The instantaneous force-velocity relations of cat papillary muscles were studied at different times in the twitch in normal and in postextrasystolic potentiated contractions. Fourteen to sixteen different loads were used to define each of the force-velocity curves. The curves were fitted by a least-squares procedure to the hyperbolic (Hill equation). The hyperbolae were extrapolated to obtain maximum velocity and isometric force and interpolated to obtain maximum power. All three of these values rose more quickly than developed force. Maximum velocity reached 77% of its peak at the earliest time studied, 20–25% of the time to peak force. Developed force achieved 22% of its final value at this time, while extrapolated isometric force and maximum power both reached 44% of their peak values. Postextrasystolic potentiation sufficient to produce a 1.5 to twofold increase in peak developed force produced less than a 20% increase in extrapolated maximum velocity. The results can be interpreted in terms of a model in which the maximum velocity of the contractile elements remains constant during the twitch. Variation in maximum velocity is attributed to a small internal load, equivalent to 6% of twitch force. Since maximum velocity is relatively constant, it does not give a good measure of changes in the force-velocity curves. By contrast, the extrapolated isometric force and maximum power are much more sensitive to changes in the force-velocity curves, and they vary in proportion to each other. The advantage of using interpolated maximum power rather than isometric force to define changes in the curves is that it can be normalized to muscle mass.