Cardiac Muscle Models

Quick-stretch and quick-release experiments were performed on right ventricular cat papillary muscles to test the applicability of the Hill model to cardiac mechanics. Series elastic component (SEC) force-length curves were calculated from stretches and releases carried out at various times during the contractile cycle. At any SEC force, the SEC elastic modulus depended on the time during the contractile cycle at which it was measured. When measured at the same time and at the same SEC force, elastic moduli obtained by releases of less than 1% of muscle length differed from those obtained by corresponding stretches. Larger stretches, in fact, appeared to yield negative elastic moduli. Thus, a unique SEC modulus could not be identified at any level of SEC force. It is concluded that the concept of the SEC as a passive elasticity appears unsatisfactory and, as a consequence, that the quantitative validity of the Hill model for cardiac muscle is questionable. Moreover, since an anatomical counterpart of the SEC has not been identified, the Hill model also appears unsatisfactory from a structural point of view.