A stress fracture is a small break in a bone caused not by trauma, but by the repeated application of forces during exercise. The forces could come from running impacts, or from muscles and tendons pulling hard on the bone. Left untreated, the bone can break completely.
Sports doctors have found that bone shape, bone density, muscle condition and shoe condition can all play a role in stress fracture risk. Beginning athletes bear the most risk, and female athletes are at higher risk than men. The tibia is the most common runner's stress fracture [source: Lovett].
It can be difficult to avoid a stress fracture since they're rare and can be caused by many things. The most important thing to do is stop running as soon as you suspect a stress fracture; runners describe it as a deep pain that worsens throughout a run, usually at a very tender place on the bone. You should end your run immediately, take a day off of training and try different shoes -- you don't want a minor stress fracture to get worse.