Despite the health benefits associated with regular physical activity only a small percentage of young people are meeting the physical activity recommendations. There is a need to further understanding of the factors that influence physical activity behaviour in young people to inform intervention programmes. This thesis provides six studies focusing on the objective measurement of young people's physical activity as well as social support for physical activity. Chapter 2.1 describes a systematic review of quantitative research examining parental influences on different types and intensities of physical activity in young people. Chapter 2.2 describes a systematic review of qualitative research examining the role of parents in young people s physical activity. Both reviews were conducted to examine the state of the current literature focused on parental influences on young people s physical activity and were used to inform the direction of the research in later chapters. Chapter 3 describes two cross-sectional studies examining the effects of key decisions researchers must make when using accelerometers on accelerometer ouput in children and adolescents. Chapter 3.1 describes a study examining the effect of epoch length on physical activity intensity in children and adolescents. Chapter 3.2 describes a study examining the impact of accelerometer processing decision rules, such as cut-points and non-wear period, on children s and adolescents physical activity. The purpose of these studies was to systematically explore the pre- and post-data collection decisions associated with accelerometer use on accelerometer output in young people and inform accelerometer use in chapters 4 and 5. Chapter 4 was designed to explore activity-related parenting practices and children s (7-10 years) objectively measured physical activity. Chapter 5 describes a study examining five sources of social support and adolescent s physical activity measured two ways. This thesis demonstrated that parents play in key role in their child's physical activity through a variety of support avenues and in adolescence support for physical activity provided by peers appears to be important in shaping physical activity behaviour. Targeting such facets of the social environment offers a potentially useful avenue for interventions designed to increase physical activity. Finally, this thesis also demonstrated that there are a number of challenges with accelerometer use particularly in the area of processing data. The rich information provided by accelerometers makes them an invaluable tool to understand the complex nature of young people's physical activity behaviour but further work needs to be conducted on standardising methods for cleaning, analysing and reporting accelerometer data.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.