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.

Objectives: Canadian adolescents' sedentary behaviour (SB) is poorly understood and greatly understudied compared to physical activity (PA). Accumulating evidence suggests that SB poses long-term health risks regardless of PA levels. To design effective interventions that target SB, it is critical to first understand adolescents' sedentary time (ST) trajectories in a Canadian context. Therefore, we examined longitudinal trajectories of Manitoba students' ST from 2008 to 2011 and identified associated factors in the context of a province-wide physical education (PE) policy.

Methods: Secondary schools offering grades 9 through 12 were randomly selected in blocks to represent the urban and rural geography of Manitoba. In each selected school (n = 31), a convenience sample of grade 9 or 10 PE classes was recruited, leading to a final sample of 447 students. To assess ST, participants wore accelerometers on 7 consecutive days at baseline (2008) and during at least one follow-up period (2009, 2010 and 2011).

Results: At baseline, students accumulated an average of 540 minutes/day of ST. Over the course of secondary school, students' ST trajectories remained stable. Females compared to males had a slightly higher rate of decline in ST (p = 0.035), adjusting for socio-demographic variables. ST trajectories were not associated with baseline PA, body mass index and school neighbourhood socio-economic status.

Conclusion: Adolescent ST remained high throughout secondary school. SB may be well established by early adolescence and track through late adolescence. Our findings suggest the potential need for additional interventions to reduce SB before and over the course of secondary school.