Stress fractures are tiny cracks in a bone. They're caused by repetitive force, often from overuse — such as repeatedly jumping up and down or running long distances. Stress fractures can also develop from normal use of a bone that's weakened by a condition such as osteoporosis. Stress fractures are most common in the weight-bearing bones of the lower leg and foot. Track and field athletes and military recruits who carry heavy packs over long distances are at highest risk, but anyone can sustain a stress fracture. If you start a new exercise program, for example, you might develop stress fractures if you do too much too soon.