Fig. 2. Mechanical performance of a budgerigar pectoralis muscle fascicle in vitro. (A) Force production during strain and activation conditions measured in vivo at 4 m s^–1 simulated flight speed. Length, unbroken lines; stress, broken lines. The bold line shows the timing of the stimulus relative to the strain cycle. (B) Work loops obtained by plotting fascicle stress against length for the data shown in A. Fascicle length is expressed relative to resting muscle length, L₀. Muscle stress is expressed relative to maximal isometric stress, P₀.