Extensibility and symmetry of actin filaments in contracting muscles

When isometrically contracting muscles are subjected to a quick release followed by a shortening ramp of appropriate speed (V(o)), tension decays from its value at the isometric plateau (P(o)) to <0. 05 P(o) with the same time course as the quick part of the release; thereafter, tension remains at a negligible level for the duration of the shortening ramp. X-ray diffraction data obtained under these conditions provide evidence that 1) at V(o) very few heads form an actomyosin complex, while the number of heads doing so at P(o) is significant; 2) relative to rest the actin filament at V(o) is approximately 0.12% shorter and more twisted, while it is approximately 0.3% longer and less twisted at P(o); and 3) the myosin heads attaching to actin during force development do so against a thin filament compliance of at least 0.646 +/- 0.046% nm per P(o).