Relation of V max to Different Models of Cardiac Muscle

The present study was undertaken to reevaluate the effects of preload on maximum velocity of shortening at zero load, V max , relative to the length-tension curve. Force-velocity relations were measured from afterloaded isotonic contractions and were calculated from isometric contractions of isolated cat papillary muscles. Results were interpreted in the light of three alternative mechanical models of muscle. V max was obtained by mathematical extrapolation of each force-velocity relation to zero load using a hyperbolic least-squares analysis performed on an IBM 360 computer. With the application of all three muscle models to isotonic force-velocity relations, V max was relatively constant at low preloads but was reduced substantially as muscle length approached L max (the length at the peak of the active length-tension curve). In force-velocity relations from isometric contractions, similar results were obtained with the two-element and Voigt models of muscle. With the Maxwell model, V max remained more nearly constant near L max . Peak developed force (isometric contraction), maximum dP/dt, peak calculated velocity of the contractile element (V CE ), and V max were compared in terms of their dependence on preload and length over the entire length-tension curve (using the Maxwell model). Peak V CE and V max were similar and were less dependent on preload than maximum dP/dt or developed force.