Meeting Abstract
Birds modulate aerodynamic force in large part by varying wing velocity, and flapping velocity can be increased either through wingstroke amplitude or wingbeat frequency. Muscle physiology studies of zebra finches, a bird with a relatively high wingbeat frequency, predicts that the birds should not increase frequency because reducing the already brief muscle shortening period does not allow sufficient time for force development and/or generates excessive negative power. We tested this prediction by recording the wing kinematics and muscle strain from the pectoralis major of zebra finches while they flew carrying a range of weights (0 to 75% body mass). We found that the birds do increase wingbeat frequency to increase wing velocity, however they do it without decreasing the duration of muscle shortening. The pectoralis began shortening 6 to 10ms (14-26% of wingbeat) before the wing began moving downward, although the muscle began lengthening at the same time the wing began to elevate. This delay between muscle and wing extends the duration of shortening allowing more time for the muscle to develop force, while simultaneously reducing the wing downstroke duration. Across the weight treatments, the amount of delay increases, so that muscle shortening duration remains unchanged but the downstroke duration becomes smaller. With stroke amplitude remaining constant or increasing slightly, the resulting wing velocity is faster despite a constant muscle velocity. This decoupling between muscle and joint motion allows the bird to overcome the limitations of muscle physiology to enhance wing velocity. Such decoupling is characteristic of power amplification through an elastic element, similar to frog jumping. However, this phenomenon is more unusual because it occurs in a cyclic contraction and there is no obvious in-series tendon to store energy.
