Fig. 6. Comparison of red muscle strain calculated from videography using beam theory (filled circles) and from sonomicrometry (continuous lines) for a yellowfin tuna swimming at 2.5 fork lengths s^-1 (FL s^-1). The image on the right of the figure is an example overhead video image of a yellowfin tuna swimming in a water-tunnel treadmill. The dots on the lateral outline of the fish mark the locations of the digitized points used to calculate body bending and thickness, which are then used to calculate muscle strain using equation 1. The approximate locations of the sonomicrometers within superficial muscle are indicated by green dots superimposed on the image, and the location of the sonomicrometer within the deep red muscle is indicated with a red dot. The association between muscle strain data collected at these locations is indicated by arrows. (A) Muscle strain calculated at 0.5 FL for superficial red muscle located in close apposition to the skin. (B) Muscle strain calculated at 0.5 FL for deep red muscle located within the myotome. (C) Muscle strain calculated at 0.7 FL for superficial red muscle in close apposition to the skin. The width of the vertical green bar indicates the magnitude of the phase shift between video and sonomicrometry estimates of muscle strain in the deep red muscle. The phase shift was calculated as the difference in argument for the principal harmonic component of a Fourier transform of the two time series and, in this case, was approximately 0.1 of a complete strain cycle, a value very close to the phase difference in muscle strain for the superficial muscle in the two longitudinal locations.

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