How does a school-based intervention impact students' social cognitions on reducing sedentary behavior over 14 months?

Despite sedentary behavior being ubiquitous in students and detrimental to health, interventions specifically targeting it are mostly restricted to leisure time screen time reduction. With six weekly sessions alongside a poster campaign and an additional teacher intervention, the Let's Move It trial delivered environmental and psychological strategies to increase physical activity (PA) and reduce sedentary behavior (SB) in vocational schools, an understudied environment for behavioral interventions. Participants in the intervention arm considerably reduced sedentary time post-intervention. To investigate how social cognitions about restricting SB, as defined by the Reasoned Action Approach, change in intervention and control arms, self-reported data on social cognitions was collected as part of a cluster-randomized controlled trial from 1166 students (59% female, m = 18.7 years, range: 16-49) in six vocational schools before, post-intervention, and 14 months post-baseline. Data were analyzed using mixed between-within repeated measures ANOVA. We found greater improvements in intention (F(1, 833) = 9.69; η = 0.01;  = .018) and descriptive norms (F(1, 831) = 13.25; η = 0.016;  < .001) in the intervention than control arm, but these effects depended on the included control variables. Generally, intervention effects leveled off from post-intervention to follow-up. The Let's Move It intervention for SB reduction showed modest, short-lived effects on social cognitions, indicating that changes in behavior are likely due to other factors like changes to the classroom environment. Optimally, SB reduction interventions should not only change behavior but produce robust changes in conscious intentions to restrict one's sitting, so that positive effects generalize to other contexts.