Force-velocity relation and contractility in striated muscles

For skeletal muscle, the physiological meaning of Hill's hyperbolic force-velocity equation and the factors affecting it, such as the active state, method of velocity measurement and mode of stimulation, have been discussed. After the development of the sliding filament theory, Hill's equation was generalized to all partially activated isometric tensions and the meaning of Hill's dynamic constants was interpreted from the kinetic analysis of the cross-bridge cycle. In cardiac muscle, determination of the precise force-velocity relation was almost impossible, but most difficulties were overcome by tetanizing the cardiac muscle. As a result, the force-velocity properties of cardiac muscle were confirmed to be very similar to those of skeletal muscle. The maximum shortening velocity under no load, v0, was once used as an index of myocardial contractility which is insensitive to muscle length, but it is now believed that at least at shorter lengths, v0 may vary with muscle length and degree of activation. As new approaches, studies on sarcomere dynamics by the laser diffraction method, observation of calcium transient and pressure-velocity measurement in whole ventricle have been introduced.