Meeting Abstract
Avian wings change shape during the flapping cycle due to the activity of a network of intrinsic wing muscles. One control aspect is elbow joint motion, which relates to wing folding for the upstroke and re-expansion for the downstroke. Muscle anatomy suggests that if the muscles are actuating then the biceps flex the elbow, and the two heads of the triceps, the humerotriceps and scapulotriceps, extend the elbow. Elbow angle control is uncertain as motor elements can have diverse functions such as actuators, brakes, springs, and struts, where specific roles and their magnitudes depend on when muscles are activated in the contractile cycle. The wing muscles best studied during flight are the elbow muscles of the pigeon (Columba livia). In vivo studies during different flight modes show variation in strain profile, activation timing and duration, and in contractile cycle frequency of the humerotriceps. This variation suggests that the pigeon humerotriceps may alter wing shape in diverse ways. To test this hypothesis, we developed an in situ work loop technique to measure how activation duration and contractile cycle frequency affected muscle work and power across the full range of activation onset times. The humerotriceps produced mainly net negative power, likely due to relatively long activation durations, indicating that it absorbs work, but the work loop shapes also suggest varying degrees of elasticity and resistance. Although we were not able to examine the effects of variation in strain profile, our results, when combined with previous in vivo studies, indicate that the humerotriceps can dynamically shift among roles of brake, spring, and strut, based on activation properties that vary with flight mode.
