In a previous post on running shoe selection, I briefly went over foot mechanics. However, it's come to my attention that a number of people don't truly understand the concept, and it's no wonder given the wealth of bad and/or oversimplified information out there (Runner's World, I'm looking at you). People aren't stupid. They don't want the watered down version that says to do a wet footprint test and buy your shoes based on that, because 90% of the time, the oversimplified version doesn't even work. They want to be able to make an informed decision and buy the right shoes. For example, high arches and a lateral wear pattern (wear on the outside of the shoe) doesn't necessarily equate to supination. High arches can contribute to supination, but they are not the only factor. And lateral wear pattern can be caused by supination...or it can be caused by forcing yourself to run unnaturally on the outside of your feet to minimize pain caused by wearing the wrong shoes (a forced and unnatural gait that will almost certainly result in injury).
Neutral pronation is the "ideal" foot motion, and is marked by the foot pronating at the right time. Pronation is your foot's natural cushioning system. Therefore, neutral pronation is the ideal combination of your foot being able to simultaneously cushion and stabilize itself, that is, it pronates normally. Neutral pronators don't need a lot of extra cushioning or stability in their shoes, though a little of each, as is often found in neutral (e.g. Brooks Ghost) and mild-stability (e.g. Brooks Ravenna) trainers, can be nice for high volume running.
Overpronation is a deformity caused by too much motion in the foot, that is, too much inward roll. (Deformity just meaning "not normal"...though since the majority of people overpronate to some extent, I'm not sure where the osteokinematic definition of normal came from). The foot still strikes on the outside of the foot in a supinated position, but then rolls into a pronated position too early. With overpronation, the foot is cushioning itself too much, which causes a lack of stability. The pronated position is called an open pack position, meaning that there's less joint congruity and the joint isn't stable. Imagine your knee collapsing whenever you took a step...you need to somehow make that knee more rigid, since it's providing too much suspension. Same with your foot. Overpronators don't need much extra cushioning in their shoes, since their feet are already providing a ton of cushioning. Instead, they need the stability in their shoes that their feet do not naturally provide. Depending on the level of overpronation (how much cushioning vs. stability the foot already provides), overpronators will be happiest in mild stability (e.g. Brooks Ravenna), moderate stability (e.g. Brooks Adrenaline), or motion control (e.g. Brooks Beast/Ariel) shoes. Overpronation, in and of itself, isn't bad, and the right pair of shoes can allow an overpronator to run just as efficiently as a neutral pronator.
Supination is a deformity caused by too much stability in the foot, that is, not enough inward roll. With supination, the foot isn't able to cushion itself. The foot strikes on the outside of the foot in a supinated position, and then never rolls over into a pronated position. The supinated position is called a close pack position, meaning there's more joint congruity, the ligaments are pulled taut, and the joint is rigid. Imagine stepping off a stair and landing with your knee straight...you need to add cushioning since your joint isn't able to do it for you. Supinators don't need extra stability in their shoes, since their feet are stable enough. However, they need extra cushioning, since their feet aren't able to do that on their own. Supinators will be best matched with a shoe with maximum cushioning (e.g. Brooks Glycerin). Like overpronation, supination isn't bad, and can be corrected with the right pair of shoes that will allow the supinator to run efficiently and without injury.
With all the confusion about foot types, so many people have no idea what kind of shoes they need. For example, I often hear people saying that they need "cushioning for their knees." This is really common in older runners and beginner runners. Sometimes they do...but more often than not, knee problems are caused by a lack of stability and too much cushioning, which causes extra torque on the knee when the foot rolls inward too soon. And "cushioned shoes" are usually labelled "cushioned" because they lack the extra stability, not because they have a ton of extra cushioning, since stability and motion control shoes are also appropriately cushioned (though something like the Ravenna won't be as soft as the Glycerin, because someone who needs the Ravenna will have feet that provide more cushioning than someone who needs the Glycerin).
So why can foot motion not be determined by arch height? As I stated in my last post, arch height is a contributing factor, since it's more common for someone with the pes cavus deformity (high arches) to supinate than someone with normal arches, and it's more common for someone with the pes planus deformity (low arches) to overpronate than someone with normal arches. However, it's still possible for a high arched person to have their arches collapse to the point of overpronation, and for someone with low arches to have rigid feet and supinate. I'll leave you with an example. Two of my friends have very high arches. However, when my first friend is weight bearing, her arches collapse, and when walking or running, she is a mild to moderate overpronator, and shoes without enough stability give her knee problems. My second friend, on the other hand, has rigid feet and underpronates to the point she can't run barefoot (you can hear her feet slapping a mile away), and shoes with too little cushioning cause her peroneal pain. Moral of the story is that the best way to determine what foot type you have is to get a gait analysis done by a professional. However, if that's impossible for some reason, look at all of the factors that contribute to foot motion, not just arch height or wear pattern.