In North America, up to 90% of adolescents do not meet recommended physical activity (PA) guidelines (≥60 min moderate-to-vigorous PA/day). The importance of mechanical loading through PA on bone mass accrual, especially in peripubertal years, is well documented. However, we know less about the response of bone strength, structure and density to PA and inactivity during adolescence.
PURPOSE: To compare bone strength, structure and density between adolescents who were meeting recommended PA guidelines (Active) and those who do not (Inactive).
METHODS: Adolescents (n=183, 83 boys, 15.3±0.3 yrs) were participants from the Health Promoting Secondary Schools (HPSS) study. We assessed height (cm) and weight (kg) using standard methods. We assessed bone strength (bone strength index; BSI mg2/mm4) at the 8% tibia and polar strength strain index (SSIp, mm3) at the 50% tibia using peripheral quantitative computed tomography (pQCT). We report total (Tt), trabecular (Tb) and cortical (Ct) bone area (Ar, mm2), density (Dn, mg/mm3), cortical thickness (Ct.Th, mm), muscle cross-sectional area (MCSA, mm2), periosteal circumference (Ps.Pm, mm) and endosteal circumference (Es.Pm, mm). We assessed PA (min/day) using a validated PA questionnaire for adolescents (PAQ-A), muscle mass by total body dual energy X-ray absorptiometry (DXA) and maturity and calcium intake by questionnaire. We conducted sex-specific multivariable regression analysis (adjusted for maturity, muscle, ethnicity and tibial length).
RESULTS: SSIp was 6% higher (+126.4 mm3, SE: 63.2 mm3, p<0.05) in Active boys (140±71 min PA/day) compared with Inactive boys (37±19 min PA/day). Active boys had 9% greater Tt.Ar (+70.9, SE: 27.5 mm2) at the 8% tibia; 6% greater Ct.Ar (+20.2, SE: 7.4 mm2) and Ct.Th (+0.35, SE: 0.1 mm) at the 50% tibia compared with Inactive boys (p<0.05). Meeting PA guidelines explained 5-10% of the variance in Tt.Ar (8% tibia), SSIp, Ct.Ar and Ct.Th in boys (p<0.05). SSIp, Ct.Ar and Ps.Pm were similar between active (111±41 min PA/day) and Inactive girls (34±16 min PA/day) after adjusting for covariates. There was no difference in BSI between Active and Inactive adolescents of either sex.
CONCLUSION: Adolescent boys who meet recommended PA guidelines have greater tibial bone strength and size compared with boys who do not meet guidelines.