Muscle function and physical activity in pre-pubertal school children

Abstract: Background: Regular participation in physical activity or organized exercise is regarded as one important lifestyle factor that could improve musculoskeletal health, fitness and body composition. For children and adolescents physical activity is important not only in training of the neuromuscular system, coordination and balance, but in a wider perspective, for adopting a lifestyle that possibly reduces the risk of coronary artery disease, obesity, osteoporosis and other contributors to morbidity and mortality in the population. Previous studies have investigated the effects of moderate physical activity on fitness, obesity and bone health. The pre- and early pubertal period is usually considered as the best opportunity to enhance skeletal strength, but whether the same applies to neuromuscular function has seen less evaluation. In addition, muscular strength in pre-pubertal children has been shown to benefit from resistance training but whether similar benefits can be achieved by a moderately intense exercise programme is unclear. Hence, before we can recommend physical activity as prevention against low muscle strength and impaired neuromuscular function, we ought to show effects of intervention programmes on a population-based level with moderately intense exercise so that virtually all children can participate. School has been regarded as the ideal and perhaps only arena that could include all children within a defined population. This is the reason why this thesis was designed to present normative data for muscle strength and soft tissues in pre-pubertal children, and also to evaluate whether a general school curriculum-based moderately intense physical education intervention programme and mode of transportation to school could influence the development of neuromuscular function. Materials and Methods: The participants in this thesis were collected from the Pediatric Osteoporosis Prevention (POP) cohort. A school-curriculum-based exercise intervention programme comprising 40 minutes of physical education per school day was initiated and 68 boys and 53 girls aged 6 to 8 from one school were included in the intervention group. Age- and gender-matched children in the control group (46 boys and 50 girls) from three neighbouring schools were assigned to the general Swedish school curriculum of 60 minutes of physical education per week. When evaluating the effect of the mode of school transportation, comparisons were made between children who reported to have walked or cycled to school, and those who had commuted by bus or car. Physical activity was measured objectively by accelerometers worn for four consecutive days. Accelerometer measurements, however, were not done until two years after study start. In the cross-sectional normative data report (paper IV) 246 boys and 190 girls aged 6 to 12 in Tanner stages I and II were included. Measurements were conducted before any intervention was initiated. Neuromuscular function was assessed by concentric isokinetic peak torque of the knee extensors and flexors at 60° and 180°/s and with vertical jump height test. Total body soft tissue composition was evaluated by dual energy X-ray absorptiometry and body mass and stature by standard measurements. Level of physical activity was subjectively estimated through questionnaires. In the prospective part of the study (papers I, II and III), neuromuscular and soft tissue development was followed for one year in the intervention group and control group. Results: The normative evaluation reported improved performance and higher values in all anthropometric measurements with advancing ages, and with no constant gender differences across the age groups. Both boys and girls with increased school-based physical educational classes had higher annual gains in different muscle strength indices, but not in vertical jump height, than the control group. Walking or cycling to school was not associated with beneficial gain in muscle strength indices or vertical jump height in either boys or girls. The level of physical activity measured by accelerometers was so high in these children that all reached the level recommended by the United Kingdom Expert Consensus Group of 60 minutes of moderate-to-vigorous daily physical activity. Conclusion: Normative gender-specific data on muscle strength, lean and fat mass and vertical jump height show that in Swedish children aged 6 to 12 in Tanner stage I and II there seems to be an increase with age and no structural gender differences. Daily physical education classes in school could be used in pre-pubertal boys and girls as a strategy to increase muscle strength, at least in the short term. These findings suggest increased physical education in pre-pubertal children as one possible strategy to improve neuromuscular function on a population level in society. In contrast, the mode of school transportation seems not to influence the measured traits, at least not in young children with a generally high level of everyday physical activity.