Teun Remmers

General introduction | 7 The fourth aim of this thesis was to apply these optimized methodologies in investigating the influence of the objective environment on children's PA patterns, and to investigate the development of these patterns in the transition from childhood to adolescence. Outline of this thesis The studies presented in this thesis have used data from various cohorts. First, the KOALA study is a birth cohort study that started in the year 2000 with the recruitment of around 2800 healthy pregnant women from the general Dutch population (68). Initial focus of this cohort was on the development of children's asthma and allergies, but subsequently measurements of growth and (un)healthy behaviors such as PA were also performed. Second, the PHASE-kids study is a cohort study that aimed to investigate the development of PA patterns of Dutch children transitioning from primary to secondary school, and relationships with the environment. Furthermore, this thesis also used data from the Be Active Eat Right study, which is a cluster-randomized controlled trail on the effectiveness of an overweight-prevention protocol in 44 Youth Healthcare centers in the Netherlands (69). In addition, data was used from the Active Living study, which is a school-tailored multicomponent intervention study focusing on improving attributes of the physical and social environment in the vicinity of 21 Dutch primary schools (70). Finally, this thesis also used data from the Patterns of Habitual Activity across Seasons (PHASE) study, which used a repeated measures design to examine seasonal changes in children’s activity levels in Melbourne, Australia (71). The current thesis reports on a series of studies to improve our understanding of individual and environmental determinants of PA in childhood and adolescence (see Table 1). Chapter 2 reports on a study that investigated the longitudinal relationship between PA and the development of Body Mass Index (BMI) in 4-9-year-old children. Chapter 3 focuses on the association between PA enjoyment and PA behavior, and discusses moderating influences of several socio-demographic factors and personality traits. Chapter 4 describes a longitudinal study on children's parent-reported outside play and determinants of the social- and perceived physical environment, whereas Chapter 5 also focuses on moderating factors in the relationship between the social- and physical environment and children's subjectively measured outside play. Chapter 6 presents a study on the influence of daily weather on children's objectively measured PA across the four seasons, and whether this relationship was moderated by day of the week (i.e. weekday versus weekends), socio-demographics, and children's BMI. Chapter 7 and 8 focus on relationships between objectively measured attributes of the physical environment and accelerometer-measured afterschool PA in 8-12-year-old children. Finally, Chapter 9 describes a longitudinal study on context-specific PA patterns of children in the transition from primary to secondary school, and investigates whether PA patterns differ by gender and differences in home-school distances as a result of the change to secondary school.