High Performance Muscle Power Output
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There is now a general understanding of the metabolic costs of vertebrate hovering flight, as we can perform respirometry on hovering hummingbirds and bats. There are also expertly derived mathematical models on the aerodynamic costs of vertebrate hovering flight. However, there are virtually no empirical measurements of the power output of hummingbird or bat muscles during hovering flight. As a result, it is currently impossible to confidently bridge the gap between our understanding of the metabolic costs of hovering flight and the aerodynamic forces that result.
Dr. Robert Josephson and others have successfully recorded workloop data from isotonically exercising insect and bird flight muscles. Such data, obtained by causing sinusoidally oscillations in the length of a target muscle while phasically stimulating the muscle with implanted electrodes, permits estimation of the amount of work performed by the muscle. By implementing oscillations and stimulus patterns that approximate those observed in vivo, it is possible to quantitatively relate the actual mechanical power output of these muscles to their metabolic power input.
Hummingbird flight muscles represent a unique target for the application of the workloop technique. The homogenous fiber makeup of the hummingbird pectoralis corresponds with simple observed muscle excitation patterns (determined via EMG recordings). This fact permits very accurate approximation of stimulus patterns in situ. Further, because these muscle are specialized for extremely high speed (~50 Hz wingbeat frequencies) operation, high aerobic capacity, and high power output, we can begin to quantify muscle power output in muscles operating near theoretical performance maximums.
We may examine how muscle power output changes as a result of changes in stimulus or stretch regimes that mimic changes observed in vivo corresponding to changing flight behaviors or aerodynamic power output requirments. In addition, because these muscles are so specialized for aerobic activity, we could combine respirometry and the workloop technique to draw distinct relations between changes in muscle mechanical power output and metabolic power input, and relate these factors to the metabolic fuel (carbohydrates versus fatty acids) in use.