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 byindependent force or movement generators,which can be identified with cross bridges,i.e. with myosin heads interacting withactin. This implies that isometric tensionis dependent on the degree of filamentoverlap, i.e. on sarcomere length, and on activation level, whereas shorteningvelocity at zero load is not (Huxley,1957, 1974). Whereas the dependenceof isometric tension on the degree offilament overlap was demonstrated by thelength clamp experiments published in1966 (Gordon et al. 1966; see companionClassical Perspective by Allen &Westerblad,2007), the definitive demonstrationthat shortening velocity at zero load isindependent of sarcomere length was onlygiven in 1979 (Edman, 1979)