Why do runners get stress fractures?
According to a 1996 review in Sports Medicine, stress fractures result from repetitive, cyclic loading of bone which overwhelms the reparative ability of the skeletal system. Stress fractures can occur from either increasing the load on the bone (e.g. adding speedwork, switching from trail running to sidewalks, or wearing less supportive shoes), or from increasing the number of times the bone is stressed (i.e. increasing your mileage). Running causes more stress fractures than any other sport because we tend to run on hard surfaces and land with forces of 4 to 6 times bodyweight. The impact forces of running make our bones stronger, however, our bones can only adapt at a gradual rate. That is why you may get a stress fracture from increasing your mileage too quickly from 20 to 40 miles per week, but a few years later may handle 60 miles per week with no problems.
