What role does exercise play in bone health over a person's lifespan?
As a personal trainer, you probably know that exercise has been shown to increase bone strength, as measured by bone mineral density (BMD), in people of all ages. But did you know that the degree to which exercise improves BMD depends on a variety of factors, including age, reproductive hormone status, nutritional status and the nature of the exercise?
The exact relationship between bone loading and gains in BMD is not completely understood. This article will highlight critical concepts covered in a review of the research on exercise for the improvement of BMD, primarily as it relates to women throughout their life-span. (To date little data exist for men due to the focus on osteoporosis in women.)
Article Reviewed: Beck, R.B., & Snow, C.M. 2003. Bone health across the lifespan—Exercising our options. Exercise and Sport Sciences Reviews, 31 (3), 117-22.
The following five training principles should be taken into account when designing an exercise program to increase bone mass (Drinkwater 1994):
1. Specificity. The exercise program should be designed to load specific target bone(s) or body sites. For example, if gains in bone mass in the hip and femur are desired, exercises that load those specific sites, such as running or jumping, are indicated.
2. Overload. To stimulate BMD gains in a particular bone, an exercise must overload that bone, and the load imposed must be substantially greater than that experienced during normal activities of daily living. In other words, a certain threshold of loading must be reached to produce a bone mass gain. More research is needed to identify the exact intensities necessary for optimal bone adaptation.
3. Reversibility. Any gains adults make in BMD during an exercise program will likely be lost if the program is stopped. On the other hand, various studies with children and teenagers (whose skeletons are still growing) suggest that at least some of the gains they achieve from increased mechanical loading during an exercise program may be retained even if they discontinue the program.
4. Initial Values. Subjects with below-average initial BMD values will show greater gains in bone mass with exercise than persons with average or above-average values.
5. Diminishing Returns. The greatest gains in BMD will be seen early in an exercise program. After the initial increase, gains will continue, but at a slower pace.
An understanding of these five principles is critical to designing an exercise program that will help your clients maximize bone mass gains in critical regions of the body. To prevent these gains from being lost, reinforce the importance of lifetime exercise, especially for your adult clients.
A properly designed exercise program will increase bone strength in a wide variety of populations. However, the specific circumstances of each individual—such as age, hormonal status and nutrition—also play an important role.
Adults. Numerous studies have demonstrated that adults who exercise have greater bone mass than those who are less active. The degree of increase in bone density depends on:
- the mode and intensity of exercise
- the number of years training
- the age at which training began
The principle of specificity (discussed above) is validated by research demonstrating that higher BMD has been found in athletes at the body sites loaded during their respective sports. Studies have also indicated that certain activities may not apply a sufficient enough overload to cause an increase in bone mass. Athletes who perform moderate- to high-intensity impact activities—such as gymnastics, power lifting and jumping—have been found to have greater BMD than those who perform low-intensity or nonimpact activities, such as swimming and cycling. Some elite swimmers have even been found to have a lower BMD than nonexercisers, apparently due to the unloading of their bones that takes place during extended periods in a buoyant environment.
Children. Research with children and adolescents supports a direct relationship between physical activity and increased bone mass. For example, studies have consistently found higher BMD values in dominant limbs when compared to nondominant limbs. Other investigations 8 have found that young athletes who load their dominant limbs preferentially while exercising (such as tennis players) have even greater disparities in BMD between their dominant and nondominant sides.
Jumping and other weight-bearing activities have been found to provide a significant increase in bone density at the spine and hip in prepubescent children. Some recent evidence suggests that exercise provides the best long-term benefits in BMD when it is initiated before puberty. As with adults, exercise interventions with children should take into account the training principles discussed above, although, unlike adults, children have been found to maintain at least some of their BMD gains from weight-bearing exercise even if they discontinue their exercise program.
Postmenopausal Women. Due to the decline in circulating estrogen and the resulting bone loss commonly seen in postmenopausal women, weight-bearing exercise to stimulate BMD and bone strength is especially important for this population. Resistance training and weight-bearing cardiovascular exercise have been shown to increase and/or maintain BMD in postmenopausal women when compared with control groups. Research has also found that low-impact activities, such as walking, are not effective exercise interventions for preventing bone loss in this population. Thus a program for postmenopausal women should include exercises that provide a more substantial load on bone, such as jogging and weight training, which may increase bone density in a site-specific manner.
Studies of women who incorporate exercise with hormone replacement therapy (HRT) have had mixed results in the area of bone loss prevention. In some studies, exercise has been shown to enhance the effect of HRT, whereas in other investigations no interaction has been observed.
The effects of combining bone loading with adequate calcium intake are not fully identified and appear to vary during one’s lifespan. In any case, calcium is a major mineral that is vital to the bones and is needed for the heart, muscles and nerves to function properly. Adequate calcium intake is especially important for women and girls, since women can experience rapid bone loss in the years immediately following menopause. General recommendations for intake are 1,300 milligrams (mg)/day for girls 9 to 18 years, 1,000 mg for women 19 to 50 years, and 1,200 mg for women over 50 years (Gottlieb 2002).
The ultimate goal of increasing bone mass is to prevent osteoporosis-related fractures. Unfortunately—and perhaps surprisingly—many studies have shown little or no reduction in this type of fracture between those who exercise and those who don’t. It seems that while increased physical activity does stimulate gains in BMD, being more active also elevates the likelihood of falls, which often cause fractures. In fact 50 percent (%) of spine fractures and 90% of hip fractures are attributable to falls. For this reason, preventive interventions should try to not only minimize bone loss and improve peak bone mass but also reduce the risk of falling. Any exercise program designed to increase BMD should also incorporate intervention strategies for balance, coordination, postural stability, muscular fitness and functional mobility to prevent future falls.