In a study out of Germany, Kemmler et al. followed 85 postmenopausal women for 12 years to determine if supervised long-term exercise training had an effect on bone mineral density and the incidence of fractures. The women participating in the study all had low bone density (osteopenia) but were not using medication. The exercise program consisted of two group sessions/week and two home training sessions/week. Bone mineral density was assessed through DXA scans. The authors concluded that engaging in a consistent exercise regimen plus supplementing with calcium and vitamin D improves bone strength (increased bone mineral density) and prevents fractures.
Kemmier et al. 2012. Exercise and fractures in postmenopausal women: 12-year results of the Eriangen Fitness and Osteoporosis Prevention Study (EFOPS). Osteoporosis International 23(4):1267-1276.