Fast-start muscle dynamics in the rainbow trout Oncorhynchus mykiss: phase relationship of white muscle shortening and body curvature.

Muscle length changes of the lateral myotomal fast fibers of rainbow trout (Oncorhynchus mykiss) were measured using sonomicrometry during induced fast-starts. Simultaneous high-speed videography allowed for the analysis of midline kinematics to estimate the degree of muscle strain that occurs during body deformation. Comparison of these data was used to examine the phase relationship between local muscle shortening and local body bending during unsteady, large amplitude maneuvers. Our analysis finds that muscle shortening is temporally decoupled from body bending, probably due to the influence of hydrodynamic forces. The phase shift was such that midline curvature lagged behind muscle shortening at both the anterior (0.4 L, where L is fork length) and posterior (0.7 L) axial positions. Stronger escape responses were correlated with high peak strains and rapid strain-wave velocities, but not faster curvature-wave velocities. Under these conditions of high strain, the phase shift at the posterior position is significantly increased, whereas the anterior position fails to be affected. Curvature lag was still observed at both axial locations under conditions of low strain, suggesting that hydrodynamic forces are still significant during weaker escape responses. These data support a previous model that suggests fast-start body bending is determined by the interaction between muscle torque and hydrodynamic resistance along the body.