The effects of a recent long-term randomized controlled prospective study of high intensity resistance training in young women indicated that regional BMD at the femoral and trochanteric sites can be increased by resistance training exercise (17). TBBMD did not change significantly over the 18 months of this trial despite these regional changes and significant increases in maximal and isokinetic strength. Lohman et al. (17) speculate that increases in strength and lean tissue may be greater than increases in BMD in premenopausal women and that in young women there may be a site-specific redistribution of bone mineral rather than a total body increase in BMC. Similarly, Snow-Harter et al. (32) noted significant increases in lumbar BMD in young women completing either a progressive aerobic training program (jogging) or a progressive resistance training program, when compared with a control group. The resistance training group showed significant strength increases when compared with the aerobic trained women; however, the increases in bone density were not significantly different between the two exercise groups (32). This result is consistent with the site-specific principles of mechanical loading as both groups of women performed weight-bearing exercise stressing the lower body and spine. Differences might not necessarily be expected between exercise groups due to the short duration of the protocol and the physiologic limits of bone formation and remodeling.
Conversely, resistance training has been reported to have either no effect or a negative impact on bone in two studies in premenopausal women (28,34). Vuori et al. (34) found that unilateral resistance training did not have significant impact on BMD or BMC in physically active young women except at the patella. However, there was a trend towards increased BMD and BMC at several sites in the trained limb. In another study (28), lumbar spine BMD decreased in an exercise group completing an intensive weight training regimen and remained unchanged in the control group. This study is difficult to interpret because of a nonrandomized design and baseline physical differences between the women in the control and resistance training groups. The women in the control group had greater body weight and body fat, which is typically associated with higher BMD, when compared with the resistance trained women who had lower body weight.
The effects of resistance training and bone density have also been reported in older populations (19,20,22,24,25). A commonality of these studies is a moderate- to high-intensity resistance training protocol and significant gains in strength. Our laboratory has completed a 1-yr randomized controlled trial of high-intensity resistance training in postmenopausal women (20). The results of the study demonstrated that women in a 2 d·wk−1 resistance training program gained an average of 1% in BMD of the femoral neck and lumbar spine whereas the control group lost 2.5% and 1.8% at these sites, respectively. In addition, the resistance-trained women tended to maintain TBBMC of the skeleton whereas the women in the control group had a 1.2% decline in TBBMC. Furthermore, the resistance-trained women had a 35-76% increase strength, 14% improvement in dynamic balance, and a 1.2-kg increase in total body muscle mass and a 27% increase in physical activity unrelated to the intervention whereas the control group showed declines in all of these parameters. The overall findings of our study indicate that resistance training in postmenopausal women can decrease the risk for osteoporosis by simultaneously influencing multiple risk factors for osteoporotic fractures.
In a study of surgically postmenopausal women receiving estrogen replacement therapy, Notelovitz et al. (22) found that a resistance training group increased their bone density at multiple sites, whereas the control group maintained their bone density. This study clearly demonstrates the benefits of resistance training on the skeleton and has important implications for women receiving only hormone replacement therapy to improve bone density.
Strength training has also been shown to be an effective osteogenic agent for glucocorticoid-induced bone loss. Braith and colleagues reported that the significant decrease in lumbar spine BMD secondary to heart transplant surgery and antirejection drug therapy can be mitigated by postoperative strength training in middle-aged male patients (1). In a prospective randomized controlled trial, eight patients who participated in resistance training were able to recapture almost all of their presurgical total body, femoral neck, and lumbar spine BMD as measured by dual energy x-ray absorptiometry whereas eight male transplant recipients in the control group continued to lose bone (1). These results suggest that resistance training exercise may be an effective strategy for preventing secondary bone loss due to other medical conditions.
Three studies involving less intense resistance training programs have also found that resistance training has a positive impact on bone through either maintenance or formation in postmenopausal women (26,27,29). However, it has not been shown that low-intensity or home-based resistance training is an effective stimulator of bone formation in women by several others investigators (3,23,30).
The research completed to date indicates that resistance training is positively associated with high BMD in both young and older adults and that the effect of resistive exercise is relatively site specific to the working muscles and the bones to which they attach (1,4,5,11,13-16,19,20,22,24,26,27,29,31,32,34). Although aerobic exercise and weight bearing physical activity are important in maintaining overall health and healthy bone, resistance training exercise seems to have a more potent impact on bone density (12). The positive effect on bone is most convincingly demonstrated by randomized, controlled trials using a high-intensity resistance training protocols. This indicates that progressive resistance training may have significant clinical application as a prevention and treatment for osteoporosis and other degenerative bone diseases in a wide range of individuals. Resistance training may help to achieve the highest possible peak bone mass in premenopausal woman and may aid in maintaining or increasing bone in postmenopausal women. The frail elderly may also benefit from progressive resistance training to help preserve bone density in addition to increasing muscular strength and potentially improving agility and balance. There are many interesting directions for future research in this area. There is certainly a need for additional randomized controlled trials of progressive resistance training possibly in conjunction with other novel exercise modalities and calcium and vitamin D supplementation (6). Furthermore, the potential mechanisms responsible for the positive changes in bone due to these interventions warrants further investigation.
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