A stress fracture is a hairline crack in a bone. In some cases, the crack may be so small that it is not visible on an X-ray.

Stress fractures are usually caused by repeated stress on a bone rather than by a specific injury. They can occur in any bone that repeatedly bears weight. For instance, stress fractures in the small bones of the foot are common during intensive training for sports that involve lots of running or jumping. A stress fracture is also more likely to occur in a bone that is not accustomed to or conditioned for a particular activity, such as when a person starts a new sport.

The most common symptom of a stress fracture is persistent pain at the site of the fracture. Pain may improve temporarily during exercise but gets increasingly worse after each exercise session.

If the activity that caused the stress fracture is stopped, the bone will heal and symptoms will go away. The activity can then be resumed gradually until the bone becomes conditioned to the repeated movements.