Bone Stiffness and its Association with Physical Activity in Children : an Epidemiological Perspective

Monitoring and promotion of skeletal development by physical activity (PA) in early life may optimize bone strength during growth in order to delay age-related bone loss and prevent osteoporotic fractures. In contrast to commonly applied x-ray methods, the assessment of calcaneal bone stiffness (SI) by quantitative ultrasound (QUS) may be an alternative method to examine bone health in children over several years, without using ionized radiation. This thesis aims to enhance knowledge on the age-specific distribution of SI, and to investigate the role of PA on SI in 2-10-year-old European children from the IDEFICS study (Identification and prevention of dietary- and lifestyle- induced health effects in children and infants). The aims of this cumulative thesis were investigated and discussed in one narrative review and three original peer-reviewed publications. Based on more than 10,000 children, this thesis provides the derivation of the first age-, sex- and height-specific reference values of SI, which point to a decline in SI until the age of 5-6 years, while an increase in SI has been observed from the age of 6 years onwards. Cross-sectional and case-control analyses of the IDEFICS data indicate a positive association between PA and SI in children and support the beneficial osteogenic effect of vigorous and weight-bearing PA as observed in previous studies, which examined bone health with x-ray methods. These comparable results as well as the developed reference data, which form the basis for comparing bone health of children with a healthy reference population, support the use of QUS in further epidemiological studies and clinical settings for monitoring bone health for instance during interventions.