Physical inactivity is a major public health concern. This Perspective advocates for a complex systems approach to physical activity, emphasizing dynamic, interdisciplinary strategies to design more effective, theory-informed interventions.

Effort plays a crucial role in shaping behavior. We propose that physical effort – and its perception – modulates people’s engagement across different stages of behavioral regulation: before, during, and after engagement. We demonstrate that individuals tend to avoid effort before engagement, minimize effort during task performance, and derive a sense of reward from effort after engagement due to an effort-justification mechanism. This dynamic and stage-specific approach moves beyond static models of effort (de)valuation, offering a more nuanced understanding of how effort shapes behavior. Focusing on physical activity, we explore how these specific effort-related mechanisms could promote physical activity by fostering conditions where individuals engage in active behaviors because of the potential rewards effort brings, despite its associated cost.