High-impact activities and weight training have the greatest impact on BMD. Gymnasts, who regularly experience high-impact loads upon landing, have a greater BMD compared to runners. If you’d rather weight train than somersault on a balance beam, you only need one set of a heavy weight to increase BMD, since research has shown that the magnitude of the stress on the bone is more important than the number of times you repeat the stress.
Exercising before you reach skeletal maturity is also important. The greatest impact on bone mass is achieved when a substantial exposure to mechanical stress occurs before puberty. As you age, the capacity of bone to respond to loading decreases.
As is the case with muscles, tendons, and ligaments, too much stress on bones can cause injuries. Weight-bearing sports like running are more likely to incur bone injuries, the most common of which is shin splints (medial tibial stress syndrome). As its medical name implies, pain is felt along the medial (inner) border of the tibia (shin bone) and feels like someone has kicked you repeatedly. Another common and more serious injury is a stress fracture which, contrary to the dull, poorly-localized pain of shin splints, is characterized by a sudden, sharp pain at a specific point on the bone.
Contrary to popular belief, women don’t have a greater risk for stress fractures than men do, as long as they don’t have one or more of three associated characteristics—menstrual irregularities, disordered eating, and osteoporosis—collectively called the female athlete triad. High training volumes can cause irregular or even absent menstrual cycles (amenorrhea), which increase the risk for osteoporosis and stress fractures since women with irregular menstruation or amenorrhea have lower BMD than women with normal menstruation. Osteoporosis, which literally means “porous bones,” is a reduction in BMD 2½ standard deviations below the average for healthy young adults at the age of peak bone mass. Disordered eating, common among female athletes due to external or self-imposed pressure to lose weight, may result in caloric restriction, and is independently associated with both irregular menstruation and low BMD. In other words, you could have normal menstruation and still have low BMD if your dietary habits are inadequate to meet your caloric needs. Many amenorrheic women have a diet low in calories and are therefore not meeting their needs for calcium and vitamin D. If you have any of the characteristics of the female athlete triad, you should regularly have your BMD assessed.
A decrease in estrogen level over a long duration is also associated with a decrease in BMD, which explains why a woman’s risk for osteoporosis and fractures increases dramatically with amenorrhea and after menopause, when there is a lack of estrogen. Indeed, estrogen deficiency caused by amenorrhea is the most significant risk factor for osteoporosis in active women. Many women do not absorb enough calcium due to lack of estrogen and low vitamin D intake, which limits the effectiveness of exercise on bone health.
