A foot orthosis is used to align and support the foot; to prevent, correct, or accommodate foot deformities; and to improve overall function of the foot.
During walking, the normal foot changes from a supple, shock-absorbing structure to a rigid lever for push off.
A flat foot that is supple and does not invert and become rigid does not effectively form a rigid lever for push off.
Just the opposite occurs with a cavus, or high-arched, foot. A cavus foot is rigid and lacks shock absorption. During walking, the high-arched foot remains locked and fails to become supple for a comfortable stance.