Maximum Velocity as an Index of Contractility in Cardiac Muscle

The use of maximum velocity of shortening (V max ) as an index of contractility has been based on the assumption that V max of muscle fibers is equivalent to V max of the contractile element. It is shown here that such equivalence applies to the two-element model of skeletal muscle but not to three-element models of cardiac muscle. Analysis of published data in terms of the Voigt and Maxwell three-element models shows that the V max of the contractile element, unlike that of the muscle fibers, is not independent of fiber length but increases at least 50% for a 25% increase of fiber length. Moreover, V max of the contractile element is seen to be even more highly dependent on fiber length when correction is made for the nonuniform contribution of levels of active state to the force-velocity curves giving rise to V max of the contractile element. It appears that in cardiac muscle an inotropic shift of V max of the contractile element cannot be distinguished from a shift due to change in fiber length, thus invalidating it as an index of contractility.