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.

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• The study of strips of cardiac muscle has provided much valuable information regarding those factors which determine the force generated during contraction. Interrelationships between force, velocity of shortening, di&stolic stretch, duration of systole, and contractility have been delineated. "5 These findings are not, however, directly and siiiaply applicable to the physiology of the intact heart because the force generated by the myocardium and the corresponding pressure are by no means synonymous. In order to relate the physiology of muscle strips to that of the intact ventricle, the quantitative relation between force in the wall of the ventricle and pressure in the cavity must be known.The importance of such considerations in cardiac physiology has long been recognized. Burton, 10 and Linzbaeh. 11 More recently, Levine and Wagman have discussed the possible influence of the size and shape of the heart on myocardial oxygen consumption, emphasizing the important distinction between the pressure developed by the ventricle and the tension exerted by the fibers. Supported by American Heart Association grant. Work was done during Dr. Hefner's tenure as an Established Investigator of the American Heart Association and Dr. Sheffield's tenure of a TJ. S. Public Health Service Research Fellowship (HF-9775).Received for publication March 30, 1962. lows: Gravitational effects are ignored since they are unimportant in this situation. Consider a static left ventricle of any shape and size containing blood under a given pressure, as in figure 1. Now visualize an imaginary plane through this ventricle as shown in the figure. This imaginary plane divides the ventricle into two parts and passes through a rim of myocardium and a cross-sectional area of the cavity. From elementary hydrostatics it is known that the pressure of the blood in the cavity creates a force in a direction perpendicular to the imaginary plane exactly equal to the product of the pressure (which is force per unit area) times that cross-sectional area of the cavity included in the imaginary plane. The shape of this cross-sectional area of the cavity in our imaginary plane is immaterial, and the size and shape of the rest of the ventricle are also completely without effect on this relation. Since we began by postulating a static ventricle, Newton's laws of motion require that the force mentioned above tending to separate the two parts of the ventricle on each side of the plane be exactly balanced by an equal and opposite force. This equal and opposite force must exist in the rim of myocardium included in the imaginary plane, shown by the stippled area in figure 1. Note that this force exists in the rim of myocardium determined by the imaginary plane, its direction is perpendicular to the plane, and its magnitude is determined only by the pressure and cross-sectional area of the cavity and is independent of the thickness of the wall, the shape or cross-sectional area of the rim, the size or shape of the remainder of the ventricle, the distribution of f...