Everyone's feet are not completely symmetrical. The instep height of the two feet is different, and the top of the instep will contact with the tongue of the shoe, which will cause discomfort, especially when exercising. It is also related to the long-established walking habits. When everyone walks, the center of gravity of their feet is different. Some people are used to tilting their feet to one side, and their tongues will shift accordingly, which makes them uncomfortable to walk.