Testing the HAPA model for predicting daily physical activity of women survivors of breast cancer

The power of the Health Action Process Approach (HAPA) for understanding interindividual health outcomes is well-documented. However, its applicability in explaining intraindividual evolution of constructs over time remains under-researched. This study addresses this gap by conducting nine longitudinal N-of-1 studies to estimate how the HAPA constructs predict physical activity (PA) at the intraindividual level in breast cancer survivors over-time. PA actual behaviors (<i>N</i> = 338) of nine women survivors of breast cancer were observed over 6-weeks, and associations between HAPA constructs and PA were analyzed longitudinally. Time-series regression analyses revealed that self-efficacy and social-support significantly predicted participants' intention to engage PA. Recovery self-efficacy emerged as predictor of PA in three studies, while planning was predictor in one. Future interventions targeting PA after breast cancer should consider individual and daily variations, and should focus self-efficacy, social-support, and planning. This highlights the importance of tailoring interventions to promote PA to specific and individual needs.