Family-based physical activity (PA) interventions have proven effective in modifying PA; yet, the underlying mechanisms are currently unclear. In this study, we follow-up upon prior trial data that showed changes to child moderate-to-vigorous PA (MVPA) as a result of a family education+planning intervention compared to an education-only condition. We examined parents' perceived family PA frequency, family PA social cognitions, and family PA habit over 26 weeks between the two conditions. One hundred and two parents (of children aged 6 to 12 yrs), were recruited through advertisements and randomized to either the planning + education condition (n = 52) or an education-only condition (n = 50). Self-reported family PA, and measures of theory of planned behaviour, planning intention, and habit were completed by the contact parent at baseline, six-week, 13-week, and 26-week time-periods. The education+planning intervention increased planning intention (p <.01) and family PA (p =.06) compared to the© education-only group. The effect of condition on the putative mediators did not explain these changes (all 95% CIs crossed 0), and family PA was associated with child MVPA only at 13 weeks (p <.01). Intervention 20 assignment had a small indirect effect on family PA (favouring©the education+planning group) via Habit. Perceived behavioural©control and habit were consistent predictors of family PA (p <.05), but not PA intention or planning intention (p >.05). The planning+education intervention showed improved family PA and planning intention, yet null mediation results suggest that other variables from the family system (e.g., child motivation, other parent's motivation and plans) are likely needed to fully understand the intervention effects. The promotion of perceived behavioural control and habit to assist in family PA is recommended for future research in this population.
Keywords: Planning; exercise; habit; theory of Planned Behaviour.