Starling's law of the heart is explained by an intimate interaction of muscle length and myofilament calcium activation.

The results of several different types of investigations over the last decade clearly indicate that muscle length modulates the extent of myofilament calcium ion (Ca2+) activation. Similarly, the fiber length during a contraction, which is determined in part by the load encountered during shortening, also determines the extent of myofilament Ca2+ activation. Thus, "contractile" or "inotropic" state as it refers to the extent of myofilament activation can, in theory, no longer be considered independent of the muscle length, as was formerly thought to be the case. Accordingly, terms such as preload, afterload and myocardial contractile state as they pertain to cardiac muscle properties lose part of their significance in light of current knowledge.