Adult milkfish (Chanos chanos) swam in a water-tunnel flume over a wide range of speeds. Fish were instrumented with sonomicrometers to measure shortening of red and white myotomal muscle. Muscle strain was also calculated from simultaneous overhead views of the swimming fish. This allowed us to test the hypothesis that the muscle shortens in phase with local body bending. The fish swam at slow speeds [U<2.6 fork lengths s-1 (=FL s-1)] where only peripheral red muscle was powering body movements, and also at higher speeds (2. 6>U>4.6 FL s-1) where they adopted a sprinting gait in which the white muscle is believed to power the body movements. For all combinations of speeds and body locations where we had simultaneous measurements of muscle strain and body bending (0.5 and 0.7FL), both techniques were equivalent predictors of muscle strain histories. Cross-correlation coefficients for comparisons between these techniques exceeded 0.95 in all cases and had temporal separations of less than 7 ms on average. Muscle strain measured using sonomicrometry within the speed range 0.9-2.6 FL s-1 showed that muscle strain did not increase substantially over that speed range, while tail-beat frequency increased by 140 %. While using a sprinting gait, muscle strains became bimodal, with strains within bursts being approximately double those between bursts. Muscle strain calculated from local body bending for a range of locations on the body indicated that muscle strain increases rostrally to caudally, but only by less than 4 %. These results suggest that swimming muscle, which forms a large fraction of the body volume in a fish, undergoes a history of strain that is similar to that expected for a homogeneous, continuous beam. This has been an implicit assumption for many studies of muscle function in many fish, but has not been tested explicitly until now. This result is achieved in spite of the presence of complex and inhomogeneous geometry in the folding of myotomes, collagenous myosepta and tendon, and the anatomical distinction between red and white muscle fibers.

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