Flat feet can be distinguished from normal feet by appearance, function, and secondary changes. 1. Appearance and morphology: In flat feet, the arch is low and collapsed, which means that the medial side of the foot is low and flat, and the whole foot is widened and turned out. When standing, the inner side of the foot often touches the ground, while the inner side of a normal foot does not touch the ground. 2. Function: Flat feet, due to the low and collapsed arches, will have a decrease in the ability of the foot to cushion the impact, decrease in elasticity, and decrease in strength, and so on. It will lead to the lack of foot movement ability, such as standing and walking for a long time, and corresponding pain symptoms will appear. 3. Secondary changes: flat feet will lead to unbalanced weight-bearing of the foot and even the ankle joint. Therefore, it is more likely to cause secondary bone injury, strain, or even traumatic arthritis of the ankle joint, leading to ankle pain and dysfunction. When a patient suffers from flat feet, no treatment is needed when there are no obvious symptoms, and when the symptoms are more obvious, treatment is needed.