When the foot rolls inward and the natural arch flattens out, it's called pronation. If your foot pronates too much and it's noticeable, you may be called by the more common term of flatfooted. Mild pronation usually isn't a problem. When bone on the inside edge of the arch (the navicular bone) drops 10 mm or more, it's considered severe pronation.

Severe pronation can lead to changes in the foot. The foot can become unstable. Ligaments and muscles can get stretched out. Painful conditions of the lower leg can occur. Most cases of flat foot don't cause any symptoms. More people seek help for how it looks than for how it feels.