Analyzing theoretical mechanisms of physical activity behavior change in breast cancer survivors: Results from the Activity Promotion (ACTION) Trial

Background: We previously reported that a physical activity (PA) behavior change intervention based on the theory of planned behavior (TPB) increased PA and quality of life in breast cancer survivors. Purpose: To examine the effects of our interventions on TPB variables and to determine if PA at 12 weeks follow-up was mediated by TPB variables at 4 weeks. Methods: Breast cancer survivors (N=377) were randomly assigned to receive either a standard public health recommendation for PA (SR group), a step pedometer alone, or one of two TPB-based behavior change interventions consisting of print materials (alone or combined with a step pedometer). For the purpose of this study, we compared the two TPB-based intervention groups (INT group) to the SR group. Results: Compared to the SR group, the INT group reported more favorable changes in instrumental attitude (mean difference=0.13; 95% CI=-0.01 to 0.23; d=0.19; p=0.077), intention (mean difference=0.33; 95% CI=0.10 to 0.56; d=0.33; p=0.006), and planning (mean difference=0.39; 95% CI=0.04 to 0.73; d=0.26; p=0.027). Mediation analyses indicated that both planning and intention partially mediated the effects of the intervention on PA at 12 weeks. Conclusions: Our TPB-based behavior change intervention resulted in small improvements in the TPB constructs that partially mediated the effects of our intervention on PA behavior. Additional research with the TPB is warranted.