Mediators of physical activity behavior change: findings from a 12-month randomized controlled trial.

OBJECTIVE: To examine putative mediators of a 12-month motivationally tailored physical activity (PA) promotion intervention.

DESIGN: We randomly assigned 239 healthy, underactive adults (moderate-vigorous physical activity <90 min/week; mean age = 47.5 years; 82% women) to receive (a) print-based feedback, (b) phone-based feedback, or (c) contact control.

PRIMARY OUTCOME: PA at baseline, 6, and 12 months, as measured by the 7-day physical activity recall interview. MEDIATORS: Four TransTheoretical Model constructs explicitly targeted by the intervention (i.e., self-efficacy, decisional balance, cognitive and behavioral processes of change), as well as four additional constructs linked to PA behavior change (i.e., social support, outcome expectancy, PA enjoyment, exercise-induced feelings).

RESULTS: Multivariate mediation analyses were used to analyze longitudinal PA outcomes. Changes in behavioral processes and one aspect of exercise-induced feelings (revitalization) satisfied both action theory (i.e., treatment effects on mediators) and conceptual theory (i.e., mediator effects on PA) tests at 6 and 12 months and emerged as statistically significant mediators of treatment effects on PA across delivery channels (ps <.014). Cognitive processes, self-efficacy, decisional balance, and social support for PA participation satisfied Action Theory tests at both 6 and 12 months, but failed conceptual theory tests. Delayed intervention effects were observed on other aspects of exercise-induced feelings, PA enjoyment, and outcome expectancies, but these variables failed mediation testing at 12 months.

CONCLUSION: Findings are consistent with previous research illustrating the importance of behavioral processes of change, but also indicate that affective response to PA may warrant more attention as a potential target of behavior change programs.