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.

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