One in four adults in the U.S. has flat feet or fallen arches. Some people are born with flat feet, while others acquire it as they get older. The foot may be flat all the time or it may lose its arch when the person stands (“flexible flatfoot”). Many people with flat feet don’t experience any symptoms. Others, however, suffer from heel or ankle pain, tired feet, bunions, arthritis in the foot or ankle, foot or ankle deformity, knee or back pain, or other problems that need professional treatment.