A stress fracture is an overuse injury. When muscles are overtired, they are no longer able to lessen the shock of repeated impacts. When this happens, the muscles transfer the stress to the bones. This can create small cracks or fractures. The most common sites of stress fractures are the second and third metatarsals of the foot. Stress fractures are also common in the heel (calcaneus), the outer bone of the lower leg (fibula), and the navicular, a bone on the top of the midfoot.