Habitual levels of vigorous, but not moderate or light, physical activity is positively related to cortical bone mass in adolescents

Abstract

Context: The intensity of habitual physical activity (PA) needed to affect skeletal development in childhood is currently unclear.

Objective: To examine associations between light PA, moderate PA, and vigorous PA (as assessed by accelerometry), and tibial cortical bone mass (BMC(C)) as measured by peripheral quantitative computed tomography.

Design/setting: Cross-sectional analysis based on the Avon Longitudinal Study of Parents and Children.

Participants: A total of 1748 boys and girls (mean age 15.5 yr) participated in the study.

Outcome measures: We measured BMC(C), cortical bone mineral density, periosteal circumference, and endosteal circumference by tibial peripheral quantitative computed tomography.

Results: Multivariable models, adjusted for height and other activity levels, indicated vigorous PA was positively related to BMC(C) (P = 0.0001). There was little evidence of a relationship with light PA or moderate PA (both P ≥ 0.7). In path analyses, the relationship between vigorous PA and BMC(C) [0.082 (95% confidence interval [CI]: 0.037, 0.128), P = 0.0004] (SD change per doubling of vigorous PA) was minimally attenuated by adjusting for body composition [0.070 (95% CI: 0.026, 0.115), P = 0.002]. In analyses adjusted for body composition, the relationship between vigorous PA and BMC(C) was explained by the periosteal circumference pathway [0.043 (95% CI: 0.004, 0.082), P = 0.03] and the endosteal circumference adjusted for periosteal circumference pathway [0.031 (95% CI: 0.011, 0.050), P = 0.002], while there was little contribution from the cortical bone mineral density pathway (P = 0.3).

Conclusions: Vigorous day-to-day PA is associated with indices of BMC(C) and geometry in adolescents, whereas light or moderate PA has no detectable association. Therefore, promoting PA in childhood is unlikely to benefit skeletal development unless high-impact activities are also increased.

Figures

Fig. 1.

The association of light, moderate, and vigorous activity on BMCC (top) and SSI (bottom), adjusted for age, height, average recording time, and other levels of exercise intensity in boys (n = 778) and girls (n = 970). y axis represent β coefficients with 95% confidence limits, expressed as sd change in pQCT variable per sd increase in light activity and per doubling in moderate or vigorous activity. BMCC, P values for light activity P(ALL) = 0.68 (B = 0.81, G = 0.68), for moderate activity P(ALL) = 0.93 (B = 0.57, G = 0.75), and for vigorous activity P(ALL) < 0.00001 (B = 0.007, G = 0.002). SSI, P values for light activity P(ALL) = 0.49 (B = 0.87, G = 0.33), for moderate activity P(ALL) = 0.69 (B = 0.21, G = 0.64), and for vigorous activity P(ALL) = 0.003 (B = 0.06, G = 0.01). No evidence of any sex differences were observed for light, moderate, or vigorous PA (P ≥ 0.2 in all instances). B, Boys; G, girls.

Fig. 2.

Theoretical relationships between PA, body composition, bone shape, and density. Direction of effect is indicated by +ve (positive) effects, −ve (negative) effects, and ?ve (direction not currently clear) effects. PC, periosteal circumference; EC, endosteal circumference.

Fig. 3.

The association of vigorous PA (vigorous) with BMCC via LM, FM, and an independent bone pathway (top, body composition pathways). The association of vigorous PA with BMCC via periosteal circumference (PC), endosteal circumference adjusted for periosteal circumference (ECPC), and BMDC pathways (bottom, independent bone pathways). Bootstrap path coefficients (with 95% confidence limits) represent sd change per doubling in vigorous activity (P values are also indicated adjacent to each path). Sex interactions (differences between the sexes) for the association between vigorous PA and BMC via body composition paths: LM (P = 0.0672), FM (P = 0.8291), bone (P = 0.7024), total (P = 0.8686), and via independent bone paths PC (P = 0.3619), ECPC (P = 0.7197), and BMDC (P = 0.1266). Arrow sizes are proportional to the strength of the association, with solid arrows indicating positive association and dashed arrows negative association. Bold formatting indicates P < 0.05.