The diffusion process in the myocardium is difficult to investigate because of the unqualified sensitivity of diffusion measurements to cardiac motion. We introduced a mathematical formalism to quantify the effect of tissue motion on the diffusion NMR signal. The presented model is based on the Bloch-Torrey equations and takes into account the cardiac deformation according to the laws of continuum mechanics. Approximating this model by using a finite element method, numerical simulations can predict the sensitivity of the signal to cardiac motion under the influence of different preparation schemes. Our model identified the existence of two time points of the cardiac cycle, called "sweet spots", on which the diffusion is unaffected by the cardiac deformation. This study also demonstrates that the sweet spots depend on the type of diffusion encoding schem.