Physical Activity, Bone Mass and Bone Structure in Pre-pubertal Children

Abstract: Low bone mineral density (BMD) is a risk factor for fractures and much of the fracture preventive effort today is devoted to preventing low BMD. Physical activity (PA) has been described as one of the best strategies to optimize skeletal development in the growing child. However, most exercise intervention studies in children have involved the use of volunteers and specifically designed high impact exercise programs. This thesis evaluates whether a general school-curriculum-based, moderately intense exercise intervention program and the mode of transportation to school, could influence skeletal development in a population-based cohort of pre-pubertal children. The study subjects were recruited from first two years’ evaluations of Paediatric Osteoporosis Prevention (POP) Study. 81 boys and 53 girls aged 7–9 years at baseline were included in a curriculum-based exercise intervention program comprising 40 minutes of PA per school day. Age and gender-matched boys (57) and girls (50), assigned to the general Swedish school curriculum of 60 minutes PA per week, served as controls. Furthermore, the effect of the mode of school transportation was compared between children who walked or cycled to school and those who traveled by bus or car. Bone mineral content (BMC) and BMD were evaluated by dual energy X-ray absorptiometry and hip structure analysis software evaluated the structural properties of the femoral neck (FN). Level of PA was estimated through questionnaires and accelerometers. Both boys and girls in the intervention group had significantly higher accrual of BMC and larger gain in bone size in the third lumbar vertebrae. No exercise-induced effects were observed at the FN. Walking or cycling to school was not associated with higher accrual of bone mineral or beneficial gain in bone size in boys or girls. The PA measured by accelerometers was high such that all children reached the international recommended level of 60 minutes of moderate to vigorous PA per day. Children who participated in the exercise intervention groups were reported to experience more of the highest intensities of physical activities. This thesis has identified that a school-based exercise intervention program in pre-pubertal children enhances the skeletal benefits at the lumbar spine but not bone mineral accrual or structural changes at the femoral neck. In contrast, the mode of school transportation does not influence skeletal traits in this cohort with a generally high level of everyday PA and a short journey distance to school.