Stress fractures occur when there are tiny cracks in a bone. These cracks are usually caused by repetitive force, often from activities like jumping or running long distances. Stress fractures can also happen when a bone is weakened by conditions like osteoporosis. The weight-bearing bones in the lower leg and foot are the most susceptible to stress fractures. Athletes in track and field, as well as military recruits who carry heavy packs over long distances, are at a higher risk. However, anyone can experience a stress fracture, even if they’re starting a new exercise program and push themselves too hard too quickly.