In a number of laboratories, the mechanical properties of myotomal muscle are being investigated under conditions which approximate to those believed to be operating in vivo during steady swimming. The aim of these studies is to learn how myotomal muscle is used to generate swimming movements. To perform these experiments, the muscle length changes during steady swimming (the strain cycle) and the pattern of muscle activation (activation cycle) must be known. Electromyographical (EMG) recordings from several points along the length of swimming fish have provided information on the activation cycles. Strain cycles of superficial muscle fibres have to date been determined primarily by calculation from the changes in shape of the fish plan outline. Simultaneous EMG and kinematic analyses yield the relation between strain and activation cycles. By imposing cyclical strains on isolated muscle, and stimulating at different phases in the strain cycle, a wide range of possible in vivo situations can be simulated, and muscle performance under these conditions can be compared to values under conditions which yield, for example, maximum power, work or stress. Valuable insights are thus gained into design constraints and the mechanisms by which muscle function has been optimized by selective pressures.