Extensibility and symmetry of actin filaments in contracting muscles

When isometrically contracting muscles are subjected to a quick release fol lowed 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 co urse as the quick part of the release; thereafter, tension remains at a neg ligible level for the duration of the shortening ramp. X-ray diffraction da ta 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 similar to 0.12% shorter and more twisted, while it is similar to 0.3% longer and l ess 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.