Body bending during fast-starts in fish can be explained in terms of muscle torque and hydrodynamic resistance

Fish fast-starts are rapid events caused by the simultaneous onset of muscle activity along one side of the body. Spine curvature and the strain and electromyographic activity in white muscle were measured for fast-starts in the common carp Cyprinus carpio. The first bend of the fast-start was powered by muscle on the concave side: muscle fibres on this side were activated and began shortening simultaneously between the length-specific longitudinal sites 0.3L and 0.56L, where L is total body length. However, there was an increasing delay in the timing of the first peak in body curvature and muscle strain along the length of the body. Modelling studies related the rate of body bending to the muscle torque and hydrodynamic resistance of the fish. The muscle torque produced on the spine was greatest in the central region of the trunk, and this acted against the moments of inertia of the fish mass and added mass of water. It was concluded that a wave of body bending can be generated as a result of the hydrodynamic resistance of the fish despite the initiation of that bending being simultaneous along the length of the body.