The second level includes the passive mechanical properties of myocardium as well as coupling a 3D view of active force generation into mechanical tissue analysis. In contrast to the situation for electrophysiological models of a cardiac cell, fundamental aspects of how sarcomere length affects the biophysical process of force generation remain unclear. Today, even 100 years after its first description by Otto Frank, the molecular basis of the Frank-Starling law remains undecided. These uncertainties impair our ability to model with full confidence. Recruitment implies that a length change alters the number of attached myosin crossbridges ac ting in parallel within a half-sarcomere.
Distortion implies an altered internal stretch within each attached crossbridge.
Recruitment provides the molecular basis of Starling's Law. However, the mechanism by which a sarcomere senses its altered length is not completely certain, nor is the mechanism of recrui tment. If narrower radial spacing increases the equilibrium for attachment of myosin to actin, then the pCa50 of the force-pCa relationship will shift so that the sensitivity to calcium ions becomes greater. Coo perativity is an essential feature of the recruitment response to sarcomere length. As suggested above, cooperative interaction among neighboring tropomyosin molecules along the thick filament may be the essential ingredient providing the hypothesized mechanism of Starling's Law.