We examined the influence of objectively measured moderate-to-vigorous physical activity (MVPA) and vigorous PA (VPA) and sedentary time (SED) on bone strength, structure and density in post-pubertal girls.
We had 63 healthy girls (15.3±0.3 years) from the Health Promoting Secondary Schools (HPSS) study with valid accelerometry data and tibia scans. We assessed the left tibia using peripheral quantitative computed tomography (pQCT, XCT 3000, Stratec). We derived bone strength index (BSI, mg2/mm4), total bone area (Tt.Ar, mm2) and density (Tt.Dn, mg/cm3) at the distal tibia (8% of tibial length), and polar strength-strain index (SSIp, mm3), cortical area (Ct.Ar, mm2), density (Ct.Dn, mg/cm3) and thickness (Ct.Th, mm) at the 50% site. We assessed PA objectively using accelerometers (GT1M, Actigraph) and considered accelerometry data valid if participants had a minimum of 3 days, 10 h/day of wear time with non-wear time defined as 60 min of continuous zero counts. We defined SED as ≤100 counts/min (cpm), MVPA as ≥2296 cpm, and VPA as ≥4012 cpm 1. Using nested, multivariable regression models, we examined the influence of SED and PA on bone outcomes; controlling for ethnicity, tibia length, age at menarche and whole body lean mass (by dual-energy X-ray absorptiometry (DXA, Discovery-A, Hologic)). We also controlled for total wear time when we added SED in the models.
Girls had 44.0±20.7 min/day of MVPA and more than 10 h/day of SED (611.7±69.4 min/day). Initially, MVPA explained 6% and 7% of the variance in Tt.Dn and BSI at the distal tibia, respectively (P<0.05). When we added SED to the model, MVPA no longer predicted for Tt.Dn or BSI. In contrast, VPA remained a significant predictor of BSI even after SED was added to the model (4% of the variance; P<0.05). MVPA and VPA were not significant predictors of bone strength, structure or density at the tibial shaft.
There appears to be independent consequences of unloading on the growing skeleton in girls through sedentary behaviours, that cannot be compensated for by MVPA. However participation in more vigorous activities may mitigate the negative influence of these behaviours on bone strength in post-pubertal girls.
1. Evenson KR, Catellier DJ, Gill K, Ondrak KS & McMurray RG. Calibration of two objective measures of physical activity for children. J Sports Sci. 2008 26 (14):155765.
This study was funded by Canadian Institute for Health Research (227967).
Disclosure: The authors declared no competing interests.
27 - 30 Jun 2015