This paper offers an analysis of the recruitment and normalized power output of the slow oxidative (red) axial muscles of the carp, during swimming in two different modes. Recruitment patterns of the muscle fibres and swimming movements were measured by synchronized electromyography and cinematography. The ultrastructure of the muscle fibres along the trunk was measured by electron microscopy. Strain fluctuations and normalized power along the trunk were estimated using a model, as direct measurement was impossible. The model takes into account structural parameters of the sarcomeres, modulation of sarcomere force as a result of changing sarcomere length, modulation of cross-bridge force owing to the force-velocity relationship, and rate of tension rise and of tension decline as a result of fibre properties and stimulation. In continuous swimming, the amplitude of the fluctuations in the strain was constant along the trunk, owing to a remarkable coherence between the amplitude of body curvature and the position along the body of the red muscle fibres with respect to the vertebral column. However, the strain range and speed of contraction at which the fibres were active changed along the trunk in continuous swimming as well as in ‘kick and glide swimming’. During continuous swimming, the fibres along the trunk had a period of negative active power production followed by a period of positive power production. Anteriorly, the positive phase was most important, with a close to optimal power production in a plateau phase, so that net positive work was produced in a swimming cycle. Posteriorly, net negative work was done. In the anal region, the amounts of positive and negative work almost balanced each other. During the first tail strike in intermittent swimming, peak positive normalized work for an activated fibre was produced in the anal region. For this initial tail strike, fast muscle fibres were calculated to work more effectively than slow fibres. Fast fibres were needed to generate kick and glide swimming.
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