Figure 1.

Mechanical performance of pectoralis muscle fascicles in vitro. (a,d) The imposed strain trajectory(dashed line), stimulus and resulting force production(solid line), relative to peak isometric tetanic force, P₀. The bold line indicates the period of stimulation. (b,e) Instantaneous power output. (c,f) The force–length relationship or ‘work loop’. The work loops are anticlockwise (indicated by the arrows) and therefore represent net positive work. Data are typical examples obtained from (a–c) zebra finch and (d–f), budgerigar pectoralis muscle.