Here's are some excerpts from a great article on stress fractures from the American Orthopedic Society of Sports Medicine.
Each day, the body produces new bone to replace the bone that is broken down by the stress of everyday living. Usually, this process is balanced, with the body replacing the amount of bone lost. However, this balance may become upset with excessive physical training. The body, due to several factors, may not produce sufficient bone. As a result, microcracks, called “stress fractures,” can occur in the bone.
Learn more about diagnosis, treatment, & prevention by downloading the complete publication.