Multiscale Characterization of Left Ventricle Active Behavior in the Mouse

This study presents a comprehensive multiscale characterization of myocardial contractile behavior in the murine left ventricle, spanning fiber-, tissue-, and organ-level mechanics. While papillary muscle fibers exhibited rapid, high-magnitude active stress generation, left ventricular free wall (LVFW) tissue showed markedly slower and lower-magnitude active contraction, highlighting the role of extracellular matrix content and fiber architecture in force transmission. Biaxial tissue testing revealed anisotropic active stress generation, with greater longitudinal than circumferential contractility, and demonstrated that active stress magnitude and relaxation behavior depend strongly on initial passive stretch. Passive viscoelastic relaxation differed fundamentally from active relaxation, underscoring distinct mechanisms governing myocardial contraction and relaxation. These findings establish benchmark experimental assays for multiscale cardiac contractility and provide critical insight into how myocardial mechanics translate from fiber to organ-level function.