Abstract
Both activation and relaxation times of rainbow trout Oncorhynchus mykiss red muscle were shorter in parr than in older juveniles. Furthermore, parr red muscle had a faster maximum shortening velocity than that of older fish, as estimated with the force‐clamp technique. Parr swam with higher tailbeat frequencies and lower tailbeat amplitude than did older fish across a range of length‐specific steady swimming speeds. The developmental shift in contraction kinetics of red muscle and steady swimming kinematics was associated with a reduction from two or three myosin heavy chain isoforms in parr to one in older juveniles. This transition provides a mechanism to explain the variations in muscle contraction kinetics and swimming performance.