A Mechanical Model for the Rapid Body Flexures of Fast-starting Fish

To understand the mechanical behavior of fast-starting fish, we modeled the pumpkinseed sunfish, Lepomis gibbosus, as a longitudinally-loaded beam that behaves as an elastic column eccentrically-loaded by lateral musculature. Since bending moments are a function of the shape of the body, we used real sunfish to determine, (1) from cross-sections, the effective moment arm of the lateral musculature, and, (2) from high speed video, the points about which the body bends. Since electromyography of faststarting fish revealed simultaneous activation of both sides of the lateral musculature in stage 1 of a Ctype start, we used this pattern to model the internal forces from which the reactive loads were determined. The model predicts that fast-starting sunfish produce a standing wave of bending moment posteriorly that then becomes a traveling wave of bending moment moving caudally from the anterior region. At the time of the standing moment wave, the body reaches its maximal effective flexural stiffness and, as a result, would be able to rapidly transmit bending moments along the body axis.