It will bend and bend, but gets more brittle, and eventually cannot take another trauma, so it breaks. How much stress a bone can absorb before developing a stress fracture varies from person to person. People who dive into a running or walking program, rather than gradually work up to appropriate distances and time, are more at risk for stress fractures. Rapid increases in the intensity, duration, or frequency of training are prime risk factors. Regardless of conditioning, there comes a point when bone repair cannot keep up with repeated structural trauma.
Most stress fractures occur in the foot, ankle, leg, and hip.
Hard, unforgiving surfaces have been implicated in these injuries because they transfer more traumatic shock to the bones. Shoes that do not provide good cushioning similarly deliver more trauma. Protein and calcium are among the most important, although experts debate the preventive effects of calcium doses beyond 800-1,500 milligrams a day. Insufficient general caloric intake is also suspected as a contributing cause in young women gymnasts as they attempt to keep their weight low. Similarly, both women gymnasts and long-distance runners may experience cessation of their menstrual cycles, which is tied to lower estrogen levels and a decreased ability to form and maintain strong bones.