Muscle contraction is caused by attachment–detachment cycle between the cross-bridges on the thick filament and the thin filament coupled with ATP hydrolysis

cycle between the cross-bridges on the thick filament and the thin filament coupled with ATP hydrolysis (A. F. Huxley, 1957; H. E. Huxley, 1960; Bagshaw, 1994). The mechanical efficiency with which chemical energy derived from ATP hydrolysis is converted into mechanical work in demembranated muscle fibres has been estimated recently by measuring the amount of ATP utilized for work production, using fluorescence of a phosphate-binding protein (He et al., 1997, 1999) or NADH (Reggiani et al., 1997; Sun et al., 2001). During myofilament sliding, however, the cross-bridges not only attach to the thin filament to perform their powerstrokeproducing positive forces, but also produce negative forces before being detached from the thin filament (A. F. Huxley, 1957). On this basis, the overall mechanical efficiency of muscle fibres may be much smaller than that of individual cross-bridges during their powerstroke, since positive forces are always opposed by negative forces, due to asynchronous cross-bridge activity. To accurately estimate the mechanical efficiency of individual cross-bridges when they perform their powerstroke-producing positive force, it is necessary to perform experiments under conditions in which the crossbridges start their powerstroke synchronously. The present work was undertaken to estimate the maximum mechanical efficiency of the cross-bridge powerstroke in demembranated muscle fibres containing ATP molecules almost equal in number to the cross-bridges (Sugi et al., 1998). The results obtained suggest that the maximum mechanical efficiency of the cross-bridge powerstroke may be close to unity.