Meeting Abstract

32.4  Friday, Jan. 4  Neuromuscular control of mechanical power output in hummingbirds I: EMG recordings from pectoral muscles WELCH, D.B.*; WELCH, K.C.; ALTSHULER, D.L.; Univ. of California, Riverside; Univ. of California, Riverside; Univ. of California, Riverside danwelch@mac.com

Electromyogram recordings (EMGs) of exercising muscle from most vertebrate organisms, including birds, display compound waveforms representing the summed activity of multiple motor units. Hummingbirds are unique among vertebrates for their insect-like ability to sustain hovering flight, and their pectoral EMGs display relatively simple waveforms that resemble single motor unit recordings. Histochemical analysis has revealed that hummingbird pectoral muscles are composed exclusively of fast-twitch oxidative-glycotlytic fibers. Here we take advantage of this simplified anatomy and highly coherent electrophysiological signal to describe how vertebrate muscles are modulated to increase mechanical power output. Specifically, we compare EMGs from three behaviors differing in power requirements and duration: sustained hovering in air, sustained hovering in hypodense heliox (21% oxygen, balance helium), and transient load lifting. When hovering in heliox, hummingbirds increased stroke amplitude by up to 20% with little or no change in wingbeat frequency, relative to hovering in air. Comparison of the pectoral EMG patterns between hovering in air and in heliox revealed an increase in waveform amplitude with no increase in waveform number. When transiently lifting weights, hummingbirds increased both stroke amplitude and wingbeat frequency by 20% above hovering values. The pectoral EMG patterns displayed an increased number of waveforms per cycle and increases in waveform amplitude. These results indicate that hummingbirds use spatial recruitment of pectoral muscle fibers to increase contractile amplitude and use temporal recruitment of those fibers to increase contractile frequency.