Physical activity, but not sedentary time, influences bone strength in late adolescence.

Physical activity is essential for optimal bone strength accrual, but we know little about interactions between physical activity, sedentary time, and bone outcomes in older adolescents. Physical activity (by accelerometer and self-report) positively predicted bone strength and the distal and midshaft tibia in 15-year-old boys and girls. Lean body mass mediated the relationship between physical activity and bone strength in adolescents.

PURPOSE: To examine the influence of physical activity (PA) and sedentary time on bone strength, structure, and density in older adolescents.

METHODS: We used peripheral quantitative computed tomography to estimate bone strength at the distal tibia (8% site; bone strength index, BSI) and tibial midshaft (50% site; polar strength strain index, SSIp ) in adolescent boys (n = 86; 15.3 ± 0.4 years) and girls (n = 106; 15.3 ± 0.4 years). Using accelerometers (GT1M, Actigraph), we measured moderate-to-vigorous PA (MVPAAccel ), vigorous PA (VPAAccel ), and sedentary time in addition to self-reported MVPA (MVPAPAQ-A ) and impact PA (ImpactPAPAQ-A ). We examined relations between PA and sedentary time and bone outcomes, adjusting for ethnicity, maturity, tibial length, and total body lean mass.

RESULTS: At the distal tibia, MVPAAccel and VPAAccel positively predicted BSI (explained 6-7% of the variance, p < 0.05). After adjusting for lean mass, only VPAAccel explained residual variance in BSI. At the tibial midshaft, MVPAAccel , but not VPAAccel , positively predicted SSIp (explained 3% of the variance, p = 0.01). Lean mass attenuated this association. MVPAPAQ-A and ImpactPAPAQ-A also positively predicted BSI and SSIp (explained 2-4% of the variance, p < 0.05), but only ImpactPAPAQ-A explained residual variance in BSI after accounting for lean mass. Sedentary time did not independently predict bone strength at either site.

CONCLUSION: Greater tibial bone strength in active adolescents is mediated, in part, by lean mass. Despite spending most of their day in sedentary pursuits, adolescents' bone strength was not negatively influenced by sedentary time.