The prospective study of older women by Gregg and coworkers in this issue provides additional documentation of the benefits of physical activity in lowering the risk for osteoporotic fracture [3]. Almost 1000 nonblack women older than 65 years of age were categorized according to the total energy expended in walking, daily chores, and sport and recreational activity assessed at baseline by using the Harvard Alumnae Questionnaire. During follow-up, which averaged 7.6 years, women who had higher levels of leisure time and sport activity, did more household chores, and spent fewer hours sitting daily had a significantly reduced relative risk for hip fracture, even after adjustment for age, dietary factors, and health status. Women who were moderately and vigorously active had a 40% reduction in risk for hip fracture and a 33% reduction in risk for vertebral fracture compared with inactive women. The impact of that amount of prevention on morbidity and mortality in the 1.3 million U.S. women who have osteoporotic fractures yearly, especially hip fracture, and on attendant health care costs has not been subjected to formal cost–benefit analysis, but the improvement in quality of life and the savings in costs are likely to be very large.

The two targets of every successful program for prevention of osteoporotic fractures are reduction of bone loss and reduction of risk for falls in older women. Physical activity has been shown to enhance bone formation in response to forces generated by muscle contraction. Activity also may contribute to reduced risk for falling by improving muscle strength, balance, and mobility. Gregg and colleagues did not design their study to explain how physical activity prevents fracture, although they did measure bone mineral density at the calcaneus and assessed hip abduction strength and analyzed those factors as potential mediators of the relation between physical activity and hip fracture prevention. Adjustments for calcaneal bone mineral density and hip muscle strength accounted for only 3% to 11% of the preventive effect of physical activity on hip fracture. Previous work has clearly demonstrated the benefits of certain forms of exercise on the maintenance of muscle mass and strength in elderly populations [4]. Some prevention of age-related loss of muscle mass and strength, termed sarcopenia, has been attributed to strengthening or resistance exercise. Various antigravity exercises have been associated with beneficial effects on maintenance of bone mass [5, 6].

The population in the study by Gregg and colleagues was older than 65 years of age; thus, we are learning that it is never too late to reap the benefits of exercise, whether the measures of success are increased strength and mobility, better quality of life, or prevention of falls and fractures [4]. It is sobering that 70% of the women reported low-intensity sport and recreational activity (50%) or none at all (20%), whereas 67% reported more than 6 to 8 hours of sitting per day. A great deal more must be done to reverse this trend, which also contributes significantly to the current epidemic of obesity.

Physicians have a much greater role in the application of emerging knowledge of the benefits of physical activity and exercise than they currently play. Internists are almost uniformly convinced of the benefits of physical activity for their patients, and controlled trials of physician counseling demonstrate that physicians can promote increased activity, especially in sedentary patients [7]. However, a recent survey of generalist physicians in Massachusetts found that 60% assessed physical activity habits in most of their patients but less than 50% counseled that same proportion of patients on physical activity (Economos CD. Personal communication). Factors that seem to be related to low frequency of counseling include a perceived lack of success, lack of conviction that exercise is important, and higher physician heart rate (as a measure of the physician's personal exercise frequency) [8]. Less than 50% of physicians report having enough time to counsel in a typical office visit, and only 30% report that reimbursement is sufficient for that time (Economos CD. Personal communication).

To apply the knowledge of the benefits of physical activity and exercise in preventive practice will require a greater understanding by physicians and, ultimately, their patients of the specific types and amounts of exercise that are prescribed and designed to achieve the goals of disease prevention. Aerobic forms of exercise have been shown to reduce cardiovascular morbidity and mortality but have little effect on the maintenance of muscle mass and strength. Strengthening exercise, on the other hand, improves skeletal muscle mass and strength and thereby improves balance and reduces risk for falling. This basic and general understanding must be supplemented with a deeper and broader definition of recommended forms, amounts, and intensities of exercise, the basis for meaningful exercise prescriptions.

The Centers for Disease Control and Prevention and the American College of Sports Medicine [1] have issued clear recommendations that physicians can use to counsel patients. The conclusion of that task force, which was reinforced in the year 2000 health objectives of the U.S. Department of Health and Human Services [9] and in the U.S. Surgeon General's Report on Physical Activity and Health [2], is that "every U.S. adult should accumulate 30 minutes or more of moderate intensity physical activity on most or preferably all days of the week." That amount of activity corresponds to an expenditure of more than 1000 calories per week in the study by Gregg and colleagues and has clearly beneficial effects on fracture prevention and potential benefits in other areas of disease prevention. Other sources of information, including an increasing number of useful sites on the Internet [10-12], can provide physicians and their patients with guidance on specific exercise in graded amounts and methods. Scientific and professional meetings, such as the annual meeting of the American College of Physicians, will need to represent this area of investigation more intensely. We can do better than declaring "just do it," but our embrace of exercise recommendations as an integral part of preventive practice, as well as management and rehabilitation of patients with disease, will require greater intensity and specificity. The health benefits of changing both patient and physician behavior with regard to exercise prescriptions, measured in quality of life and health care costs, will extend far beyond the prevention of osteoporotic hip fracture.