Exercise: Moving in the Right Direction

concerning the beneficial effects of exercise on the skeleton highlights the remarkable manner in which the importance of mechanical loading on skeletal integrity, long understood by bioengineers, has been recognized by the osteoporosis community in a relatively few years. More than indicating the present high level of interest in exercise, these papers illustrate the degree to which this field has matured. It has been 10 years since most studies in this area did little more than compare bone mineral density (BMD) of athletes to that of sedentary controls. Not until the early 1990s did intervention trials begin to appear in which allocation of participants was randomized and in which exercise protocols themselves were described in precise and quantifiable terms. Results of those studies confirmed the general optimism that an imposed exercise program can modestly increase BMD. During this same era, insights were drawn from animal and epidemiological studies that now permit the design of clinical trials that do not simply ask whether exercise increases bone mass, but actually probe specific hypotheses about the nature of the skeletal response. The current papers are excellent examples of this approach. Exercise intervention trials frequently demonstrate increases in lumbar spine BMD of about 1.5%, but only a few have been able to show improvement at the proximal femur, despite the fact that loading conditions appeared suitable for achieving such a response. One plausible explanation for these disappointing results is that the incremental loads imposed by training are low compared with habitual loads experienced at the hip during the course of daily activities. During a relaxed walk, each step imposes a load on the axial skeleton of 1 body weight. Load magnitudes increase to 3–4 body weights from jogging and about 5 body weights from jumping hurdles. Since an average person ambulates from 4–8 h each day, adding a few thousand walking or jogging steps, typical of most exercise trials, constitutes only a slight change in the daily mechanical history and might fail to initiate an adaptive response. Bassey and colleagues report the effects of a jumping program on BMD in three groups of healthy women: premenopausal, postmenopausal estrogen deficient, and postmenopausal women on hormone replacement therapy. The exercise program consisted of 50 vertical jumps each day, with an average height of 8.5 cm and loads equivalent to 3–4 body weights per jump, as estimated by measurement of ground reaction forces. Five months after beginning the trial, premenopausal women showed an increase in femoral neck and trochanteric BMD of 2–3%. By contrast, postmenopausal women, regardless of hormone replacement therapy status, showed no change in BMD, even among those women who completed a full year of training. Thus, jumping exercise improved BMD in young but not in older women, and the failure of the latter to respond to impact loading appears independent of estrogen deficiency. The results of this study pose two important questions: Should we consider proven the concept of deficient skeletal adaptation in older people? Do the BMD increases observed in the younger women confirm the view that highimpact activity is optimal for increasing BMD? The answer to both questions is no. Although evidence for attenuation of skeletal responsiveness with age does exist, it is incorrect to conclude that older women fail to undergo skeletal adaptation at the hip. Using a different strategy to train older postmenopausal women, Kerr et al. conducted a year-long trial of bicycle exercise in which one leg served as the control and the other was subjected to a progressive increase in resistance. Results showed a gain of ;2% in the trained hip as opposed to no change on the control side. To understand the disparate results of these two protocols some appreciation of the complex nature of mechanical loading is required. With jumping, two types of loads are transmitted to bone: those due to impact absorption and those due to muscle-generated forces. A dismount from parallel bars clearly exposes a gymnast to enormous impact force, about 11 body weights, which may partially explain why the hip BMD of gymnasts significantly exceeds that of other athletes. However, in the jumping protocol of Bassey et al., the measured