Predictive ability of the theory of planned behavior on physical activity in newly diagnosed women with breast cancer: a prospective cohort study

Purpose: Few studies assess how well the Theory of Planned Behavior (TPB) predicts physical activity over time after a breast cancer diagnosis. This study evaluated TPB’s prediction, measured at diagnosis, on women’s physical activity 1 year later. Methods: The Alberta Moving Beyond Breast Cancer (AMBER) study follows women recently diagnosed with breast cancer (n = 1528; average age 55.4). Participants completed questionnaires on TPB variables about recreational activity shortly after diagnosis. At 1 year, actiGraph GT3X+® devices measured light and moderate-vigorous activity, and self-reported activity over the past year was assessed with the Past Year Total Physical Activity Questionnaire. Structural equation models evaluated TPB variables’ ability to predict physical activity after 1 year. Results: At diagnosis, positive attitudes (β = 0.51; p < 0.001) and greater perceived behavioral control (β = 0.16; p < 0.001) were associated with greater intentions for physical activity derived from device-measured (R² = 0.51, p < 0.001) and self-reported questionnaire (R² = 0.54, p < 0.001). The TPB at diagnosis was associated with greater device-measured MVPA at 1 year (β = 0.13; p < 0.01). The TPB was not associated with device-measured light-intensity physical activity (β = 0.07; p = 0.08) or self-reported recreational physical activity (β = 0.08; p = 0.07). Conclusion: The variance explained by the TPB for physical activity at 1 year after breast cancer diagnosis was minimal. These findings highlight the need to identify additional factors influencing intentions and long-term activity. Implications for Cancer Survivors: Incorporating behavior change strategies that influence attitudes toward physical activity may positively impact intentions at diagnosis. To ensure long-term activity among breast cancer survivors, supportive care should include interventions addressing factors beyond initial intentions.