Flat feet, also referred to as pes planus, is a foot deformity that is characterized by the loss of the medial longitudinal arch of the foot. Flat feet can cause problems with walking and running. The foot's arch helps to disperse the shock of impact as you walk or run. Loss of this arch can lead to pain in the feet, ankles, calves, and knees. Flat feet can also make it difficult to balance properly.