Muscles work together in balance. They work like a pulley system. Imagine a rope looped around a pulley, with equal lengths. If there is an equal pull on each side the pulley will not move. If the pulley is pulled on only one side, then the pulley rotates to that side.
Muscles work on the body in a similar fashion.
They move the foot up and down. If the muscle that pulls the foot up is lost due to nerve damage, then the healthy or stronger muscle on the opposing side over powers the weak muscle and pulls the foot downward. This principal holds true for the arch of the foot. The Type I foot has a high arch, because the muscle that normally helps maintain an arch is unopposed, thus raises the arch further. This muscle is called the Posterior Tibialis.