Patterns of Sedentary Behaviour and Physical Activity Among Adolescents in the United Kingdom: Project STIL Trish Gorely Æ Simon J. Marshall Æ Stuart J. H. Biddle Æ Noel Cameron Accepted: July 31, 2007 / Published online: 22 August 2007 Ó Springer Science+Business Media, LLC 2007 Abstract The purpose of this study was to use ecological momentary assessment to investigate the patterning of physical activity and sedentary behaviours in UK adoles- cents and to examine if different lifestyle groups differ on key explanatory variables. A total of 1,371 (38% boys, mean age 14.7 years) adolescents completed diaries every 15 min for 3 weekdays outside of school hours and 1 weekend day. Cluster analysis yielded five-cluster solu- tions for both boys and girls to explain the grouping of sedentary behaviours and physical activity. The clusters demonstrated that adolescents engage in many leisure time behaviours but have one activity that predominates. Active adolescents spend more time outside and more time with their friends. Few demographic and environmental vari- ables distinguished between clusters. The findings suggest a potential need for different behavioural targets in inter- ventions to reduce sedentary behaviour in sub groups of the adolescent population. Further research is required to examine the modifiable determinants of different sedentary lifestyles among young people. Keywords Ecological momentary assessment Á Youth Á Sedentary behaviour Á Physical activity Introduction The magnitude of sedentary behaviours in youth popula- tions and the consequent effect on health is a growing public health concern (UK Department of Health 2004). There is agreement that a sedentary lifestyle is related to negative health outcomes such as heart disease, diabetes, and obesity (UK Department of Health 2004). Even small reallocations of time spent in sedentary behaviours in favour of more physically active behaviours have been shown to have significant positive health implications (Epstein and Roemmich 2001). Increasing physical activity and decreasing sedentary behaviours are therefore key factors in promoting health (Nelson et al. 2005). Despite the importance of understanding sedentary behaviours, and the factors that influence them, few studies have addressed these issues (Henning Broderson et al. 2005; Marshall et al. 2002; Nelson et al. 2005). Understanding the relation- ship between sedentary behaviours and physical activity may lead to the development of more effective strategies for promoting healthy physical activity levels. The most common approach to the study of inactive lifestyles has been to focus on highly visible and prevalent sedentary behaviours such as television viewing and use of other screen based media (see for example, Henning Broderson et al. 2005; Nelson et al. 2005, Schmitz et al. 2002). Although investigating individual behaviours is important, this focus is inadequate for understanding pat- terns of behaviour or how single behaviours impact the overall lifestyle of contemporary youth (Nelson et al. 2005). In particular, this single behaviour approach fails to capture the diversity and variability in young people’s sedentary behaviour (Marshall et al. 2002). Measurement approaches are needed that can capture the multiple sed- entary and active behaviours adolescents engage in. T. Gorely (&) Á S. J. H. Biddle Á N. Cameron British Heart Foundation National Centre for Physical Activity and Health, School of Sport & Exercise Sciences, Loughborough University, Loughborough, Leics LE11 3TU, UK e-mail: p.j.gorely@lboro.ac.uk S. J. Marshall San Diego State University, San Diego, USA 123 J Behav Med (2007) 30:521–531 DOI 10.1007/s10865-007-9126-3