GOLDBOGEN, Jeremy; SHADWICK, Robert; Scripps Institution of Oceanography, University of California – San Diego; Scripps Institution of Oceanography, University of California – San Diego: Fast-start muscle dynamics in the rainbow trout Oncorhynchus mykiss.

Recent investigations regarding the muscle dynamics of fish locomotion have provided evidence that the wave of muscle shortening traveling along the body is in phase with changes in local midline curvature, as in a homogeneous bending beam. However, studies involving the red muscle of steady swimming of tuna (R.E. Shadwick et al. 1999, J. Exp. Biol. 202:2139; S.L. Katz et al. 2001, Nature 410:770) and the white muscle in fast-start responses of rainbow trout (J.W. Covell et al. 1991, J. Exp. Biol. 156:453) have suggested a temporal decoupling of muscle length and backbone kinematics, such that muscle shortening in the mid-body region is in phase with curvature at more posterior locations. Here we address this disparity in our understanding of fish swimming mechanics by revisiting the phase relationship of muscle strain and midline curvature in the fast-start behavior of Oncorhynchus mykiss. We used sonomicrometry to measure shortening in the lateral myotomal fast fibers at 0.4L and 0.7L (where L is fork length) of 5 fish (L = 31-35 cm) during induced escape responses. Muscle strains in the initial stage of the fast-start progress along the body, ranging from 13%lo � 5% (where lo is muscle resting length) at 0.4L to 15%lo � 8% at 0.7L. Simultaneous high-speed video analysis (filming frequency: 250Hz) was used to calculate the midline curvature, which also increases as bending progresses posteriorly. Comparison of these data shows conclusively that contractions of white fibers are in phase with local curvature during fast starts, as they are for steady swimming.