Evidence for high mechanical efficiency of cross-bridge powerstroke in skeletal muscle.

Abstract

Muscle contraction is caused by attachment-detachment cycles 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 fibers can now be estimated 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 powerstroke producing 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 fibers 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.