Fitness Q&A by Richard Weil
Osteoporosis is a disease characterized by loss of bone that increases the risk of fracture, particularly in the wrist, spine, and hip. More than 10 million adults in America have osteoporosis, and 34 million more are estimated to have low bone mass, putting them at increased risk for the disease. Eight million of the 10 million people who have osteoporosis are women. Exercise can have a positive effect on bone mineral density and the strength of bone, but it is a complicated relationship because of many factors including genetics, age, gender, hormone and menopause status, activity level, and nutrient deficiencies. Age applies to everyone when it comes to the effect of exercise on building bone strength, and so I will review some of the age-related differences and then let you know what the guidelines are.
In youth, the time of peak bone mineral content velocity (the time around puberty when bone rapidly forms) is an important time for children and early teens to be exercising because:
2. Research shows that active youth have higher bone density than inactive children
3. Research suggests that denser bone formed during childhood may track into adulthood. That is, if children and teens exercise enough to create denser bones than their sedentary peers, then they may have thicker bones as adults.
The effect of exercise on bone density is most evident in athletic kids who participate in sports that cause the highest levels of ground reaction or impact forces like gymnastics and weight lifting, and less pronounced in low impact sports like swimming. Weight lifting is interesting because it's not always the ground reaction force that creates the increase in bone density, but simply the force applied where the muscle attaches to the bone and pulls on it during the lift. For instance, if you do lots of lifting with your biceps, which does not involve impact on the ground, the bone density in your arm may show some increase. An example of this is a professional tennis player's dominant arm (the arm they play with). Typically the dominant arm has bones that are larger than the non-dominant one because the athlete swings a racket with it all the time and so there is a training effect.
As for young adults, it is generally believed that peak bone density is reached by the time we reach our late 20s or early 30s, and research suggests that athletes in this age group have higher bone mineral density than sedentary individuals. Importantly, the effect of exercise on bone density is site-specific. That is, the exercise effect on bone density appears primarily in the bone that is involved in the activity. For example, if you do jumping exercises, then bones in your legs will show the greatest effect, whereas bones in your arms will show no effect. Bones in the spine will also show some effect with jumping, but perhaps not as much as the bones in the leg and hips that absorb the majority of the impact.
As for middle-aged and older adults, bone density decreases by about 5% per decade after the age of 40. Some scientists believe that during these times of life it may not be realistic to expect substantial increases in bone density, but rather we should expect a reduction, or attenuation, of the loss of bone that we experience as we age. These scientists may be right because there is little evidence that walking, weight lifting, or any other type of exercise, substantially increases bone density in older adults. In some studies, there is a trend or suggestion of more density, but the results seem to be small and clinically insignificant. What you can say with some certainty is that active individuals in these age groups do maintain their bone density while their sedentary peers do not. And that's a good thing.