Force-Velocity Relationship of Cat Cardiac Muscle, Studied by Isotonic and Quick-Release Techniques

Contractions of isolated cat papillary muscle were studied using a lever system with an electromagnetic load which allowed an on-line computer to control the experiment and to process all the data. Isotonic force-velocity curves were determined in 17 cat papillary muscles; the curves were not hyperbolic. Force-velocity curves at constant time in the contraction and constant contractile element length were obtained with a systolic quick-release technique in 9 muscles. The velocity of shortening after release to low force was almost always less than the maximum recorded following release to slightly higher force. When quick-release force-velocity curves determined at different times in the contraction were compared, the maximum velocity occurred at approximately 60% of the time to peak isometric force. The fall in velocity at lower forces was more marked later in the contraction. The shape of the quick-release force-velocity curves was found to depend on muscle length. At a constant time of release, and ignoring the low force end of the curves, the quick-release force-velocity relationships were not hyperbolic at muscle lengths appreciably below optimum, but near the optimal length the curves were hyperbolic. When these quick-release force-velocity curves were corrected for the presence of an elastic element in parallel with the contractile and series elastic elements, it was found that none of the contractile element force-velocity curves was hyperbolic.