Introduction:​ Osteoporosis and related fracture are significant societal health burdens. Physical activity during childhood can result in significant bone health benefits, which may reduce the risk for osteoporosis later in life. Aim:​ The primary aim was to evaluate the effectiveness of a school-based physical activity model, Action Schools! BC (AS! BC), for enhancing bone mass and strength in boys and girls using novel bone imaging technologies.

Methods:​

Design and Participants:​ This was a 16-month cluster randomized, controlled, school-based intervention. Ten schools were randomized to Intervention (INT, 7 schools) or Control (CON, 3 schools). The bone-loading component of AS! BC was a daily, high-impact jumping program (Bounce at the Bell) plus 15 minutes/day of classroom physical activity. Participants were 514 children aged 9-11 years at baseline.

Bone Measurements:​ Tibial bone strength was assessed with peripheral quantitative computed tomography. Dual energy x-ray absorptiometry and hip structure analysis were used to assess femoral neck bone mineral content (BMC) and bone strength, respectively.

Results

Part 1:​ Cross-sectional Comparisons In pre- and early pubertal boys tibial bone strength was 5-15% greater than in pre- and early pubertal girls. After adjusting for tibial length, muscle cross-sectional area was the primary explanatory variable of bone strength in both sexes.

Part 2:​ 16-Month Change - Tibial Bone Strength Intervention boys (n = 147) tended to have greater gains in tibial bone strength than CON boys (n = 64);​ however, the intervention effect was only significant for prepubertal boys at the distal tibia. Action Schools! BC was not effective for increasing tibial bone strength in girls (n = 137 INT, 65 CON).

Part 3:​ 16-Month Change - Femoral Neck Bone Mass and Strength Intervention girls tended to have greater gains in femoral neck bone strength and BMC than CON girls;​ however, the difference in change between groups was not significant. Action Schools! BC was not effective for increasing femoral neck bone mass or strength in boys.

Summary:​ Skeletal adaptations to the AS! BC intervention were sex-, maturity- and site-specific. Action Schools! BC offers promise as a simple and inexpensive strategy for increasing bone mass and strength in boys and girls.