Active force as a function of filament spacing in crayfish skinned muscle fibers.

Filament spacing is shown to have a pronounced effect on active force in skinned striated muscle fibers of crayfish. At constant filament overlap and constant ionic strength, the separation between the myofilaments (measured by low-angle X-ray diffraction) was adjusted by application of osmotic pressure. Force was induced by a calcium-containing activating solution. In the absence of compression, calcium-activated force in skinned fibers was approximately 80% of that in normal intact fibers. In fibers compressed somewhat beyond the dimension of intact fibers, force was maximal. With further compression, force was reduced and then abolished. The filament spacing-force relation reported here suggests that, at any instant, the distance between the myosin filaments and actin filaments affects either the axial force per cross bridge or, more likely, the number of cross bridges in the force-generating state.