Bone Abstracts

Objectives: Activity duration and the daily patterns of activity during childhood and adolescence could contribute to long-term bone health. We examined cross-sectional associations between 11 and 12 year old children’s bone health and (1) durations, (2) patterns, and (3) combined durations and patterns of moderate-vigorous physical activity (MVPA) and sedentary behaviour.

Methods: Design: Population-based cross-sectional study nested within the Longitudinal Study of Australian Children. Participants: 11–12 year olds attending the Child Health CheckPoint physical module. Exposures: MVPA and sedentary behaviour (7-day wrist-worn accelerometry) yielding; (1) daily average durations (hours/day), and (2) patterns (power law alpha, representing the relationship between frequency and length of bouts of activity). Outcomes: Peripheral quantitative computerised tomography, yielding tibial bone density (cortical and trabecular), geometry (endosteal and periosteal circumference) and strength (polar stress-strain index (SSI)). Analysis: Multivariable regression models adjusting for sex, age, height, puberty, neighbourhood disadvantage, body fat percentage and muscle cross-sectional area (Aims 1–2), and mutually for durations and patterns (Aim 3). Interaction tests also assessed the effect of child sex.

Results: Of the 3,764 eligible children, 866 (23%) had both bone and accelerometry data available (mean age 11.4 years (S.D. 0.5); 49% boys). On average, children accumulated 0.6 (S.D.: 0.5) hours/day) of MVPA and 11.1 (S.D.: 1.2) hours/day of sedentary behaviour. Each additional daily hour of MVPA was associated with small bone health benefits, including larger periosteal and endosteal circumference (standardised effect sizes 0.26 (95% CI 0.10, 0.43) and 0.22 (95% CI 0.02, 0.41), respectively) and greater bone strength as evidenced by higher SSI (0.29 (95% CI 0.15, 0.42). Duration of sedentary behaviour showed little association with bone health. In mutual models, bone health was slightly better with patterns of longer continuous MVPA and shorter fragmented sedentary behaviour, but these largely attenuated after adjusting for duration. There were no interactions for sex.

Conclusions: In early adolescence, more time spent in MVPA is associated with better bone health. While small, these associations are of population level importance. Activity guidelines to optimise adolescent bone health may need to focus explicitly on increasing daily duration of MVPA, rather than on its pattern or on sedentary behaviour.

Conflicts of interest: The authors declare no potential conflicts of interest.

Funding: This work has been supported to date by the National Health and Medical Research Council (NHMRC) of Australia (Project Grants 1041352, 1109355), The Royal Children’s Hospital Foundation (2014-241), Murdoch Childrens Research Institute and The University of Melbourne. Research at the Murdoch Childrens Research Institute research is supported by the Victorian Government’s Operational Infrastructure Program.

Disclosure: The authors declared no competing interests.