Pectoralis muscle performance during ascending and slow level flight in mallards (Anas platyrhynchos)

In vivo measurements of pectoralis muscle length change and force productio n were obtained using sonomicrometry and delto-pectoral bone strain recordi ngs during ascending and slow level flight in mallards (Anas platyrhynchos) . These measurements provide a description of the force/length properties o f the pectoralis under dynamic conditions during two discrete flight behavi ors and allow an examination of the effects of differences in body size and morphology on pectoralis performance by comparing the results with those o f a recent similar study of slow level flight in pigeons (Columbia livia), In the present study, the mallard pectoralis showed a distinct pattern of a ctive lengthening during the upstroke. This probably enhances the rate of f orce generation and the magnitude of the force generated and, thus, the amo unt of work and power produced during the downstroke. The power output of t he pectoralis averaged 17.0 W kg(-1) body mass (131 W kg(-1) muscle mass) d uring slow level flight (3 m s(-1)) and 23,3Wkg(-1) body mass (174 W kg(-1) muscle mass) during ascending flight. This increase in power was achieved principally via an increase in muscle strain (29 % versus 36 %), rather tha n an increase in peak force (107 N versus 113N) or cycle frequency (8.4Hz v ersus 8.9Hz), Body-mass-specific power output of mallards during slow level flight (17.0 W kg(-1)), measured in terms of pectoralis mechanical power, was similar to that measured recently in pigeons (16.1 W kg(-1)). Mallards compensate for their greater body mass and proportionately smaller wing are a and pectoralis muscle volume by operating with a high myofibrillar stress to elevate mechanical power output.