Despite the numerous available clinical investigation tests, the associated alteration of quality of life and the socioeconomic cost, it remains difficult for physicians to identify the pathophysiological origins of defecation disorders and therefore to provide the appropriate clinical care. Based on standardized dynamic X-ray defecography, we developed a 2D patient-specific computational fluid dynamic model of rectal evacuation. X-ray defecography was carried out in a sitting position with a standardized paste whose yield stress matched that of soft human feces. The flow was simulated with lattice-Boltzmann methods for yield stress fluids and moving boundary conditions. The model was applied for a patient with a normal rectoanal function. We deduced from the flow field that the main flow resistance during the defecation was due to the extrusion of the paste through the anal canal. We calculated also from pressure and stress fields the spatio-temporal evolution of the wall normal stress. This latter highlighted a gradient from the proximal to the distal part of the rectum. We discussed how this new set of hydrodynamical and biomechanical parameters could be interpreted to gain new insights on the physiology of defecation and to diagnose underlying evacuation disorders. Clinical relevance-If confirmed, our approach should allow clinicians to obtain other parameters from a classic clinical examination and thus better adapt the response of clinicians to the defecation disorders observed in patients.