Physical Activity and Children’s Bone Health

The benefits of weight-bearing physical activity, and the negative consequences of disuse, on the skeleton have been known for centuries. For example, Darwin (2) described how large, ground feeding birds acquired stronger legs through exercise and weaker wings from not flying “until, like the ostrich, they could not fly at all.” The skeletal adaptations that Darwin noted were manifest over generations; however, positive effects of weight-bearing physical activity on the growing bone have been observed over much shorter periods. In the current issue of Exercise and Sport Sciences Reviews, Gunter and colleagues (3) provide compelling evidence that physical activity is integral for developing and maintaining a strong and healthy skeleton. Growing bone may have a far greater capacity to adapt to weight-bearing exercise than does older adult bone. Thus, optimizing bone accrual during growth, particularly during the prepubertal and early pubertal years, may be the best means to reduce adult fracture risk (1). Gunter and colleagues focused on school-based physical activity interventions that resulted in persistent benefits to bone health (up to 8 yr postintervention) in boys and girls and programs that showed promise in terms of sustainability. Of particular interest to public health and education officials should be that simple exercise programs (∼100 jumps) required very little time in the school day (10–15 min·d−1) and were delivered by generalist classroom teachers. These exercise programs could be incorporated easily into the regular school curriculum in many jurisdictions. Importantly, what works in elementary schools may not be the appropriate strategy for secondary schools. This review suggests that a higher dose of impact activity may be required to stimulate the adolescent skeleton. Also, physical activity levels decline as children, particularly girls (5), move into the challenging period of adolescence. Thus, a research priority remains — to develop programs for secondary schools that engage youth and promote physical activity while ensuring adequate amounts of impact activity. This review underscores the fundamental knowledge of skeletal adaptations to weight-bearing activity gained from dual energy x-ray absorptiometry (DXA)-based studies. Nevertheless, DXA ignores subtle changes in bone structure that significantly impact overall bone strength. Innovative imaging instruments, such as peripheral quantitative computed tomography (pQCT), high-resolution pQCT, and magnetic resonance imaging, serve to improve our understanding of the mechanisms that underpin bone’s response to loading. For example, weight-bearing physical activity positively is associated with aspects of bone microstructure such as trabecular number (4). Previously, this could be measured only with invasive bone biopsies. These novel instruments will allow us to characterize more accurately surface-specific and architectural changes that contribute to bone strength gains. Although the benefits of physical activity are well established, how can we measure the potentially negative consequences of sedentary behavior on growing bone? Emerging evidence indicates that sedentary behavior (≤1.5 METs) is distinct from a lack of moderate-to-vigorous physical activity and is associated with different health outcomes (6). This distinction commonly is not made in the bone health literature. However, as accelerometry becomes more widely used to characterize children’s activity, researchers will be able to address this important question and inform public health guidelines. Finally, as noted in this review, it likely will be decades before we can say definitively whether benefits of childhood activity persist into late adulthood. However, based on the available evidence summarized by Gunter and colleagues, including their own elegant intervention and observational studies, quality moderate-to-vigorous physical activity including short bouts of impact activity during childhood can have a lasting positive effect on bone health. It is up to our generation to disseminate this message to policy makers, with the goal being to implement effective, sustainable, and wide-reaching physical activity programs. Heather M. Macdonald Department of Orthopaedics University of British Columbia Child & Family Research Institute and Centre for Hip Health and Mobility Vancouver Coastal Health Research Institute Vancouver, Canada Vina P.S. Tan Department of Orthopaedics University of British Columbia Centre for Hip Health and Mobility Vancouver Coastal Health Research Institute Vancouver, Canada and School of Health Sciences Universiti Sains Malaysia Malaysia

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