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Neuromuscular
Control of Hummingbird Flight |
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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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