Bridging the Intention-Behavior Gap in Physical Activity: Towards a Dynamic Within-Person Perspective

The world is facing a physical inactivity ‘pandemic’. Almost a third of the global adult population are not meeting the recommended levels of physical activity (PA) set by the World Health Organization (WHO). This insufficient PA is a leading risk factor for non-communicable diseases, such as heart disease, type 2 diabetes, and obesity, that lead mortality rankings all over the globe. While individuals frequently set goals to exercise more, a substantial gap persists between these intentions and their actual behavior. This is commonly referred to as the intention-behavior gap. Building on social-cognitive (e.g., the Health Action Process Approach) and dual-process theories (e.g., Temporal Self-Regulation Theory), this dissertation investigates when and how individuals translate their goal intentions into day-to-day PA behavior.Three empirical papers form the core of this work. Paper 1, using ecological momentary assessment (EMA), found that setting short-term PA intentions (referring to four-hour windows) was associated with a sharp increase in the likelihood of actually engaging in PA within that timeframe (~28-fold). Yet, a quarter of these intentions still went unrealized. Moreover, the translation of these short-term intentions into behavior was more likely when individuals planned more specifically, felt more self-efficacious about enacting the momentary intention, and momentary intention strength was higher. Paper 2 showed that positive, high-energetic affective states (e.g., feeling energized) and low anticipated contextual barriers (e.g., minimal scheduling constraints) facilitated the formation of such short-term intentions, although these internal and external context factors were not directly associated with actual PA engagement. Paper 3, an intervention trial testing reinforced implementation intentions in a guided mobile health approach, demonstrated significantly higher moderate-to-vigorous PA in the intervention group – potentially through fostering short-term intention formation, their enactment, and context-triggered, ‘spontaneous’ exercise bouts.The findings collectively highlight that PA behavior is shaped by an interplay of reflective (planning, self-efficacy) and automatic or impulsive (affect, environmental cues) processes that become active on a momentary level. Introducing the concept of “transitional intentions” (TIs) – situated, time-bound derivatives of goal intentions – may further illuminate how momentary fluctuations in reflective and impulsive processes contribute to the intention-behavior gap. Building on these insights, this dissertation provides practical recommendations for the design of Just-in-Time Adaptive Interventions (JITAIs) that can provide personalized PA support when it is needed most.