Flat feet, or fallen arches, occur when the arch of the foot is lower than normal, causing the sole to make full contact with the ground. This condition (known as pronation) can result from various factors, including genetics, age-related wear-and-tear, or injury to the tendons or ligaments that support the arch. 

While some people with flat feet experience no symptoms, others have pain, swelling or weakness during prolonged standing or walking. Flat feet can also lead to altered body alignment, causing knee, hip, or back pain. 

Treatment depends on the severity of symptoms and may include supportive footwear, custom orthotics, or physical therapy.