Meeting Abstract
Birds dynamically modulate their wing shape during flight. One example is folding and expanding the wing during the up- and downstroke, and involves flexion and extension of the elbow, respectively. Elbow extension is driven by the two heads of the triceps; scapulotriceps and humerotriceps. However, little is known about the role of these two muscles in actively modulating wing shape. Electromyography recordings (EMGs) in pigeons (Columba livia) show activation of the scapulotriceps beginning just prior to muscle peak length, suggesting the muscle functions as an actuator. In contrast, the humerotriceps is activated near the end of muscle shortening, suggesting the muscle could function as a brake. To test these hypotheses, we used an in situ work loop technique in pigeons to characterize the performance space of the humerotriceps by sampling a wide range of activation onset times and measuring the net work. We then compared the in vivo EMGs to this performance space to propose the in vivo role of this muscle. The muscle generated net positive work, indicating it serves as an actuator, between -40% and -15% cycle before peak length. The muscle generated negative work, indicating it functions as a brake, between -5 and 40% cycle before peak length. Comparison to the in vivo activation timing (9%) suggests that this muscle is used primarily as a brake, resisting elbow flexion and wing folding. Repeating the study with the scapulotriceps will reveal if the muscle is the primary elbow extensor, or if it also provides a braking role. Assuming the performance space is similar for both triceps, then the in vivo activation (-32%) is within positive work output range and the muscle’s primary role would be actuation. More research is required to test this hypothesis.
