Myofilament overlap in swimming carp. II. Sarcomere length changes during swimming

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Abstract
This study was performed to determine myofilament overlap during swimming in carp. By using frozen sections, we found that sarcomere lengths of the red and white muscle could be related to the curvature of the backbone. Sarcomere length (SL) during swimming was calculated from an analysis of backbone curvature in high-speed motion pictures. Because carp have the same myofilament lengths as frogs, we related force generation to SL using the frog SL-tension curve. At slow swimming speeds, the red fibers are used at a SL of 1.91-2.22 microns, where force generation is calculated to be no less than 96% maximal. If the red fibers powered the escape response they would have to shorten to 1.45-1.55 microns, where force generation would be reduced to approximately 50% maximal and the fibers damaged. Instead, the white muscle fibers are recruited and because of their helical orientation (resulting in a higher gear ratio), they shorten to only 1.75 microns, where they generate no less than 85% maximal tension. Thus, by recruitment of fiber types with different orientations, the full range of movements is powered by fibers operating at nearly maximal overlap. This suggests that myofilament overlap is an important design constraint of muscular systems.