When you run, your foot naturally rotates inward to help absorb impact as you hit the ground.  Problems occur when your foot rolls too far inward (known as overpronation), leading to shin splints or knee pain, or when your foot does not roll inward much at all (or underpronation), which can lead to stress fractures, heel and knee pain, and other injuries because the foot does not absorb impact well.

Your arch height is one of the factors contributing to this natural rotation of the foot.  Feet with a low arch tend to roll inward too much.  Feet with a high arch are more inflexible and do not roll inward much at all.  Feet with a normal arch rotate normally.

The best way to assess your running gait is to go through the SILICON COACH DYNAMIC SHOE FITTING SYSTEM.  This is a FREE service offered by Smiths Sports Shoes