Mediating Effect of Muscle on the Relationship of Physical Activity and Bone

Purpose This study analyzed prospective associations between distinct trajectories of objectively measured physical activity (PA) and late adolescent bone parameters and explored the mediating effects of lean soft tissue (LST), a surrogate of muscle mass to associations. Methods PA was measured by accelerometry starting at age 5 and continuing at 8, 11, 13, 15 and 17 years in approximately 524 participants from the Iowa Bone Development Study. Gender-specific group-based trajectory modeling was used to construct developmental trajectories of moderate-and-vigorous intensity PA (MVPA) from childhood to late adolescence. At age 17, proximal femur bone mineral density (aBMD) was assessed by dual X-ray energy absorptiometry, and its distribution was calculated by aBMD ratios. Specific geometric measures of the proximal femur were assessed using Hip Structural Analysis. 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 an increase in leg LST in both sexes. For males and females, indirect effects were observed on the total and all regional proximal femur aBMDs, and on the ratio between the inferomedial and superolateral neck aBMD. The effect on the ratio between the trochanter and the total proximal femur was specific to females, while the effect on the hip axis length was specific to males. Direct effects of MVPA on aBMDs were identified only in males. Conclusions Using robust mediation analysis, this is the first study addressing the indirect effect (through muscle) of PA across childhood and adolescence on proximal femur bone parameters. To improve bone health at the proximal femur, the results suggest PA interventions during growth that increase muscle mass, particularly in females. Corresponding author: Vera Zymbal, Postal address: Department of Sports and Health, Faculdade de Motricidade Humana, Universidade de Lisboa, Estrada da Costa, 1499-002 - Cruz Quebrada, Lisbon, Portugal, E-mail: verazymbal@fmh.ulisboa.pt, Telephone: +351 96192383 This work was supported by the Portuguese Science and Technology Foundation (FCT) (SFRH/BD/79828/2011 and PTDC/DES/115607/2009). It is also supported by the National Institute of Dental and Craniofacial Research grants R01-DE12101, R01-DE09551, and P30-DE10126, and the General Clinical Research Centers Program from the National Center for Research Resources, M01-RR00059 and UL1-RR024979. No potential conflict of interest was reported by the authors. The authors have no conflicts of interest to disclose in relation to this study. The results of the present study do not constitute endorsement by American College of Sports Medicine. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Submitted for publication November 2017. Accepted for publication August 2018. © 2018 American College of Sports Medicine

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