Modulation of pectoralis muscle function in budgerigarsMelopsitaccus undulatusand zebra finchesTaeniopygia guttatain response to changing flight speed

David J Ellerby,Graham N Askew

doi:10.1242/jeb.006296

Abstract

Flight power varies in a U-shaped relationship with flight speed, requiring the modulation of flight muscle power in order to meet these changing power demands. The power output of the pectoralis muscle can potentially be modulated by changing strain trajectory and the relative timing and intensity of muscle activity. Pectoralis muscle length change and activity patterns were recorded in budgerigars Melopsitaccus undulatus and zebra finches Taeniopygia guttata at a range of flight speeds using sonomicrometry and electromyography (EMG). The pectoralis muscles in these species contain a single muscle fibre type. Therefore, the power output is entirely determined by muscle activity and strain trajectory, rather than recruitment of motor units with different contractile properties as in many other vertebrate muscle systems. Relative EMG intensity, wingbeat frequency and muscle strain varied in an approximately U-shaped relationship with flight speed. The shape of the length trajectory varied with flight speed in budgerigars, with the proportion of the cycle spent shortening being lowest at intermediate flight speeds. In zebra finch pectoralis muscle the shape of the length trajectory did not vary significantly with flight speed. In both species the observed changes in muscle recruitment and length trajectory are consistent with meeting flight power requirements that vary in a U-shaped pattern with speed. Both species utilised intermittent flight, tending to spend relatively less time flapping at intermediate flight speeds. This supports the idea that intermittent flight is used as a simple power modulation strategy. However, the idea that intermittent flight serves to maintain a 'fixed gear' is over-simplistic and fails to recognise the plasticity in performance at the level of the muscle. Intermittent flight is only one component of a complex power modulation strategy.

Highlights

All animals tailor the mechanical power output of their muscles to the changing demands of locomotion
Where locomotion involves the transfer of momentum to a surrounding fluid, as in flying and swimming, mechanical power requirements change with speed
Avian flight power has a Ushaped relationship with speed (Rayner, 1999; Askew and Ellerby, 2007) and during swimming, the mechanical power requirements increase exponentially with speed (Alexander, 2005)

Summary

Introduction

All animals tailor the mechanical power output of their muscles to the changing demands of locomotion. Muscles differ widely in the stress that they can generate, the rate at which they are able to shorten and the degree of curvature of their force–velocity relationships (Josephson, 1993) All of these factors affect the power that can be generated, resulting in a wide range of measured skeletal muscle power outputs, from 0.5·W·kg–1 in eel slow muscle (Ellerby et al, 2001) to 390·W·kg–1 in blue-breasted quail pectoralis muscle (Askew and Marsh, 2001). Where an organism moves through a fluid, the muscular power source can be turned off periodically to control average power output This occurs in fish during ‘burst and coast’ swimming, and birds during intermittent flight. Fascicle strain trajectory and its THE JOURNAL OF EXPERIMENTAL BIOLOGY

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