Passive overstretch of myocardium occurs in the ischemic and dilated heart, but the subsequent changes in local tissue mechanics and structure remain unclear. Therefore, we estimated changes in myocardial biaxial stiffness using by solving an inverse finite element problem. The differences between the strains measured on the epicardium of the isolated rat left ventricle (LV) during overstretch and those of a prolate-spheroidal finite element model were minimized by optimizing the material parameters of an exponential constitutive law. When passive LV pressure was increased from 10 to 120 mmHg, there were progressive decreases in wall stiffness up to 93% that were equal in fiber and cross-fiber directions. Thus, myocardium softens isotropically with progressive overstretch independently of muscle fiber direction. This local strain softening may, therefore, arise from damage to structures with uniform distribution of orientation rather than components aligned with myofibrils.

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