ELLERBY, DJ; ASKEW, GN; ALTRINGHAM, JD; University of Leeds; University of Leeds; University of Leeds: Pectoralis muscle power modulation in small birds

Most vertebrates are able to modulate the power output of their muscles by recruiting motor units with different intrinsic properties (i.e. fast or slow fibres). In many small birds the pectoralis muscles, the major power source during flight, consist of a single muscle fibre type (fast oxidative glycolytic). The power requirements of flight typically show a U-shaped relationship with speed, with the upper and lower extremes of the speed range requiring the greatest power outputs. Given the restrictions imposed by having a single muscle fibre type, these species must adopt alternative power modulation strategies to tailor muscle power output to the changing power requirements of a range of flight speeds. There are a number of possibilities. The power source may simply be turned on and off to control the average power output, apparent as the intermittent flight of many species. The amount of muscle work done per wing beat could also be adjusted via changes in recruitment intensity and strain trajectory. Power output may also be controlled by changes in wing beat frequency. In order to investigate these mechanisms, pectoral muscle recruitment and strain were measured in budgerigars and zebra finches across a range of flight speeds in a wind tunnel. The birds were simultaneously filmed with a high-speed camera, and aerodynamic power calculated from wing kinematics. In both species, the muscle length change cycle and recruitment intensity were modulated in ways consistent with the modulation of work per wing beat. Wing beat frequency also changed with speed, as did the duration of flapping flight relative to bounding. In short, all possible mechanisms for modulating pectoralis muscle power were exploited.