Abstract
Introduction: Existing behaviour change theories often frame physical activity (PA) engagement and sustainment as linear, stage-like processes and treat self-efficacy or intention as stable drivers across individuals and contexts. However, PA emerges from fluctuating internal and external conditions, requiring models that capture this complexity and guide intervention.Method: Guided by participatory systems science and Theory Construction Methodology, we developed the Integrative Model of PA as a prototheory through three stages. Participatory systems modelling interviews with 26 adults elicited participant-specific accounts of everyday PA. Abductive synthesis and ontological mapping linked participant-derived concepts with behavioural constructs. Expert consensus and framework comparison integrated configurational condition-set logic, person- and context-specific activation regions, state-dependent value integration, and multi-timescale adaptive feedback.Result: Participants described PA as emerging when situational and personal features aligned, creating 'moments to act' and altering the probability of transitions among PA-related states. These narratives informed the prototheory in which state-dependent valuation and adaptive feedback shape PA across timescales.Conclusion: The Integrative Model of PA conceptualises engagement and sustainment as nonlinear, adaptive valuation processes shaped by configurational patterns and multi-timescale feedback. Integrating lived experience with prior theory increases representational precision while preserving idiographic specificity. It generates testable hypotheses and supports adaptive, person-centered interventions and cumulative theory development in PA.