Skeletal “impact” of exercise
Authors: Robert Marcus
Journal: The Lancet (1996)

In this issue, Heinonen et al report that 18 months of high-impact exercise improved bone mass, muscle strength, and other measures of physical performance in healthy premenopausal women. This paper joins a growing number of controlled trials that clearly demonstrate skeletal benefit from exercise. Physiologists traditionally distinguish two forms of physical activity— endurance, done at low intensity with much repetition, and resistance, done at high intensity with little repetition. Heretofore, most exercise trials directed towards effect on bone have used running and weight-training as models of endurance and resistance activity, respectively. Both have proven effective across a broad age range of individuals, from young adult to elderly.1Snow-Harter C Marcus R Exercise, bone mineral density, and osteoporosis.Exercise Sport Sci Rev. 1991; 19: 351-388Crossref PubMed Google Scholar As encouraging as these reports may be, the exercise-induced gains in bone-mineral density (BMD) have been modest, generally about 1–3%, and do not approach the magnitude of difference that distinguishes athletes from the people who watch them. It may be that BMD gains in published studies have been constrained by the type of exercise employed. One characteristic of exercise that has not been adequately accounted for is impact, the rate at which skeletal loads are applied. In animal models the rate at which bone is deformed by loading has been an important predictor of skeletal response (reviewed in 2Marcus R Mechanisms of exercise effects on bone.in: Bilezikian JP Raisz LG Rodan GA Principles of bone biology. Academic Press, San Diego1996: 1135-1145Google Scholar). Unexpectedly high BMD values in women gymnasts3Robinson TL Snow-Harter C Taaffe DR Gillis D Shaw J Marcus R Gymnasts exhibit higher bone mass than runners despite similar prevalence of amenorrhea.J Bone Min Res. 1995; 10: 26-35Crossref PubMed Scopus (281) Google Scholar are intriguing in this regard, since gymnastic activity produces enormous skeletal impacts (about 18 body weights with each dismount from parallel bars) with few loading cycles. Using fairly high-impact activity, Heinonen et al found BMD gains of 1·4–3·7% by 18 months. Although significant, these increases do not substantially exceed those reported for other forms of endurance or resistance activity, perhaps because the training impacts actually achieved were not very large. The authors calculate their jump-training to have induced peak forces below 6 body weights, little more than the 3–4 body-weight forces associated with jogging. To fully exploit the skeletal potential of impact loading might therefore require activities so rigorous that safety would be jeopardised. The Heinonen paper offers new information about human skeletal response. Given the sedentary nature of western societies and their generally dismal record of adherence to exercise programmes, questions remain about how practicable the findings might be. As we learn more about the nature of skeletal adaptation, new strategies to motivate and assist sedentary people incorporate physical activity into their daily lives will be equally important if scientific insights are ever to lead to important reductions in the incidence of falls and fractures. Randomised controlled trial of effect of high-impact exercise on selected risk factors for osteoporotic fracturesHigh-impact exercises that load bones with a rapidly rising force profile in versatile movements improve skeletal integrity, muscular performance, and dynamic balance in premenopausal women. If done on a regular basis, this type of exercise may help decrease the risk of osteoporotic fractures in later life. Long-term studies are required to show whether these 18-month results can be translated into long-term benefit. Full-Text PDF