Children and adolescents are encouraged to maintain a habitually active lifestyle because of the known health benefits associated with regular physical activity, but there are some reports that a high level of activity may be associated with increased fracture risk. This prospective controlled exercise intervention study in prepubertal children evaluated if a school-based exercise intervention could enhance growth-related gains in muscle strength and muscular function without affecting fracture risk.Methods
Fractures were registered in 417 girls and 500 boys age 7–9 yr in the intervention and in 836 age-matched girls and 872 boys. The intervention included 40 min·d−1 of school physical education for 2 yr, whereas the controls achieved 60 min·wk−1. In a subsample consisting of 49 girls and 80 boys in the intervention and 50 girls and 53 boys in the control group, body composition was measured by dual-energy x-ray absorptiometry, muscle strength by isokinetic peak torque (PT) of the knee extensors, and flexors at 60 and 180°·s−1 by a computerized dynamometer and neuromuscular performance by vertical jump height.Results
The rate ratio (95% confidence interval) for children in the intervention group to sustain a fracture was 1.07 (0.66–1.68). The annual gain in knee extensor PT at 180°·s−1 was significantly higher for both girls (P < 0.001) and boys (P < 0.01) in the intervention compared with the control group. Boys in the intervention group also had a greater annual gain in knee flexion PT at 180°·s−1 (P < 0.001), and girls had a greater gain in vertical jump height (P < 0.05).Conclusion
An increase in school-based physical education from 60 to 200 min·wk−1 enhanced muscle strength in prepubertal children without affecting fracture risk.