Pronation describes the inward rolling motion of your foot after it lands on the ground. There is an optimal degree (15º) of pronation, but many people either underpronate or overpronate. No type of pronation is particularly good or bad in itself, it just means you have different requirements when looking for a shoe. Once you are aware of your pronation pattern you can determine much better what you need in a running shoe.
Pronation occurs during the Initial Contact phase of the gait cycle. Initial Contact is the moment when your foot first makes contact with the ground. The movement of your foot landing and pushing off again happens roughly like this: first the heel makes contact with the ground, then the foot rolls a little inwards (pronation!), until the ball of the foot provides full support to the foot and then follows through with a push off from the big toe.
Neutral pronation is when the foot rolls about 15º inward. There is a relatively even distribution of load on the foot, followed by a forward motion. In this case the feet and legs have an efficient forward motion, little energy is lost with laterial movement which can be seen over and underpronators.
You are likely to be a neutral runner if your running shoes show even wear across the heel and under the ball of the foot. Neutral runners also tend to have normal arches. (A Wet Test is a simple way to find out what kind of arches you have – see the “know your arches” article.)
Neutral runners can run in a wide variety of shoes, but specialized neutral running shoes offering cushioning and support are most suitable. However other factors such as weight also influence shoe choice. If you are a normal pronator, but on the heavy side, you may need a structured cushioning shoe.
Underpronation (or Supination) is when the foot doesn't roll inward enough (less than 15º). The outer or lateral side of the heel hits the ground, the weight is then transferred to the outer edge of the foot and push off comes from the toes on the outer side of the foot rather than from the inner.
People with high arches tend to underpronate. This is likely the case when your running shoes mostly show wear on the outer heel and outer edges of the sole. Underpronation means certain parts of the foot are under extra pressure and if this goes unsupported, there is a risk of injury.
Underpronators should choose a neutral running shoe with plenty of cushioning. The extra cushioning will lessen the impact of landing the legs have to endure when running.
Overpronation is when the foot rolls too far inward (more than 15º). In this case much weight is transferred to the inner or medial side of the foot and as the runner moves forward, the load is borne more by the inner edge of the foot rather than the ball. Lacking a stable base, the big toe must make more effort to push off effectively.
People with low or flat arches are more likely to overpronate. The shoes of an overpronator will show extra wear on the inside of the heel and the inner edge of the forefoot.
Overpronators should consider choosing maximum support or structured cushioning shoes. Structured cushioning shoes provide a degree of stability and cushioning, whereas maximum support shoes are the most stable shoes you can get. Both these shoe categories will help your feet distribute the impact of running more effectively.