Effect of tension on the rigor cross-bridge angle

The effect of resting tension and external force on the rigor crossbridge angle was investigated in insect flight muscle (Honeybee, Apis mellifera). In the presence of resting tension, bridges were either perpendicular to the filament axis or tilted towards the M-line. In shortened, slack muscle, bridges remained perpendicular or were tilted towards the Z-line. Thus, the rigor bridge angle appears to depend on the state of the thick filament. With the thick filament under stress, the angle may be quite different than when it is slack. On the other hand, external force applied directly to the rigor bridges did not change their configuration. Bridge angle remained unchanged, irrespective of the amount of tension applied to the bridge. This implies that a very tight bond exists between the rigor bridge and thin filament, allowing essentially no rotation even with a large applied force.