Abstract
In intact single crayfish muscle fibers and frog semitendinosus muscles we have studied the tension response to sinusoidal length changes in the frequency range of 0.25-133 Hz. By this method we have resolved three processes in the interaction of myosin cross-bridges with actin in fully activated preparations. They are (A) a low-frequency phase advance, (B) a middle-frequency delay, and (C) a high-frequency advance. These processes can be used as probes to study the chemomechanical coupling of contractility. Process (B) represents net power output from the muscle preparation (oscillatory work). With maximal K or caffeine activation of crayfish muscle at 20 degrees C, it decreases to zero in the initial 45 s of maintained tension. Similar results were obtained with frog semitendinosus whole muscles. We interpret this decrease of (B) with time as a gradual decrease in actomyosin ATP-hydrolysis rate.