9603187 Josephson Muscles which are capable of high-frequency operation use one of two fundamentally different modes for controlling connection. Muscles of the two modes are termed synchronous or asynchronous, depending on whether there is or is not a 1:1 correspondence between muscle electrical activity and muscle contraction. In synchronous muscles brief contraction, and therefore the potential for high frequency operation, is achieved through the elaboration of the components within muscle fibers (the T-tubules and sarcoplasmic reticulum) which are involved in contractile activation through calcium release and rebinding. The asynchronous approach to high-frequency operation is built on shortening deactivation and sketch activation, features which are found in many muscles but which are particularly elaborated in high-frequency muscles of some insects. If one of these muscles is tetanically stimulated and subjected to cyclic shortening and lengthening at an appropriate frequency, the force at any given length during shortening is greater than the force at that length during reextension, a consequence of shortening deactivation. Thus more work is done during shortening than is required to relengthen the muscle, and there is net work output, work which allows the muscle to contract repetitively at high frequency when it is attached to an appropriate resonant load. It has been proposed that asynchronous muscles should have higher mass-specific power output and be more efficient than synchronous counterparts. They are expected to have greater power output because less space need be devoted to control components (sarcoplasmic reticulum and T-tubules) in asynchronous muscles than in synchronous ones; more efficient because calcium cycling costs are substantially less in the asynchronous muscles. Unfortunately, available information on power output and efficiency of asynchronous muscle is insufficient to know if the hypotheses are correct. These hypotheses will be tested by directly m easuring power output and efficiency of an asynchronous muscle. The muscle to be used are beetle flight muscles, chosen for being of convenient size and shape for mechanical studies. Power output will be determined using the work loop approach, in which a repetitive length change is imposed on the muscle and work output per cycle is measured as the area of the figure formed when muscle force is plotted against muscle length. Measurements will be made at a contraction frequency and muscle temperature characteristic of those during normal flight. Power measurements will be combined with measurements of energy expenditure, judged by oxygen consumption or carbon dioxide production, to determine the efficiency of the muscle in converting metabolic to mechanical energy. ***
