During initial contact or heel strike, the ankle rolls inward, causing excessive pronation or flattening of the foot. This action forces the big toe (instead of the ball of foot and all toes) to do all the work to push off the ground. When the ankle overpronates, this causes the tibia (shin bone) to rotate and the femur (thigh bone) to adduct (move toward the mid-line of the body), causing internal rotation or knee valgus (knock-kneed).
There is a relationship between biomechanics and injury that is specific to each body part. Overall though, poor mechanics will either increase the landing forces acting on the body or increase the work to be done by the muscles. Both increase the stress, which, depending on the individual and the amount of running can become excessive and cause injury.
Common conditions that develop with prolonged overpronation typically include plantar fasciitis, achilles tendonitis, shin splints, posterior tibial stress syndrome and even IT band syndrome. With long term neglect you may see the development of bunyons, foot deformities and early onset of hip and knee arthritis.
If you cannot afford to get a proper gait analysis completed, having someone observe you on a treadmill from behind will give you an idea if you are an overpronator. It is possible to tell without observing directly whether you are likely to be an overpronator by looking at your foot arches. Check your foot arch height by standing in water and then on a wet floor or piece of paper which will show your footprint. If your footprints show little to no narrowing in the middle, then you have flat feet or fallen arches. This makes it highly likely that you will overpronate to some degree when running. If you have low or fallen arches, you should get your gait checked to see how much you overpronate, and whether you need to take steps to reduce the level to which you overpronate. Another good test is to have a look at the wear pattern on an old pair of trainers. Overpronators will wear out the outside of the heel and the inside of the toe more quickly than other parts of the shoe. If the wear is quite even, you are likely to have a neutral running gait. Wear primarily down the outside edge means that you are a supinator. When you replace your running shoes you may benefit from shoes for overpronation. Motion control or stability running shoes are usually the best bet to deal with overpronation.
Non Surgical Treatment
Fortunately, there are simple things you can do to cure and correct your overpronation issues. Certain exercises help. Pull your toes back using a rolled up towel. Roll your feet over a golf or tennis ball for a minute. And do calf raises by standing up and lifting up on your toes. These all help reposition the foot and strengthen the muscles and tendons necessary for proper support. Beyond that, simple adjustments to footwear will help immensely.
With every step we take, we place at least half of our body weight on each foot (as we walk faster, or run, we can exert more than twice our body weight on each foot). As this amount of weight is applied to each foot there is a significant shock passed on to our body. Custom-made orthotics will absorb some of this shock, helping to protect our feet, ankles, knees, hips, and lower back.