Pronation is the normal movement of the foot, where the arch flattens slightly to assist with absorbing and dispersing shock during the gait cycle. It also allows the foot to adjust to changes in terrain it encounters (such as when trail running). Everyone pronates! The question is how you pronate. In order to properly assess your pronation tendency, it is necessary to observe the foot in its natural motion, whether running or walking.
Pronation tendency is related to a number of factors including arch type, ankle flexibility, foot structure and rate of pronation.
Occurs when the foot rolls inward to a very slight degree as it absorbs impact, resulting in a stable and efficient gait.
Excessive Pronation (over pronation)
Occurs when the ankle or foot collapses inward (in varying degrees) too much or too quickly and/or the arch flattens too much. The resulting unstable position places added stress on the medial (inside) portion of the foot during the mid-stance and/or toe-off phases of the gait cycle.
Inadequate Pronation (under pronation)
Occurs when the arch does not flatten enough, causing a runner or walker to place added stress on the lateral (outside) portion of the foot during the midstance and/or toe-off phases of the gait cycle.
Both over and under pronation create an inefficient gait and place the foot in an unstable position. This can cause injury due to the body overcompensating for the added stress on the foot. The degree of stability or instability that is demonstrated during an individual's gait cycle determines the degree of stability that is required from a particular shoe.
Instability at either Midstance or Toe Off is not only inefficient, but can also lead to injury, as the body may overcompensate for the lack of stability.