Flat feet (pes planus) is a fairly common condition in which the foot does not have a normal arch, and so the entire foot touches the floor when you’re standing. According to the 2012 National Foot Health Assessment conducted by the NPD Group for the Institute for Preventive Foot Health, 8 percent of U.S. adults ages 21 and older (about 18 million people) have flat feet. Another 4 percent, or about 8 million U.S. adults, have fallen arches.
Flat feet can occur as normal arches “fall” over time. Years of walking, running and standing can weaken the posterior tibial tendon, which runs along the inside of the ankle from just above the joint to the arch. This tendon is the main support for the arch; overloading the tendon can cause inflammation (tendinitis) and tearing. Once the tendon is damaged, the arch loses support and can flatten, resulting in “fallen arches.”
Flat feet and fallen arches are similar and closely related to overpronation, which is an excessive inward rolling of the foot during walking and/or running. Flat feet are normal in infants and toddlers because their arches have not yet developed. In most people, the arches develop throughout childhood, but some people never develop arches.
The main way to determine if a person truly has flat feet (as opposed to simply being an overpronator) is to observe the arches in both seated (non weight-bearing) and standing (weight-bearing) positions. In the person with flat feet, no arch will be observable in either the seated or the standing position. In a person who overpronates, the arch will be at least slightly visible in the seated or non weight-bearing position, even though it may not be observable while he or she is standing.