The instantaneous force-velocity relationship as an index of the contractile state in cardiac muscle.

The instantaneous force-velocity curve of mammalian myocardium has a hyperbolic shape like that of skeletal muscle. Similar to the situation in single skeletal muscle fibres maximum speed of shortening and maximum contractile force in myocardium are altered in parallel as the state of activity is changed. Various inotropic interventions affect the force-velocity relation in the same way as occurs when the activity is altered during the course of the contraction. These findings support the view that activator-calcium governs both the number of active crossbridges between the A and I filaments during tension development and the rate of turnover of the myosin bridges during shortening at zero load.