Effects of Flight Speed upon Muscle Activity in Hummingbirds

 Title: Effects of Flight Speed upon Muscle Activity in Hummingbirds Author: Tobalske, Bret W.; Biewener, Andrew Austin; Warrick, Douglas R.; Hedrick, Tyson L.; Powers, Donald R. Note: Order does not necessarily reflect citation order of authors. Citation: Tobalske, Bret W., Andrew A. Biewener, Douglas R. Warrick, Tyson L. Hedrick, and Donald R. Powers. 2010. Effects of flight speed upon muscle activity in hummingbirds. Journal of Experimental Biology 213(14): 2515-2523. Full Text & Related Files: Tobalske_EffectsFlight.pdf (693.7Kb; PDF) Abstract: Hummingbirds have the smallest body size and highest wingbeat frequencies of all flying vertebrates, so they represent one endpoint for evaluating the effects of body size on sustained muscle function and flight performance. Other bird species vary neuromuscular recruitment and contractile behavior to accomplish flight over a wide range of speeds, typically exhibiting a U-shaped curve with maxima at the slowest and fastest flight speeds. To test whether the high wingbeat frequencies and aerodynamically active upstroke of hummingbirds lead to different patterns, we flew rufous hummingbirds (Selasphorus rufus, 3 g body mass, 42 Hz wingbeat frequency) in a variable-speed wind tunnel $$(\textrm{0–10 m s}^{-1})$$. We measured neuromuscular activity in the pectoralis (PECT) and supracoracoideus (SUPRA) muscles using electromyography (EMG, $$N=4$$ birds), and we measured changes in PECT length using sonomicrometry ($$N=1$$). Differing markedly from the pattern in other birds, PECT deactivation occurred before the start of downstroke and the SUPRA was deactivated before the start of upstroke. The relative amplitude of EMG signal in the PECT and SUPRA varied according to a U-shaped curve with flight speed; additionally, the onset of SUPRA activity became relatively later in the wingbeat at intermediate flight speeds $$(\textrm{4 and 6 m s}^{−1})$$. Variation in the relative amplitude of EMG was comparable with that observed in other birds but the timing of muscle activity was different. These data indicate the high wingbeat frequency of hummingbirds limits the time available for flight muscle relaxation before the next half stroke of a wingbeat. Unlike in a previous study that reported single-twitch EMG signals in the PECT of hovering hummingbirds, across all flight speeds we observed 2.9±0.8 spikes per contraction in the PECT and 3.8±0.8 spikes per contraction in the SUPRA. Muscle strain in the PECT was 10.8±0.5%, the lowest reported for a flying bird, and average strain rate was 7.4±0.2 muscle lengths $$s^{−1}$$. Among species of birds, PECT strain scales proportional to body mass to the 0.2 power $$(\infty M_b^{0.2})$$ using species data and $$\infty M_b^{0.3}$$ using independent contrasts. This positive scaling is probably a physiological response to an adverse scaling of mass-specific power available for flight. Published Version: doi:10.1242/jeb.043844 Other Sources: http://www.ncbi.nlm.nih.gov/pubmed/20581281 http://dbs.umt.edu/flightlab/documents/Tobalske_Biewener_Warrick_Hedrick_Powers_2010_JEB.pdf http://www.donandtheopowers.com/Powers_GFU/Publications_files/2983JEB213_2515-2523.pdf Terms of Use: This article is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAP Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:4892998 Downloads of this work: