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Dual camera stereo photogrammetry was used to quantify the deformation of the central region of the anterior leaflet: of the mitral valve operating in a flow loop under physiological and pathological conditions. Stretches and strains were measured and analyzed. The principal stretches demonstrated a rapid rise early in leaflet closure, followed by a plateau, suggesting that the collagen fibers "locked" in place. There is a big difference between the major principal stretch and minor principal stretch. Similar to in-vitro tissue mechanics studies, the transmitral pressure-areal strain relationship during loading and unloading clearly demonstrated hysteresis. Further, this relationship demonstrated a dramatic stiffening attributed to collagen fiber locking. When the data of 3 papillary muscle positions were compared, the leaflet in the slack papillary position showed the highest areal strain rate. The valve exhibited a strongly anisotropic behavior. Papillary muscle position had a small effect on strain measured in the region.