Stress fractures are hairline fractures that occur most often in the foot and less commonly in the lower leg. They can occur elsewhere in the body as well, but this is more rare. They're usually the result of overuse - most typically from running. Normally, the connective tissues (ligaments, tendons, muscle) of the lower leg and foot absorb the impact of activity. However, once their capacity to absorb these stresses is exceeded, the bones become more responsible for absorbing and distributing the forces of impact. When the bones are overstressed and inadequate time for recovery occurs, the bone will develop a stress fracture.