Background: It is estimated that children spent 6 to 9 hours per day in sedentary behaviour, as assessed by accelerometry (1). This has important public health implications since sedentary behaviour has been associated with poor metabolic health in children and adults, and early mortality in adults (2-4). While accelerometry provides an objective estimate of total sedentary behaviour, this measure of sedentary behaviour does not provide details on how that time is spent. This is a limitation from a preventive standpoint as it is unknown what behaviours need to be specifically targeted to reduce time spent in sedentary behaviour. It is known from self-reported diaries that there is within and between gender variability regarding sedentary behaviour profiles (5, 6). However it is unknown if sedentary behaviour profiles are associated with more or less total sedentary behaviour as assessed by accelerometry.

Aim 1: To determine if spending time in specific sedentary behaviours associate with total sedentary behaviour, MVPA and adiposity at 14-years-old

Aim 2: To determine if spending time in specific sedentary behaviours at 14-years-old associate with changes in total sedentary behaviour, MVPA and fat mass from 14- to 16-years-old.

Cluster Analysis: Cluster analysis will be used to determine if children belong to similar groups based on their self-reported sedentary behaviours (separately for boys and girls). The time spent in 14 sedentary activities on a typical day was self-reported by the children: none, less than 1hr, 1-2hrs or 3 or more hrs.

Dependent Variable 1: Total sedentary behaviour (accelerometry less than 200cpm, mins/d)

Dependent Variable 2: Total MVPA (accelerometry(cubed)3600cpm, mins/d)

Dependent Variable 3: Fat mass (DXA, kg)

Independent Variable: Sedentary cluster group

Covariates: Maternal education, maternal obesity, birth weight, and length of gestation.

Statistical Analysis: Cross-sectional analysis at 14-years using analysis of covariance (ANCOVA) and Tukey adjustment for multiple comparisons. Longitudinal analysis between 14- and 16-years using linear mixed models to determine if sedentary cluster group is associated with change in total sedentary behaviour, MVPA and fat mass over time. All analyses will be conducted separately for boys and girls.

Model Building:

Model 1 - adjusted for age

Model 2 - adjusted for age + maternal factors

Model 3 - adjusted for age + maternal factors + birth factors

Target Journals: American Journal of Preventive Medicine or MSSE

References:

1. Matthews CE, Chen KY, Freedson PS, et al. Amount of time spent in sedentary behaviors in the United

States, 2003-2004. Am J Epidemiol. Apr 1 2008;167(7):875-881.

2. Ekelund U, Brage S, Froberg K, et al. TV viewing and physical activity are independently associated with metabolic risk in children: the European Youth Heart Study. PLoS Med. Dec 2006;3(12):e488.

3. Healy GN, Dunstan DW, Salmon J, et al. Breaks in sedentary time: beneficial associations with metabolic risk. Diabetes Care. Apr 2008;31(4):661-666.

4. Katzmarzyk PT, Church TS, Craig CL, Bouchard C. Sitting time and mortality from all causes, cardiovascular disease, and cancer. Med Sci Sports Exerc. May 2009;41(5):998-1005.

5. Biddle SJ, Gorely T, Marshall SJ. Is television viewing a suitable marker of sedentary behavior in young people? Ann Behav Med. Oct 2009;38(2):147-153.

6. Gorely T, Marshall SJ, Biddle SJ, Cameron N. Patterns of sedentary behaviour and physical activity among adolescents in the United Kingdom: Project STIL. J Behav Med. Dec 2007;30(6):521-531.