ORIGINAL ARTICLE Analyzing Theoretical Mechanisms of Physical Activity Behavior Change in Breast Cancer Survivors: Results from the Activity Promotion (ACTION) Trial Jeffrey K. H. Vallance, Ph.D. & Kerry S. Courneya, Ph.D. & Ronald C. Plotnikoff, Ph.D. & John R. Mackey, M.D. Published online: 12 March 2008 # The Society of Behavioral Medicine 2008 Abstract 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 recom- mendation 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. Keywords Physical activity . Quality of life . Theory of planned behavior . Breast cancer survivors Introduction Physical activity (PA) is an effective intervention to improve quality of life (QoL), cardiorespiratory fitness, physical functioning, and fatigue in breast cancer patients and survivors [1, 2]. Furthermore, a recent prospective cohort study of almost 3,000 breast cancer survivors reported that higher levels of PA were associated with reduced risks of death, breast cancer death, and breast cancer recurrence [3]. Despite the reported benefits of PA, the majority of breast cancer survivors are not meeting public health guidelines (i.e., at least 150 min/week of moderate- to vigorous-intensity PA) [4–6]. Given these statistics, interventions are needed to increase PA behavior in breast cancer survivors [7, 8]. In order to facilitate behavior change, researchers advocate that behavior change interventions should be theoretically based [9]. Application of behavioral theories can assist researchers in understanding the mechanisms through which individuals change (or do not change) their ann. behav. med. (2008) 35:150–158 DOI 10.1007/s12160-008-9019-x J. K. H. Vallance : K. S. Courneya (*) Faculty of Physical Education and Recreation, University of Alberta, E-488 Van Vliet Centre, Edmonton, AB, Canada T6G-2H9 e-mail: kerry.courneya@ualberta.ca R. C. Plotnikoff Centre for Health Promotion Studies, School of Public Health, University of Alberta, 5–10 University Extension Centre, Edmonton, AB, Canada T6G 2T4 J. R. Mackey Department of Oncology, Cross Cancer Institute, University of Alberta, Edmonton, AB, Canada T6G 1Z2