Academic journal article Canadian Journal of Public Health

Trajectories of Objectively Measured Sedentary Time among Secondary Students in Manitoba, Canada in the Context of a Province-Wide Physical Education Policy: A Longitudinal Analysis

Academic journal article Canadian Journal of Public Health

Trajectories of Objectively Measured Sedentary Time among Secondary Students in Manitoba, Canada in the Context of a Province-Wide Physical Education Policy: A Longitudinal Analysis

Article excerpt

Participation in physical activity (PA) has long been recognized as a key component to healthy living across the life span.1 Despite this knowledge, PA levels of Canadians continue to remain low and the prevalence of overweight and obesity has risen over the past several decades, particularly among adolescents.2 In Canada, less than 7% of adolescents accumulate the recommended minimum of 60 minutes of moderate-tovigorous- intensity physical activity (MVPA) per day,3 and most accumulate more than 9 hours of total sedentary time (ST) daily.4 The phenomenon of low PA coupled with high amounts of ST among young Canadians is raising alarm among local scientific communities and public health experts due to potential association with chronic disease risk factors.1

A growing body of evidence now links sedentary behaviour (SB), characterized by minimal energy expenditure (i.e., ≤1.5 metabolic equivalents) during waking hours, to adverse health outcomes, including weight gain,5,6 increased metabolic risks,6-9 and early mortality,10 regardless of PA levels.7,11,12 Moreover, individuals can meet the minimum MVPA recommendations per day but exceed SB recommendations by staying in a prolonged sitting or reclining posture throughout the remainder of the day.13 In light of the growing concern of low PA and high sedentariness among young Canadians as well as their independent health risks, public health efforts make recommendations for increasing PA as well as limiting ST.1,3 Although both PA and ST are issues, ST has been less often the subject of investigations to improve health of adolescents.14 While available evidence suggests that interventions with a focus on PA may result in a modest reduction in ST among adults,14 no existing research in Canada has evaluated the potential of population-based policies promoting PA on adolescents' ST trajectories.

Adolescence may be an important developmental period during which SB is established.15,16 According to the National Health and Nutrition Examination Survey (NHANES 2003-2004) US adolescents (aged 16 to 19) spent 2 hours more per day being sedentary as compared to US children (aged 6 to 11).15 Similarly, ST increased by 1.5 hours per day from age 12 to 16 among participants of the Avon Longitudinal Study conducted in the UK.16 However, available data tracking objectively measured (through accelerometers) ST during childhood and/or adolescence in Canada are sparse. Only recently has Canada incorporated objective measures of ST into its national cross-sectional surveys (i.e., since CHMS 2007-2009).4,17 To our knowledge, no studies in Canada have collected longitudinal data to objectively measure adolescents' ST over the course of secondary school. Furthermore, little is known about factors associated with ST trajectories in this population.

Therefore, the purpose of this study was to describe the longitudinal changes in ST among a sample of adolescents in Manitoba between 2008 and 2011 in the context of a provincewide physical education (PE) policy, and to identify student and school-neighbourhood factors associated with ST trajectories.

METHODS

Study design and population

This research was conducted as part of a prospective cohort embedded within the Manitoba Increasing Physical Activity in Secondary Students (MIPASS) study evaluating the impact of a province-wide secondary school PE policy on student PA.18 Introduced in September 2008, the PE policy mandates students accumulate a minimum of 30 minutes of MVPA per day on at least 5 days per week, in or out of classes.18 Details of the Manitoba secondary school PE policy are described elsewhere.18,19 Data were collected through a two-stage process whereby secondary schools (n = 31) offering grades 9 through 12 with enrolment greater than 100 were randomly selected in blocks to represent the urban and rural geography of Manitoba.18 In each selected school, a convenience sample of grade 9 or 10 PE classes was recruited. …

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