Relationship of bone health to yearlong physical activity in older Japanese adults: cross-sectional data from the Nakanojo Study

Introduction We determined associations between bone health and the quantity and quality of habitual physical activity in a cohort of 172 free-living Japanese aged 65-83 years (76 men, 96 women). Methods The number of steps taken and the intensity of physical activity were measured every 4 s throughout each 24-h period for 1 year, using a specially adapted accelerometer that distinguished up to 11 levels of physical activity (expressed in metabolic equivalents, METs). At the end of the year, a quantitative ultrasonic technique assessed each participant's osteosonic index (OSI, reflecting bone stiffness in the calcaneus). Results The data were significantly described by linear and exponential regression models which showed that in both sexes the OSI score increased with increasing daily physical activity, up to the observed maximum values of approximately 14,000 steps/day and 50 min/day at an intensity > 3 METs. However, when data were categorized into quartiles of physical activity, OSI scores were not significantly greater in persons exceeding recommended minimum standards of habitual physical activity (corresponding to counts of around 6,900 and 6,800 steps/day and durations > 3 METs of around 18 and 16 min/day in men and women, respectively). All who met such criteria (with the exception of a few women) had OSI scores above the threshold for a clinical diagnosis of osteoporosis. Multivariate-adjusted logistic regression analyses predicted that men and especially women who engaged in < 6,800 steps/day and < 16 min/day of moderate-intensity physical activity were, respectively, 4.9-8.4 and 2.2-3.5 times more likely to sustain fractures than those participating in > 8,200 steps/day and > 25 min/day of activity > 3 METs. Conclusion Causation cannot be inferred from a cross-sectional study. Nevertheless, we suggest that from the viewpoint of bone health, elderly people should be encouraged to engage in low- and moderate-intensity habitual physical activity, taking > 7,000 steps/day with a duration > 15 min/day at > 3 METs.