The relationship between objectively-assessed physical activity (PA) and bone mineral density (BMD) in older adults is poorly understood. As such we aimed to investigate associations of accelerometer-derived physical activity (PA) and bone mineral density (BMD) in community-dwelling older adults.
This secondary analysis of a subset of the Tasmanian Older Adult Cohort study included participants with PA assessed utilising a ActiGraph GT1M over seven days (N=209 participants, 53% female; 64.5 ± 7.2years). BMD was assessed at the total hip, lumbar spine and whole-body by DXA at baseline and approximately 2.5 years later. Indices of PA intensity were estimated via established thresholds. Relationships between PA and BMD were assessed via multivariable linear regression.
There we no differences in PA between men. Women with greater than median total hip BMD performed higher baseline steps/day, light PA and moderate & vigorous PA (MVPA) (all P≤0.01) at baseline. Steps/day were negatively associated with BMD at all sites except the legs in men (all P≤0.04). In women, steps/day (β; P 0.24; P=0.02) and MVPA (0.21; 0.03) were positively associated with leg BMD. Adjusting for body weight, positive associations for steps/day, MVPA and BMD were observed at trochanter, total hip, legs, whole body (all P≤0.05); sedentary behaviour was negatively associated with pelvic BMD (-0.20; 0.03) in women. Longitudinally steps/day was positively associated with total hip BMD in men and femoral neck BMD in women. Light PA was associated with leg BMD in women.
In conclusion, body weight mediates sex-specific associations between accelerometer-determined PA and BMD in community-dwelling older adults. Positive cross-sectional associations
for steps/day and MVPA, and negative associations for sedentary time, were observed in women only. These findings suggest that body composition differences may influence mechanical loading benefits of PA in older adults, although greater PA may have temporal positive effects in women