Biomechanics of fast-start swimming in fish

Fast-starts are high acceleration manoeuvres used by fish. Fast-start escape responses can be initiated by a nervous reflex mediated by a Mauthner neurone and result in the simultaneous activation of muscle along one side of the body. Such starts have traditionally been termed 'C' starts where the fish initially bends to a tight C shape and then subsequently gains propulsion away from the stimulus. However, close examination reveals that propulsion occurs even during the initial phase of body bending. In order to generate this forward propulsion the fish must flex with a rearward travelling wave of body bending, and power generated by the muscles in the central region of the trunk must be delivered to the water through the caudal region of the body. Despite simultaneous muscle activation along the length of the fish, a wave of body bending can result from the interaction of the muscle torque acting to bend the fish and the inertia of the body and water acting to resist this bending. The Mauthner neurone causes the muscle along one side of the fish to be activated immediately prior to shortening and so there is no difference in timing between muscle activation and onset of shortening for this initial contraction along the whole length of the body. Although the fish flexes to tighter curvatures towards its caudal region, the white muscle strain is reduced in this region due to the effect of the geometry of the body and muscle. The myotomal muscle generates greater mass-specific power outputs in the central region of the fish, and greater stresses towards the caudal region and this difference in muscle function can be entirely explained by the change in muscle strain. This change in muscle function enables the high muscle powers to be delivered to the water through the caudal region of the fish where it will cause the fast-start acceleration.