Understanding Physical Activity in Patients With Metastatic Breast Cancer: An Analysis Rooted in the Theory of Planned Behaviour

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Abstract
Objectives: This study aims to improve understanding physical activity behaviour in patients with metastatic breast cancer, based broadly on Ajzen's Theory of Planned Behaviour (TPB). We assessed the role of different pathways in the TPB and of cancer specific barriers to find points of leverage for improving exercise support and behaviour in this vulnerable population. In addition, we explored international differences in the model's pathways. Methods: We conducted an international survey ( n = 420) amongst patients with metastatic breast cancer, including items assessing attitude, injunctive subjective norm, intention, perceived behavioural control (PBC), and self‐reported physical activity. Additionally, we examined the relative importance of health‐related barriers on motivational pathways. Results: Model fit indices indicated a reasonable fit for the TPB (cfi = 0.98, rmsea = 0.08) with small (standardized absolute coefficients 0.13–0.30) but statistically significant associations in the expected directions according to the theoretical model, except for subjective norm ( β = 0.05). The interaction of PBC with intention was statistically significant, but adding this term diminished overall model fit (cfi = 0.85, rmsea = 0.13). The presence of health‐related barriers impacted behaviour ( β = −0.19) whilst acting as a mediator for PBC on intention. International differences in the strength of model pathways suggest cultural variation. Conclusions: Findings support the applicability of the TPB for understanding and supporting physical activity behaviour in patients with MBC. Targeting perceived behavioural control may be especially effective in overcoming health‐related barriers in this population. Additionally, improving affective and instrumental attitude towards physical activity could enhance intervention outcomes.

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