The National Osteoporosis Foundation (2009) states that approximately 35 million Americans have insufficient bone mass (osteopenia) and that another 10 million adults, 8 million of whom are women, have frail bones (osteoporosis). According to the U.S. Department of Health and Human Services (2004), osteoporosis will cause bone fractures in almost 1 of every 3 women and 1 of every 6 men.
Although many factors influence bone thinning, it is clear that muscle loss is closely associated with bone loss. Research reveals that men and women who do not perform resistance exercise reduce their bone density by 1 to 3 percent every year of adult life, which represents a bone loss of 10 to 30 percent every decade.
The connection between strength training and bone mass
Fortunately, weightlifting exercises, such as resistance training increase both muscle mass and bone mass. Numerous studies have shown significant increases in bone mineral density after several months of regular resistance exercise. Interestingly, the rate of improvement in bone mass resulting from strength training is 1 to 3 percent, which essentially reverses the bone loss that would otherwise be experienced by nonexercising adults.
Although most of the strength training studies related to bone mass have been conducted with women, research with men has demonstrated even greater effects of resistance exercise, with increases in bone mineral density exceeding 3 percent (Almstedt et al. 2011).
Resistance training also improves the size and strength of ligaments and tendons (Edgerton 1973; Fleck and Falkel 1986: Tipton et al. 1975). These changes may increase joint stability, thereby reducing the risk of sprains and dislocations.
How does strength training build bone density?
Although specific mechanisms via which exercise improves bone health are not fully elucidated yet, it is widely accepted that mechanical load induced by exercise training increases muscle mass, produces mechanical stress in the skeleton, and enhances osteoblast activity.
In simple terms, during strength training (especially lifting free weights), the muscles and tendons apply tension to the bones, which stimulates the bones to produce more bone tissue. As a result, bones become stronger and more dense and the risk of osteopenia, osteoporosis, and fractures decreases.
Bone mineral density loss in postmenopausal women
It is estimated that, on average, women lose up to 10 percent of their bone mass in the first five years after menopause. Performance of resistance training is associated with an increase in bone mineral density, possibly the most important adaptation to resistance training other than increases in muscular strength. There are numerous reasons why women should strength train.
As bone density in women is generally less than that of men, and because bone density decreases rapidly immediately postmenopause and then slowly thereafter, the performance of resistance training may be critical to bone health in women.
Bone mineral density of the lumbar spine and femur in premenopausal women significantly increased after 12 to 18 months of strength training (Lohman et al. 1995). Also, lumbar bone mineral density of early-postmenopausal women was improved following 9 months of strength training (Pruitt et al. 1992). In a long-term study of postmenopausal women (45-65 yr), the muscle strength and bone mineral density improved significantly (25-75%) after 1 yr of resistance training (2 sets; 6- to 8-RM; 70-80% 1 RM; 2 days/wk). Women who lifted weights consistently for over 4 yr had significant changes in bone mineral density at the femur and lumbar spine sites. The researchers concluded that women who maintained bone density lifted weights two or more times per week (Metcalfe 2010).
Since osteoporosis is a serious health threat to women, it makes sense to attempt to prevent it by making our bones as strong and as healthy as possible throughout life. Because peak bone mineral density is reached in late adolescence, prior to this period is the ideal time for women to start a strength-training program to possibly delay osteoporosis. However, you are never too old to start strength training to increase your bone mineralization.
Other types of exercises have also been shown to enhance bone density, such as aerobic exercise and weight-bearing activities (jogging, skipping, basketball, netball, tennis, dancing, impact aerobics, and stair walking); however, resistance training activities have the greatest impact on bone density, especially to those bones attached to the active muscles utilized.
Which is better for increasing bone mineral density: free weights or exercise machines?
The most important fact that women need to realize about starting resistance training to prevent osteoporosis is that lifting only with weight-training machines doesn’t cut it. Research on the benefits of Nautilus machines for bone mineral density suggests that women can obtain better results by lifting free weights. Other research has shown that after 20 weeks of training on Universal-type machines, women improved their muscular strength and defined their bodies. yet did not increase bone mineral density. So, choose free weights whenever possible.
Clearly, regular resistance training is the most productive means (as a low-cost and safe non-pharmacological intervention) for developing a strong and injury-resistant musculoskeletal system. When looking specifically at osteoporosis prevention, research indicates that strength training has a more potent effect on bone density than other physical activities (aerobic and weight-bearing), which renders resistance exercise an important lifestyle component for aging adults.