Almost every runner pronates to some degree. This is normal, but when a runner overpronates sometimes it can cause problems. There's a really simple test you can do to determine if you over-pronate. All you need is a brown paper grocery bag, a cotton ball, and some cooking oil. Lay the bag flat on the floor. Using a cotton ball, spread a thin layer of cooking oil on the bottom of both feet (bare). Next, carefully step onto the bag to make a set of footprints. Now examine your prints and compare them to the illustration. If you have a solid print, you're what is known as an overpronator. Ever heard of the term "flat-foot?" That's you. If you have slight curve in the middle of your print, you're neutral. If you have a very significant curve in the middle of your prints, you're a underpronator or supinator.
Now having said all of that, not everyone fits perfectly into a category. For example, I have running friends who have flat feet but have a neutral gate and I have running buddies who have high arches that overpronate. Having a gate analysis done where someone observes your actual running gate is the best method for determining your specific gate. Many local running stores as well as sports medicine doctors can examine your gate and help you determine the best running shoe for you.
If you determine that you do overpronate, make sure you use shoes that offer more anti-pronation features. The key word to look for in the shoe description is "stability." If you've tried stability shoes and still have problems, you may want to consult your sports doc and see if orthodics are in order. The sports doc can also help rule out other knee problems that might be causing the pain.
Another possibility for your knee pain could be leg-length discrepancy (one leg shorter than the other). My good friend Gary has been a runner for many years. But a year or so ago, he started having problems with his knees, especially when he upped his mileage. It got so bad that he had to stop running for a while. He went to his sports doc and discovered he actually had one leg longer than the other.
I did a little research and come-to-find-out leg-length difference is often a factor in knee pain. The reason it's a problem is because your body will try to compensate for the discrepancy. For example, your body may try to flatten the foot, drop the hip, or bend the knee to try and make up for the difference in length—all of which adds pressure on the knee.
Putting an orthodic such as a heel raise in the shoe of the shorter leg may be the answer, but you need to have a sports doc determine the cause of the discrepancy first. The difference in length could be something the person was born with and never really noticed until (like my friend) the mileage was upped and the pressure on the knee got to be too much, resulting in pain. Or, the pain could be due to inflexible hips or pelvic rotation, which is often the reason for leg length differences. If inflexible hips or pelvic rotation is the culprit, the doc may prescribe a series of stretches and/or exercises that may completely correct or dramatically reduce the leg-length difference. When this is done, the over compensation should end and your knee pain should go away.Now, a good friend of mine—Josh-the Barefoot Runner—will tell you that shoes in general are the culprit. Shoes do tend to make heel-strikers out of many runners. Heel-striking can jar the body causing lots of problems. You can learn to run with a mid-foot or fore-foot strike to alleviate this problem, but it is a little harder to do in shoes. I'm not a heel-striker and I wear shoes. But, if you're interested, check out my post on Josh and his barefoot running. Many barefoot runners will tell you that a whole host of ailments disappeared when they began barefoot running. Josh had several bouts with ITBS and it completely disappeared once he began barefoot running. So, it's definitely worth checking out and you may find that it's just the answer you've been looking for.
Bottom line. Having knee pain doesn't have to mean the end to your running. Take some time and find the root of your problem. Chances are you may be able to correct it and be back on the road in no time!