The dynamics of steady swimming were examined in the shortfin mako (Isurus oxyrinchus), a member of the cartilaginous fish family Lamnidae, a family known for their morphological adaptations for high-performance locomotion and their similarity in hydromechanical design to tunas. Patterns of red muscle (RM) strain (i.e. relative length change) and activation were quantified at two axial positions ( approximately 0.4 and 0.6L, where L is total body length), using sonomicrometry and electromyography (EMG), and correlated with simultaneous measurements of dorsal midline kinematics during steady swimming ( approximately 0.5-1 L s(-1)). RM strain varied longitudinally with strain amplitudes ranging from 5.5+/-1.1% (s.e.m.) in the anterior to 8.7+/-0.9% in the posterior. We found no significant longitudinal variation in patterns of RM activation, with mean onset of activation occurring at 83-84 degrees (90 degrees is peak length) and offset at 200-210 degrees at both body positions. Likewise, duty cycles were similar: 35.5+/-1.0% in the anterior and 32.2+/-1.6% in the posterior. Comparison of the timing of waves of dorsal midline curvature and predicted strain relative to measured RM strain revealed a phase shift between RM shortening and local body bending. Furthermore, when the body is bent passively, RM shortens synchronously with the surrounding white muscle (WM) and skin, as expected. During active swimming, peaks in RM strain were delayed relative to peaks in WM strain by a mean of approximately 10% of the tailbeat cycle, with one individual as high as approximately 17% in the anterior and nearly 50% in the posterior. The longitudinal consistency in the EMG/strain phase relationship in the mako is similar to that in the leopard shark, suggesting a consistent trend among sharks using different locomotor modes. However, unlike in the leopard shark, RM shortening in the mako is physically uncoupled from deformation of the surrounding body during steady swimming, a characteristic shared between the mako and tunas.