Because measurements on isolated skeletal muscles are often made with limited knowledge of in vivo kinematics, predictions of mechanical performance during natural movements are subject to considerable uncertainties. We used information on the in vivo length cycle and phase of activation of the scallop adductor during swimming at 10 degrees C to design an in vitro contractile regime that replicated the natural cycle. Replicating the in vivo length cycle and stimulation regime resulted in power output during cyclic contractions that matched in vivo performance both qualitatively and quantitatively. When sinusoidal length changes were used instead of the natural length trajectory, the adductor muscle produced a similar average power output (approximately 30 W kg-1 at 1.9 Hz), but the distribution of power throughout the cycles was quite different. We examined the instantaneous force-velocity properties during cyclic contractions and found that the muscle operated on or near its isotonic force-velocity curve for only 30-40% of the time required for shortening. During sinusoidal length cycles, the force-velocity trajectory was quite different. We conclude that during cyclic contractions the isotonic force-velocity curve of skeletal muscle sets an approximate boundary to the force-velocity trajectory, but the shape of this trajectory, and thus the distribution of power output, depends on the pattern of length change.

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