Fig. 5. Examples of data obtained from the in vitro work loop experiments. A bundle of muscle fibres was isolated from the quail pectoralis and subjected to a series of cyclical length changes. Two types of strain trajectory were studied: a wave that simulated the strain measured in vivo using sonomicrometry (A) and an asymmetrical sawtooth cycle (B). The muscle was activated 7 ms before peak length for 14 ms (indicated by the bold lines in A and B). Force was measured, and this has been used to calculate muscle stress (C,D). The instantaneous power output was calculated by multiplying force by the velocity of shortening (E,F). Work loops were generated by plotting stress against strain. Work loops from two consecutive cycles are shown in G and H. The arrows indicate the direction of the loop.