Effect of Muscle Length on the Force-Velocity Relationship of Tetanized Cardiac Muscle

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Abstract
Cat papillary muscles were tetanized with repetitive electrical stimulation in the presence of 10 mM caffeine and 10 mM calcium. Velocities were measured during the plateau of tetanus with quick releases to isotonic loads. The course of isotonic shortening was independent of time in the contraction cycle for at least 2 seconds after the attainment of peak isometric force. The force-velocity relationships were measured at different muscle lengths that had been corrected for series elastic extension. These lengths ranged from 75% to 90% of the passive length from which maximum force was developed. The data were fitted by a least-squares method with hyperbolas described by the Hill equation, each for a constant corrected muscle length. The extrapolated maximum velocities and isometric forces diminished together in almost direct proportion to muscle shortening. Corrections for the load borne by the parallel elastic elements did not significantly change the relationships between isometric force, maximum velocity, and muscle length. The results can be accounted for by two mechanisms: (1) an internal load and (2) deactivation of the contractile elements.

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