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Transitions in red muscle contraction kinetics, steady swimming kinematics and myosin heavy chain isoforms were studied in rainbow trout, Oncorhynchus mykiss, of two ages: parr and older juveniles.  Both activation and relaxation times were shorter in the parr than in older juveniles.  Further, 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 the 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.