title: 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.
  
  
  BACKGROUND
  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.
  
  
  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, mg2/mm4) at the distal tibia (8% site) and polar strength strain index (SSIp, mm3) 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 mg2/mm4; 95% CI: 672.7, 876.4) than CON boys (+650.9 mg2/mm4; 95% CI: 496.4, 805.4), but the difference was only significant in prepubertal boys (p=0.03 for group x maturity interaction). Intervention boys also tended to have a greater increase in SSIp (+198.6 mm3; 95% CI: 182.9, 214.3) than CON boys (+177.1 mm3; 95% CI: 153.5, 200.7). Change in BSI and SSIp 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.
authors.affiliation: School of Human Kinetics, and Department of Orthopaedics, Faculty of Medicine, University of British Columbia, Vancouver, Canada.
authors.forename: Heather M
authors.initials: HM
authors.lastname: Macdonald
authors.forename: Saija A
authors.initials: SA
authors.lastname: Kontulainen
authors.forename: Karim M
authors.initials: KM
authors.lastname: Khan
authors.forename: Heather A
authors.initials: HA
authors.lastname: McKay
mesh_terms: D015519:Bone Density / Q000502:physiology*; D002648:Child; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology; D010809:Physical Fitness; D012574:Schools; D013977:Tibia / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology*