SYNOPSIS. Recent research in fish locomotion has been dominated by an interest in the dynamic mechanical properties of the swimming musculature. Prior observations have indicated that waves of muscle activation travel along the body of an undulating fish faster than the resulting waves of muscular contraction, suggesting that the phase relation between the muscle strain cycle and its activation must vary along the body. Since this phase relation is critical in determining how the muscle performs in cyclic contractions, the possibility has emerged that dynamic muscle function may change with axial position in swimming fish. Quantification of muscle contractile properties in cyclic contractions relies on in vitro experiments using strain and activation data collected in vivo. In this paper we discuss the relation between these parameters and body kinematics. Using videoradiographic data from swimming mackerel we demonstrate that red muscle strain can be accurately predicted from midline curvature but not from lateral displacement. Electromyographic recordings show neuronal activation patterns that are consistent with red muscle performing net positive work at all axial positions. The relatively constant cross-section of red muscle along much of the body suggests that positive power for swimming is generated fairly uniformly along the length of the fish.