Purpose: High level of physical activity (PA) is associated with great muscle strength and high fracture risk. This prospective controlled population-based study evaluated how a pediatric PA intervention program influenced muscle strength and fracture risk.
Methods: We carried out a school-based exercise intervention program with 200 min of PA per week for 5 years in 335 girls and 408 boys aged 6-9 years at study start. An age-matched control cohort including 756 girls and 782 boys continued with 60 min of PA per week. We registered fractures during the study period and calculated rate ratio. In a sub-sample, including 74 girls and 107 boys in the intervention and 51 girls and 54 boys in the control group, we measured knee flexion and extension strength by a computerized dynamometer and leg composition by dual energy X-ray absorptiometry. Group comparisons were adjusted for differences in age, baseline value for the measured parameter and changes in height.
Results: Children in the intervention group had a rate ratio to sustain a fracture of 1.03 (0.78, 1.36) (mean and 95 % confidence interval) (p = 0.79). The annual gain in flexion peak torque muscle strength was greater in both girls (at 60°/s) [1.1 Nm (0.5, 1.8), p < 0.01] and boys (at 180°/s) [0.7 Nm (0.1, 1.2), p < 0.05] in the intervention than in the control group, while leg composition was similar.
Conclusion: Increased PA during a 5-year period, starting in the pre-pubertal period, improves the gain in muscle strength without affecting the fracture risk.
Keywords: Body composition; Fracture risk; Isokinetic peak torque; Muscle strength; Physical activity; School-based intervention.