Foot ulcers are a common complication of diabetes and are the consequence of trauma to the feet and a reduced ability to perceive pain in persons with diabetes. Ulcers appear internally when pressures applied on the foot create high internal strains below bony structures. It is therefore important to monitor tissue strains in persons with diabetes. We propose to use a biomechanical model of the foot coupled with a pressure sensor to estimate the strains within the foot and to determine if they can cause ulcer formation. Our biomechanical foot model is composed of a Finite Element mesh representing the soft tissues, separated into four Neo Hookean materials with different elasticity: plantar skin, non-plantar skin, fat and muscles. Rigid body models of the bones are integrated within the mesh to rigidify the foot. Thirty-three joints connect those bones around cylindrical or spherical pivots. Cables are included to represent the main ligaments in order to stabilize the foot. This model simulates a realistic behavior when the sole is subjected to pressures measured with a sensor during bipedal standing. Surface strains around 5 % are measured below the heel and metatarsal heads while internal strains are close to 70 %. This strain estimation, when coupled to a pressure sensor, could consequently be used in a patient alert system to prevent ulcer formation.