The instep is the top portion of the foot between the toes and ankles. It forms the large arch on the sole of the foot. Its shape is formed by the tibialis anterior from the top and various ligaments in the sole of the foot from the bottom.
The tibialis anterior runs down the shin and into the top portion of the foot, holding the apex of the instep upright. The ligaments in the sole keep that upright position in place. The entire instep is further held in place by the plantar ligament, which runs from the heel to the base of the toes and affects all the bones in between. The muscles and tendons in the sole of the foot lie flat, but when a person walks they are brought up into the space formed by the instep and are able to act as springs.