Effect of Physical Activity on Bone, Muscle and Fracture Risk during Growth

Sammanfattning: Osteoporosis and related fractures are a global health problem. Physical activity, especially during growth, has been suggested as a strategy to improve musculoskeletal health in the population. Previous prospective pediatric exercise intervention studies, however, are short-term, mostly covering less than one year and use bone traits as surrogate endpoints for fractures. The Pediatric Osteoporosis Prevention (POP) study is a prospective, controlled exercise intervention study, which was designed to annually assess musculoskeletal development and fracture risk in response to increased physical education in school in children aged 7-9 years and onwards, at a population-based level. The data presented in this thesis cover the results for the first two to four years from the POP study.
The intervention included 40 minutes/day of school physical education in one school whereas children from three schools in the same area served as controls achieving the Swedish standard of average 60 minutes of physical education per week. All children with school start between the years 1999 and 2008, assigned to the four participating schools in the POP study, were included in the fracture registration. In a subsample of children with school start during two years, annual measurements of musculoskeletal traits were done. Also, as part of this project, potential skeletal benefits achieved by consistently walking or cycling to school compared to if going by bus or car for two years were investigated.
Fractures were prospectively registered in a cohort of 2625 children for up to two years and in 2395 children for up to four years. Muscle strength was evaluated by isokinetic Peak Torque (PT) of the knee extensors and flexors at 60 and 180 º/second by a computerized dynamometer and neuromuscular performance by Vertical Jump Height (VJH) in 129 children in the intervention and 103 children in the control group, for two years. Bone mineral content (BMC; g) and bone width (cm) were followed by means of dual X-ray absorptiometry (DXA) for four years, in the measured subsample of 121 children in the intervention and 100 children in the control group.
The fracture risk was not higher in children receiving increased physical education in school compared to children in the control group either at the two-, three- or four-year evaluations. The rate ratio (RR) (95% CI) for fractures was 1.11 (0.78, 1.57) at the four year evaluation. The annual gain in knee extensor PT at 180º/ second was significantly higher for both genders in the intervention compared to the control group. Boys in the intervention group also had a greater annual gain in knee flexion PT at 180 º/second and girls a greater gain in VJH. The mean annual gain in lumbar spine BMC and femoral neck width was higher in both genders in the intervention compared to the control group at the four year follow-up. The mode of school transportation for two years did not influence either the annual gain in BMC or bone size in these pre-pubertal children.
This thesis concludes that increased physical education in school for two years enhanced muscle strength and increased physical activity for four years increased bone mass and size in these at study start seven- to nine-year-old children without affecting the fracture risk.