Pes planus may be congenital, or acquired. Acquired causes include:
People with flat feet have very sore, tired feet after a day of standing. They may also have related pain in the knee, hip, and low back. When a foot becomes flat, the leg often turns, which increases stress on the inside part of the knee. That causes poor positioning of the hips and swaying of the back, leading to pain in these areas.
People with flat feet often develop plantar fasciitis. This is an inflammation of fibrous tissue under the skin of the sole. People with flat feet are also more likely to have bunions and hammertoes.