effects of region-specific resistance and aerobic exercises on bone mineral density in premenopausal women, The

Military Medicine, Apr 2001 by McDermott, Michael T

Epidemiological, cross-sectional, and prospective studies strongly suggest that exercise has beneficial effects on bone mass in premenopausal women. We prospectively compared the effects of resistance or aerobic exercises on regional bone mass in premenopausal active duty military women ranging in age from 19 to 40 years. Subjects were assigned, by preference, to a resistance exercise group or an aerobic exercise group and instructed to exercise at least 30 minutes per day, three times per week, for a period of 1 year. Bone mineral density (BMD) was measured by dual photon absorptiometry at the lumbar spine and femoral neck and by single photon absorptiometry at the mid radius at baseline, 6 months, and 12 months. BMD increments during the study were statistically significant at all sites in both exercise groups. Comparisons between the groups showed that after 12 months, BMD increased similarly in the lumbar spine (2.296 resistance vs. 1.8% aerobics, p = not significant) but more in the resistance group in the femoral neck (5.096 vs. 2.796, p

Introduction

Peak bone mass attained during the first two to three decades of life is an important determinant of the risk of developing osteoporosis in the postmenopausal years. Genetics appears to be the predominant factor governing the level of peak bone mass achieved,2-5 but modifiable factors such as exercise and calcium intake may also make a significant contributions6-11

There is an extensive body of literature regarding the skeletal effects of exercise in both premenopausal and postmenopausal women.12.13 Cross-sectional studies in premenopausal women, although clearly flawed by self-selection bias, have generally shown that women who exercise regularly have significantly greater bone mass than their sedentary counterparts14-21 and that resistance training may be somewhat more effective than aerobic work.20,21

Prospective studies in premenopausal women 22-32 have tended to agree that exercise is beneficial but have not been entirely consistent in this regard. For example, regimens consisting of weight training, aerobics, or high-impact exercises have been shown to increase regional bone mineral density (BMD) in several reports22-27 but to have no effect28-31 or to decrease BMD 32 in others. In the only studies comparing resistance and aerobic exercises to one another, both forms of exercise had similar effects on BMD.22,29

For the present study, our hypotheses were that an exercise program would increase BMD in active premenopausal women and that resistance exercises would be more effective than aerobic work. To test these hypotheses, we designed a prospective study to examine changes in BMD during a 1-year program of either resistance or aerobic exercises in a group of premenopausal active duty military women.

Methods

Subjects were active duty members of the U.S. Army and the U.S. Air Force stationed at one of three military bases operating in the metropolitan Denver area at the time of study initiation in 1995. Recruitment took place through advertisements on bulletin boards in barracks, dining halls, and work areas at the military bases. Subjects who responded were invited to an interview during which the purpose and design of the study were explained, and they were screened for exclusion criteria. Exclusion criteria included pregnancy, menopause, renal disease, liver disease, hyperparathyroidism, hyperthyroidism, known osteoporosis or other metabolic bone disease, the use of glucocorticoids, and the use of suppressive doses of levothyroxine. This study was reviewed and approved by the investigational review board at our institution. All subjects gave written informed consent.

During the recruitment period, 82 premenopausal women (aged 19 to 40 years) who met all of the eligibility criteria were enrolled. All participants had a complete history and physical examination and completed a dietary recall questionnaire for estimation of daily calcium intake. Those whose estimated their calcium consumption to be less than 1,000 mg/day were advised to increase their intake to 1,000 mg/day by the addition of low-fat dairy products or calcium carbonate supplements. Because of their active duty status, most subjects were already maintaining a sufficient level of physical fitness to pass the semiannual military weigh-in and physical training tests. The Army test consists of timed push-ups and sit-ups and a 2-mile run; at the time, the Air Force test consisted of bicycle ergometry.

Subjects were assigned to either the resistance exercise group or the aerobic exercise group according to their own preference; 44 participants chose the resistance program and 38 selected aerobics. A nonexercise control group was not used because of the requirement of all active duty personnel to maintain a high level of physical fitness. A randomized design was not used because it was felt that the study subjects could not be prohibited from selecting resistance training if they felt it was necessary to help them pass the strength portions of their semiannual physical training tests.