BACKGROUND: Osteoporotic fractures among the elderly are common, and without preventive measures the burden of these fractures on health-care systems will increase further. The purpose of this randomised controlled study was to evaluate, in premenopausal women, the effects of high-impact loading on several determinants osteoporotic fractures. METHODS: 98 healthy, sedentary female volunteers aged 35 to 45 years were randomly assigned to either a training (n = 49) or a control group (n = 49). Progressive high-impact exercises were done three times per week for 18 months. We measured bone mineral density (BMD) in specific axial and lower-limb sites, by dual-energy x-ray absorptiometry, at baseline and after 12 and 18 months. Maximum isometric strength, muscular and cardiovascular performance, and dynamic balance were also assessed. FINDINGS: BMD at the femoral neck, a weightbearing site, increased significantly more in the training group (mean 1.6% (95% CI 0.8 to 2.4)) than in the control group (0.6% (-0.2 to 1.4), p = 0.006). By contrast, at non-weightbearing sites, such as the distal radius, there was no significant difference between the training and control groups (-1.5% (-2.7 to -0.3) versus -0.7% (-1.9 to -0.5), p = 0.60). In the training group there was a significant improvement in vertical jump and predicted oxygen consumption per min at maximum exercise compared with controls. INTERPRETATION: High-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.

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