Three-dimensional actuation of hummingbird wings requires diverse effects from the primary flight muscles

Hummingbirds have evolved to hover and maneuver with exceptional flight control. This is directly enabled by their musculoskeletal system that successfully exploits the agile motion of flapping wings. Here, we reveal novel principles of hummingbird wing actuation that provide insights into the evolution and robotic emulation of hummingbird flight. We develop a functional model of hummingbird musculoskeletal system, which predicts instantaneous, three-dimensional torque produced by primary (pectoralis and supracoracoideus) and combined secondary muscles. It also reveals primary muscle contractile behavior, including stress, strain, elasticity, and work. Results show that the primary muscles (i.e., the flight “engine”) function as diverse effectors, as they do not simply power the stroke, but also actively deviate and pitch the wing. The secondary muscles produce controlled-tightening effects, by acting against primary muscles in deviation and pitching. The diverse effector capacity of pectoralis is associated with the evolution of a comparatively enormous bicipital crest on humerus. ### Competing Interest Statement The authors have declared no competing interest.