Muscle strain histories in swimming milkfish in steady and sprinting gaits

Adult milkfish (Chanos chanos) swam in a water-tunnel flume over a wide ran ge of speeds. Fish were instrumented with sonomicrometers to measure shorte ning of red and white myotomal muscle, Muscle strain was also calculated fr om simultaneous overhead views of the swimming fish, This allowed us to tes t 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 FLs(-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 mus cle strain and body bending (0.5 and 0.7 FL), both techniques were equivale nt predictors of muscle strain histories. Cross-correlation coefficients fo r comparisons between these techniques exceeded 0.95 in all cases and had t emporal separations of less than 7 ms on average. Muscle strain measured us ing sonomicrometry within the speed range 0.9-2.6 FL s(-1) showed that musc le strain did not increase substantially over that speed range, while tail- beat frequency increased by 140 %, While using a sprinting gait, muscle str ains 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 ros trally to caudally, but only by less than 4 %, These results suggest that s wimming 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 homoge neous, continuous beam, This has been an implicit assumption for many studi es of muscle function in many fish, but has not been tested explicitly unti l now. This result is achieved in spite of the presence of complex and inho mogeneous geometry in the folding of myotomes, collagenous myosepta and ten don, and the anatomical distinction between red and white muscle fibers.