HabitWalk: A micro-randomized trial to understand and promote habit formation in physical activity.

Habit is a key psychological determinant for physical activity behavior change and maintenance. This study aims to deepen the understanding of habit formation in physical activity and identify promotion strategies. We examined the habit formation trajectory and its relationships with cue-behavior repetition (a cue-triggered 15-minute brisk walk) and unconditional physical activity (daily steps). We also tested whether the behavior change techniques (BCTs) 'commitment' and 'prompts and cues' promote habit, cue-behavior repetition, and daily steps within persons. This micro-randomized trial included a 7-day preparatory and a 105-day experimental phase delivered via the HabitWalk app. Participants (N = 24) had a 50% probability of receiving each BCT daily, leading to four conditions. Habit strength was assessed daily using the Self-Report Behavioral Automaticity Index, while cue-behavior repetition and steps were measured via an activity tracker. Person-specific growth functions indicated that habit strength trajectories were highly idiosyncratic. Multilevel models indicated a positive effect of cue-behavior repetition on habit strength, but not vice versa. The effect of habit strength on daily steps varied by the operationalization of cue-behavior repetition. Tentative findings suggest that commitment and prompts and cues are effective habit-promotion strategies when delivered together.