Fig. 1. In vivo measurements of mechanical work and power output from the pectoralis, the primary downstroke muscle of the avian wing are accomplished using surgically implanted strain gauges calibrated to measure force from bone strain on the deltopectoral crest of the humerus and using sonimicrometry crystals to measure muscle length (A). Similar methods are employed for the primary upstroke muscle, the supracoracoideus (not shown), which is located deep to the pectoralis (Tobalske and Biewener, in press). (From Hedrick et al., 2003.) (B) A `work loop', the area of which represents in vivo mechanical work in the pectoralis of a cockatiel Nymphicus hollandicus during one wingbeat (adapted from Tobalske et al., 2003). Arrows indicate the progression of contractile behavior. Electromyography (EMG) activity in the pectoralis indicates that the muscle functions to decelerate the wing at the end of upstroke and accelerate the wing during the first third of downstroke.