Bone Abstracts

Objectives: This study analysed prospective associations between two distinct developmental trajectories of objectively-measured physical activity and late adolescent bone parameters (age 17 yr) by exploring the mediating effects of lean soft tissue (LST), a surrogate of muscle mass.

Methods: In approximately 349 participants (191 girls) of the Iowa Bone Development Study, physical activity was measured by accelerometry starting at age 5 and continuing at 8, 11, 13, 15 and 17 years. Gender-specific group-based trajectory modelling was used to construct developmental trajectories of moderate-and-vigorous intensity physical activity (MVPA) from childhood to late adolescence. Bone parameters were assessed by dual X-ray energy absorptiometry and included bone mineral density (aBMD), aBMD distribution, and specific geometric measures of the proximal femur.

Results: A significant portion of the total effect of MVPA from age 5 to 17 yr on bone parameters at age 17 was explained by changes occurring in leg LST in both genders. These indirect effects were observed on all regional aBMDs (neck, trochanter, intertrochanter, inferomedial and superolateral neck), on the ratio between the inferomedial and superolateral neck aBMD, and on the hip axis length (HAL). The effects of MVPA mediated by leg LST were 43–49% on regional aBMDs in girls (P<0.01) and 27–32% in boys (P<0.05). On the ratio between the inferomedial and superolateral neck aBMD the effect of MVPA mediated by leg LST was 30% in girls (C=−0.011, bootstrap 95%CI: −0.027; −0.001) and 41% in boys (C=−0.013, bootstrap 95%CI: −0.029; −0.002). Regarding HAL, the effect of MVPA mediated by leg LST was 34% in boys (C=0.083, bootstrap 95%CI: 0.016; 0.172) but inconsistent in girls (the sign of the coefficient of the mediated effect differed from that of the direct effect). Direct effects of MVPA were identified only in boys on all regional aBMDs of the proximal femur.

Conclusion: To improve proximal femur bone parameters, physical activity interventions during childhood and adolescence should also focus on increasing muscle mass, particularly in girls.

This work was supported by the Portuguese Science and Technology Foundation (FCT) (PTDC/DES/115607/2009 and SFRH/BD/79828/2011).

DisclosureThe authors declared no competing interests.