Is a School-Based Physical Activity Intervention Effective for Increasing Tibial Bone Strength in Boys and Girls?

Abstract

This 16-month randomized, controlled school-based study compared change in tibial bone strength between 281 boys and girls participating in a daily program of physical activity (Action Schools! BC) and 129 same-sex controls. The simple, pragmatic intervention increased distal tibia bone strength in prepubertal boys; it had no effect in early pubertal boys or pre or early pubertal girls.Introduction: Numerous school-based exercise interventions have proven effective for enhancing BMC, but none have used pQCT to evaluate the effects of increased loading on bone strength during growth. Thus, our aim was to determine whether a daily program of physical activity, Action Schools! BC (AS! BC) would improve tibial bone strength in boys and girls who were pre-(Tanner stage 1) or early pubertal (Tanner stage 2 or 3) at baseline. Materials and Methods: Ten schools were randomized to intervention (INT, 7 schools) or control (CON, 3 schools). The bone-loading component of AS! BC included a daily jumping program (Bounce at the Bell) plus 15 minutes/day of classroom physical activity in addition to regular physical education. We used pQCT to compare 16-month change in bone strength index (BSI, mg 2 /mm 4 ) at the distal tibia (8% site) and polar strength strain index (SSI p , mm 3 ) at the tibial midshaft (50% site) in 281 boys and girls participating in AS! BC and 129 same-sex controls. We used a linear mixed effects model to analyze our data. Results: Children were 10.2 ± 0.6 years at baseline. Intervention boys tended to have a greater increase in BSI (+774.6 mg 2 /mm 4 ; 95% CI: 672.7, 876.4) than CON boys (+650.9 mg 2 /mm 4 ; 95% CI: 496.4, 805.4), but the difference was only significant in prepubertal boys (p ‫ס‬ 0.03 for group × maturity interaction). Intervention boys also tended to have a greater increase in SSI p (+198.6 mm 3 ; 95% CI: 182.9, 214.3) than CON boys (+177.1 mm 3 ; 95% CI: 153.5, 200.7). Change in BSI and SSI p was similar between CON and INT girls. Conclusions: Our findings suggest that a simple, pragmatic program of daily activity enhances bone strength at the distal tibia in prepubertal boys. The precise exercise prescription needed to elicit a similar response in more mature boys or in girls might be best addressed in a dose-response trial.

Effects of a Specialist-Led, School Physical Education Program on Bone Mass, Structure, and Strength in Primary School Children: A 4-Year Cluster Randomized Controlled Trial

“…(30) Another interesting observation from our study was the finding that the SPE program tended to improve mid-shaft cortical vBMD at the tibia in boys and radius in girls. Although similar findings were reported in prepubertal and early-pubertal girls at the tibial mid-shaft following a 16-month school-based PA intervention (6) and in prepubertal boys at the mid-femur following an 8-month school-based, weight-bearing program, (30) several other intervention trials in children have reported no effect of exercise on cortical vBMD. (26,27) In the absence of any significant exercise-induced gains in total bone area, the trend for greater gains in cortical vBMD in our study implies that there was less bone turnover (remodeling) leading to greater mineralization within the periosteal envelope of the bone.…”

“…(3) However, of the four school-based exercise interventions that used pQCT to assess bone geometry, all failed to observe any significant exercise-induced benefits on measures of tibial bone structure, (6,(26)(27)(28) which may relate to the relatively short duration of these trials (9 to 16 months). In contrast, we observed that our 4-year generalized PE program focusing on core movements, coordination and agility, skill activities, movement challenges and games, and dynamic movement control improved midshaft cortical area and/or thickness in young girls and boys, but this did not translate into greater gains in bone strength.…”

“…Previous research has shown that the osteogenic effects of loading can vary considerably along the length of the bone. (6,23) Finally, the cohort of children recruited were mainly white, from an affluent, developed country with a well-established government-funded education system, and thus the findings may not be generalizable to other populations.…”

“…Several other school-based exercise intervention have also observed a tendency for a greater gain in total bone area at certain skeletal sites in children randomized to the non-exercise arm. (6,29) Although these findings are difficult to explain, it is possible that periosteal expansion may not always represent a positive response in that less active children may experience greater endocortical resorption and thus more periosteal expansion as a compensation mechanism. Previous cross-sectional and unilateral studies in athletes (eg, tennis players) have reported that bone structural adaptations to loading during growth are maturity-dependent and sexspecific.…”

“…(3,5) With the introduction of more advanced imaging tools such as peripheral quantitative computed tomography (pQCT) that can provide a three-dimensional assessment of bone, there is also evidence that targeted weight-bearing impact exercise programs in children can improve bone structure (cross-sectional area and/or cortical thickness) and estimates of bone strength at loaded sites. (3,6,7) Collectively these findings provide important information about what can be achieved with targeted bone loading programs, but whether more generalized PA opportunities within the physical education (PE) curriculum offered to all school-aged children can enhance bone structure and strength remains uncertain.…”