This article presents the results of numerical simulations and physiological experiments on the dependence of intramyocardial slow force response on the value of afterload in heterogeneous myocardium. We used muscle duplexes as the simplest experimental and theoretical model of heterogeneous myocardium. It is shown that a degree of slow force response increases with the level of afterload and the time of stimulation delay between the elements of the duplex. According to the conducted research, this is due to an increase in the time of the mechanical interaction between the elements of the duplex in the isometric phase of the contraction. This leads to the change in individual contractility, configuration of action potential as well as characteristics of the kinetics of intracellular calcium in cardiomycetes of interacting muscles.