Pronation is the term used for a normal movement of the foot as it loads to the floor and adapts to the terrain the foot is loading onto.

When the foot pronates there is an elongation and flattening of the arch. Excessive pronation can occur because the foot is compensating for another underlying problem (imagine if the wheels of a car always drove on an adverse camber, they initially would be able to compensate but before too long some mechanical failure would occur).

Excessive pronation of the foot and the detrimental forces to the foot can be reduced using orthoses (insoles). They can be relatively simple in design or prescriptive to the individual and his/her needs.