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Department of Physiology, School of Medicine, Teikyo University, Tokyo, Japan.

1. The force-velocity (P-V) relation of single frog skeletal muscle fibres was examined in non-steady-state conditions in which the load on the fibre changed continuously with time. Two such types of condition were used; in one type, the load was increased from zero to the maximum isometric force (P0) (auxotonic condition), while in the other type the load was decreased form P0 to zero at constant rates (ramp decrease in load). 2. The P-V curves obtained in the auxotonic condition were convex upwards and always below the hyperbolic P-V curve obtained in the isotonic condition. Different curves were obtained depending on the compliance of auxotonic load. The shortening velocity for a given amount of load increased with increasing compliance. 3. Qualitatively similar P-V relations were obtained irrespective of whether the fibre was made to shorten auxotonically at the onset of stimulation or after the development of P0. 4. If the force at any time after the onset of auxotonic shortening was normalized relative to the isometric force at the same time after the onset of isometric force development, the normalized force versus velocity curves were found to fit well to the hyperbolic P-V curve in the isotonic condition except for the low-force region. 5. The P-V curves obtained during the ramp decrease in load were hyperbolic in shape except for the humps at the high-force region and always above the P-V curve in the isotonic condition. Different curves were obtained depending on the rate of load decrease. The maximum shortening velocity increased with increasing rate of load decrease. 6. The above features of the P-V relations could well be simulated by a simplified Huxley contraction model, indicating that the kinetic properties of the cross-bridges are the same in both steady- and non-steady-state conditions.

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