Neuromuscular Control of Hummingbird Flight

Many flying insects exhibit remarkable flight agility owing in part to high wingbeat frequencies and hovering ability. Thus insects have for decades served as models for the neuromuscular control of high performance motor behavior. Similarly, hummingbirds are unique among vertebrates for their ability to sustain both hovering posture and relatively high wingbeat frequency during flight. The combination of experimentally tractable and highly complex flight behavior in hummingbirds makes them an ideal model for examining mechanisms of neuromuscular control in a vertebrate. We have studied hummingbird wingbeat kinematics and muscle activity during hovering and maneuvering flight using synchronized high-speed digital video and EMG recordings from the flight power muscles. Wingbeat kinematics can be described as the time course of three wing angles, defined with respect to body: stroke position, stroke deviation, and kinematic angle of attack. Throughout the wing beat cycle, the time course of stroke deviation and angle of attack is strikingly similar to those found in fruit flies and bees, indicating that hovering is achieved using almost identical kinematics despite vast differences in the structure and function of the sensorimotor and musculoskeletal systems. EMG recording were made from the downstroke and upstroke power muscles, which are the largest muscles in hummingbirds, representing 17% and 8% of the body mass, respectively. Unlike most vertebrate EMG recordings, which display compound wave forms representing the activation of multiple motor units, hummingbird EMGs display relatively simple wave forms reminiscent of insect flight muscle activation patterns. The temporal structure of the hummingbird EMGs suggests that they comprise either very few individual motor units or, the tightly synchronized activation of a larger group of motor units. Thus, the neuromuscular control of hummingbird flight appears to be highly insect-like in that muscles are activated very briefly and produce highly complex patterns of wing kinematics.

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