Elsevier

Bone

Volume 112, July 2018, Pages 153-160
Bone

Full Length Article
Bone health, activity and sedentariness at age 11–12 years: Cross-sectional Australian population-derived study

https://doi.org/10.1016/j.bone.2018.04.011Get rights and content

Highlights

  • We examined physical activity and sedentariness with bone health at age 11–12.

  • Moderate-vigorous physical activity (MVPA) was strongly associated with bone health.

  • Total MVPA duration seemed much more important than fragmentation or sedentariness.

  • Activity guidelines for bone health should focus on increasing duration of MVPA.

Abstract

Aim

To examine cross-sectional associations of children's bone health (size, density, strength) with moderate-vigorous physical activity (MVPA) and sedentary behaviour by considering: (1) duration of activity, (2) fragmentation, and (3) duration/fragmentation combined.

Methods

Design: Population-based cross-sectional study. Participants: 11–12 year-olds in the Longitudinal Study of Australian Children's Child Health CheckPoint. Exposures: MVPA and sedentary behaviour (7-day accelerometry), yielding (1) daily average durations (min/day) and (2) fragmentations (the parameter alpha, representing the relationship between activity bout frequency and bout length). Outcomes: Tibial peripheral quantitative computed tomography (bone density, geometry, strength). Analysis: Multivariable regression models including activity durations and fragmentations separately and combined.

Results

Of 1357 children attending the CheckPoint, 864 (64%) provided both bone and accelerometry data (mean age 11.4 years (standard deviation (SD) 0.5); 49% male). Mean daily MVPA and sedentary behaviour durations were 34.4 min/day (SD 28.3) and 667.9 min/day (SD 71.9) respectively for boys and girls combined. Each additional daily hour of MVPA was associated with small bone health benefits comprising greater periosteal and endosteal circumference (standardised effect sizes 0.25, 95% CI 0.10 to 0.40 and 0.21, 95% CI 0.03 to 0.39, respectively) and bone strength (0.26, 95% CI 0.14 to 0.38). Sedentary duration and fragmentation of either MVPA or sedentary behaviour showed little association with bone health.

Conclusions

In early adolescence, MVPA duration showed associations with better bone health that, while modest, could be of population-level importance. MVPA fragmentation and sedentary behaviour duration and fragmentation seemed less important.

Introduction

Peak bone mass is reached in the early twenties and accounts for approximately 60% of osteoporosis risk [1,2]. Individuals with good bone health have bones that are generally bigger, denser and hence stronger. Genetics are considered responsible for 60–80% of bone mass variation leaving only a small but important modifiable component [3]. Pre- and peri-pubertal years are considered a critical window when modifiable factors, such as physical activity, can influence bone mass accrual [2,4,5].

Structured exercise programs demonstrate small improvements in bone measures (e.g. 5% greater bone mass) [6], but may not reflect effects of habitual activity. Studies investigating habitual activity consistently show small associations between larger durations (minutes/day) of moderate-vigorous (MVPA) or vigorous (VPA) physical activity and better bone health [[7], [8], [9], [10], [11]], while larger durations of sedentary behaviour show small associations with poorer bone health [[12], [13], [14], [15]]. However, most studies have evaluated bone with dual x-ray absorptiometry (DXA) that measures areal bone density, which does not necessarily reflect true volumetric bone density due to variations in growth and bone size [16].

Peripheral Quantitative Computed Tomography (pQCT) evaluates volumetric density, as well as differential measures of bony compartments (cortical vs trabecular), geometry and strength [17]. One study has examined activity associations with bone health using pQCT in a large paediatric population sample, reporting small associations between larger durations of VPA and greater bone content [10]. However, trabecular bone was not considered, and at age 15 participants had passed peak bone mass accrual (around 12.5 years for girls and 14 years for boys) [4], reducing the potential for exercise interventions on bone [18,19]. Recent findings from a moderate-sized cohort study, using high-resolution (HR) pQCT across a wide age range (9–20 years) also suggested MVPA was associated with improved bone strength and size, especially peripubertally, while sedentary behaviour was negatively associated with bone size and cortical density [15]. However, only VPA frequency, rather than duration, seemed important for bone strength [11].

Activity fragmentation should also be investigated, as the same total duration of activity can be accumulated in different ways. For example, cardiometabolic health guidelines recommend small bouts of sedentary behaviour broken up by frequent activity [20,21]. Animal studies suggest a long period of rest between short bursts of exercise may optimise bone formation [22]. In the only study to investigate activity fragmentation and bone health in humans, Chastin et al. [12] reported longer bouts of activity interspersed with extended periods of sedentariness were associated with better bone health in 1348 8–22 year-olds – suggesting optimal fragmentation of activity could differ between cardiometabolic and bone outcomes. However, the broad age range limits generalizability to the ages of greatest interest to bone health.

This study utilised a unique opportunity to examine associations between activity accumulation and bone health using pQCT in Australian children prior to peak mass accrual. In a population-derived study of 11–12 year-olds, we aimed to examine whether bone health was cross-sectionally associated with a) physical activity and b) sedentary behaviour, considering:

  • 1.

    Duration (minutes/day),

  • 2.

    Fragmentation, and

  • 3.

    Duration and fragmentation combined.

Section snippets

Study design and participants

The Child Health CheckPoint (CheckPoint) was a cross-sectional wave within the national population-based Longitudinal Study of Australian Children (LSAC) [23]. CheckPoint data collection took place February 2015–March 2016 between LSAC's 6th and 7th waves, when children were aged 11–12 years. The project was approved by the Royal Children's Hospital Ethics Committee (HREC33225) and the Australian Institute of Family Studies Ethics Committee (AIFS14-26). A parent/guardian provided written

Sample characteristics

Fig. 1 shows the participant flow, and Table 2 the sample characteristics.

Of 3764 eligible children, 1874 (50% of Wave 6) participated in CheckPoint; 864 (23%) had both useable accelerometry and pQCT data. This analytic sample was similar in terms of age and sex to CheckPoint participants without accelerometry and pQCT data, but came from less disadvantaged neighbourhoods (mean disadvantage index 1030 (SD 59) vs 1017 (SD 60); national mean 1000 (SD 100)). As expected, girls were further through

Principal findings

This is the first study to investigate how duration and fragmentation of physical activity and sedentariness are mutually associated with bone health in a population-derived sample of young adolescents. MVPA was more strongly associated with bone health than sedentariness, with the association driven more by duration than fragmentation; nonetheless, even MVPA duration explained <5% of the variance in bone health parameters. Associations of MVPA duration with greater bone strength appeared to

Conclusions

We conclude that MVPA has a stronger relationship with bone health in early adolescence than sedentary behaviour, which showed only small negative associations. These MVPA associations were driven by duration rather than fragmentation; adolescents with longer bouts of continuous MVPA and shorter fragmented bouts of sedentary behaviour are simply those who spend more time in MVPA. This suggests that guidelines for improving paediatric bone development via activity should focus on increasing the

Acknowledgements

We would like to gratefully acknowledge all the families who gave their time to participate in the study, Julie Briody, Senior Scientist at Westmead Hospital, Sydney, for her advice and assistance with pQCT analysis, and all members of the CheckPoint team for their contributions to the project.

Competing interests

The authors declare no potential conflicts of interest, including no specific financial interests relevant to the subject of this manuscript.

Funding

WO and KL had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. 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 (Grant 2014–241), Murdoch Children's Research Institute and The University of Melbourne. Research at the Murdoch Childrens Research Institute research is supported

Contributions

MW conceived the CheckPoint study with the wider CheckPoint team including PS and TO. KL was a post-doctoral researcher and student supervisor of WO. FF prepared the data from accelerometry recordings. JV was involved with initial data collection and analysis. WO and NI prepared the data from pQCT images. FM and JM advised on statistical issues and helped to conduct the analyses. PS and TO were student supervisors and provided expert advice throughout the study bone health and, physical

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