The aim was to analyze daily physical activity during a 12-month exercise trial to determine the duration of regular exercise needed to obtain positive bone changes in previously sedentary women. Physical activity was continuously assessed with a waist-worn accelerometer in 34 premenopausal women (35–40 years) participating in an exercise intervention (Vainionpää et al. 2005) with progressive high-impact training three times a week. The average daily numbers of impacts for each subject were calculated at five acceleration levels (from 0.3g to 9.2g, see Vainionpää et al. 2006) during time intervals of 0–3, 0–6, and 0–12 months. Impact activity data were correlated with 12-month bone density and geometry changes measured by dual x-ray absorptiometry, quantitative ultrasound and quantitative computed tomography. Femoral neck bone mineral density (BMD) changes were significantly correlated with impact activity at six and twelve months at high intensity levels (>3.9g). Trochanter BMD changes were associated with exercise even during the first three months at impacts exceeding 1.1g. Similarly, mid-femur bone geometry and calcaneal ultrasound changes were associated with impact activity within the three months of exercise. Six months of high-impact activity training were positively associated with bone changes at the femoral neck, while a shorter period was related with changes in trochanter, mid-femur and calcaneus. The results can be utilized in the design of training programs to prevent bone loss in premenopausal women.

References: 

Vainionpää, A., Korpelainen, R., Leppäluoto, J. & Jämsä, T. 2005. Osteopor Internat 16, 191–197. Vainionpää, A., Korpelainen, R., Vihriälä, E., Rinta-Paavola, A., Leppäluoto, J. & Jämsä, T. 2006. Osteopor Internat 17, 455–463.