Characterization of myocardial stress-length relations.

For several decades, some muscle physiologists, cardiac physiologists and cardiologists have normalized systolic load-dimension relations to a positive-strain reference dimension such as the muscle length which is optimal for stress development or the chamber volume resulting in normal end-diastolic stress. Some have viewed isometric stress development at reference length as a contractility index, and some have viewed shortening from reference length in the absence of afterload as an important property (shortening ability or mobility). These intuitive choices together constitute a coherent elastic theory which is more appropriate for activated muscle than is the classical theory, wherein slack length is the reference and slope of the stress-strain relation is the main parameter. The stress-length relation is readily modeled by functions in which contractility and shortening ability are parameters, entirely separate from each other and from those representing shape of the load-dimension relation.