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Flatfoot deformity is characterized by loss of the medial longitudinal arch, forefoot abduction, hindfoot eversion, and often Achilles tendon contracture. Our objectives were to validate a cadaveric flatfoot model that involves selective ligament attenuation and to determine if Achilles tendon overpull is associated with increased pes planus severity. We measured the three-dimensional (3D) orientation of the bones of interest in the unloaded, loaded, and Achilles tendon overpull conditions. A flatfoot model was created by attenuating ligaments involved in the pes planus deformity followed by cyclic axial loading, and bone orientations were acquired in the three conditions. Significant differences seen between normal feet and flat feet were consistent with those seen with the pes planus deformity. The first metatarsal dorsiflexed and abducted relative to the talus. The navicular abducted relative to the talus. The calcaneus everted relative to the tibia. The talus plantar flexed and adducted. Achilles overpull resulted in first metatarsal-to-talus dorsiflexion and navicular-to-talus abduction. Thus, selective ligament attenuation followed by cyclic axial loading can create a cadaveric flatfoot model consistent with the in vivo deformity. Longitudinal arch depression, hindfoot eversion, talonavicular joint abduction, forefoot abduction, and talar plantar flexion were seen. Simulated Achilles tendon contracture increased the severity of the deformity, particularly in arch depression and forefoot abduction.