Fig. 2. Work loops from internal red muscle of a skipjack tuna. Results from two different longitudinal body locations and three different cycle frequencies are shown. Muscle length was cycled in a sinusoidal fashion, and the muscle was stimulated phasically during the length cycle. Loops are plots of muscle force against length for a complete length cycle and are traversed in a counterclockwise direction. Net work is equal to the area inside the loop. Left-hand panels: muscle from an anterior (ANT) location (0.45L); resting length 4.0 mm, strain ±5.5 %. Right-hand panels: muscle from a posterior (POST) location (0.69L); resting length 4.5 mm, strain ±8 %. Both muscles are from the same fish. Work loops generated using stimulus parameters that maximized work output (solid lines) are shown together with loops generated when the muscle was stimulated using parameters measured from swimming fish (broken lines). Values inside loops are the net work done using stimulus parameters from swimming fish (broken loops) as a percentage of the maximum (solid loops). Note the similarity between the two types of loop. Only maximal loops are shown at 8 Hz because fish do not normally swim at this speed using only red muscle.