A stress fracture occurs when changes in physical activity produce a site-specific mechanical ''load." This load results in an acute imbalance between the rate of osteoclastic resorption and the rate of osteoblastic new bone formation. Bone metabolism and remodeling are regulated by a large number of modulators including hormones, growth factors, and cytokines that interact at the level of the osteoblast, osteoclast, and other cells to regulate bone remodeling. Systemic hormones that affect bone remodeling include parathyroid hormone, calcitonin, vitamin D, estrogen, progesterone, growthhormone, thyroid hormone, glucocorticoids, and androgens. Stress fractures are diagnosed using a combination of clinical symptoms and results from bone imaging studies.
Estimates of stress fracture rates in military women during basic training range from 1 to 20 percent, while rates in military men range from less than 1 to 9 percent. This variation likely reflects both true variation in rates and methodological differences between the studies. Rates in civilian female athletes show less variation and do not substantiate the theory that a true difference exists. Stress fractures are more common in military women than in men, with estimates in women being 1.2 to 11 times higher than in men in the various studies performed to date. The difference in findings between military trainees and civilian athletes may reflect differences in training, footwear, and initial fitness levels between these two groups. Female military trainees appear to be more likely to suffer stress fractures in the upper leg and pelvis than are males. These fractures are more costly in terms of rehabilitation and potential disability than those that occur in the lower leg. Female military trainees also appear to be more likely to enter training with low fitness levels, which increases their risk of incurring a stress fracture, presumably due to premature muscle fatigue.