Bone Abstracts

Objectives: Weight bearing physical activity (PA) is thought to be beneficial to bone mineral accrual in childhood, but the influence of PA and sedentary time on bone microarchitecture is less well described. We examined the relationships between self-reported PA and volumetric bone mineral density (vBMD) and bone microarchitecture at the distal tibia, assessed using high resolution peripheral quantitative computed tomography (HR-pQCT).

Methods: Healthy children aged 7–17 years were invited to participate. Participants were asked how many hours per day they spend asleep or doing activities whilst lying or seated (‘sedentary time’) and doing exercise during which their heart rate increases or they sweat (‘strenuous PA’). Habitual PA was also assessed using the Physical Activity Questionnaire (PAQ). vBMD and bone microarchitecture of the tibia were measured by HR-pQCT (XtremeCT, Scanco Medical AG, Switzerland). Associations between measures of PA and bone outcomes were assessed using linear regression adjusting for age, sex and height.

Results: 88 children participated; mean age 12.5 years (SD 2.4), 61% male. Reported sedentary time (median 19.0 hours/day, IQR 17.0–21.5) correlated negatively with strenuous PA (median 1.0 hours/day, IQR 0.5–1.5), r=−0.36, P=0.001. Sedentary time was negatively associated with total vBMD, cortical area, cortical vBMD, cortical thickness and trabecular thickness, but not trabecular number or separation. In contrast, strenuous PA was positively associated with total vBMD, cortical area, cortical thickness, and trabecular thickness, with similar positive relationships between habitual PA (PAQ score) and total vBMD, cortical area and thickness. When both sedentary time and strenuous PA were include in the regression model, sedentary time was significantly negatively associated with cortical area (β=−7.3 mm² per 2 hours/day, P=0.003), cortical thickness (β=−0.07 mm per 2 hours/day, P=0.01), cortical vBMD (β=9.91 g/cm³ per 2 hours/day, P=0.02) and trabecular thickness (β=−0.002 mm per 2 hours/day, P=0.05), whereas the associations with strenuous PA were no longer statistically significant.

Conclusion: High levels of sedentary time might be detrimental to tibia microarchitecture, whereas strenuous physical activity, after adjustment for sedentary time, is not associated with tibia vBMD or microarchitecture.

Disclosure: The authors declared no competing interests.