Background. Digital interventions integrating gamification features hold promise to promote physical activity (PA). However, results regarding the effectiveness of this type of intervention are heterogeneous. This study aimed to examine potential moderators of the effectiveness of a gamified intervention in a large-scale sample and in real-life conditions. Specifically, we tested (1) whether a gamified intervention enhanced daily steps during the intervention and follow-up periods compared to baseline, (2) whether this enhancement was higher in participants to the intervention than in nonparticipants, and (3) what participants characteristics or intervention parameters moderated the effect of the program. Methods. Data from 4812 individuals who registered for a Kiplin program between January 1st, 2019, and January 2nd, 2022 were analyzed. PA was assessed via the daily step count of participants. Exposure to the intervention, the intervention content, and participants' characteristics were included in multilevel models to test the study objectives. Results. Compared with nonparticipants, participants who benefited from the intervention had a significantly greater increase in mean daily steps from baseline during the same period (b = 0.55, p <.0001). Daily steps of participants with lower baseline steps significantly improved from baseline both during the intervention and during follow-up periods, whereas participants with >7500 baseline daily steps had no improvement or significant daily step decreases during the intervention. Age (b = 0.05, p <.0001) and exposure (b = 0.37, p <.0001) positively moderated the intervention effect. Discussion. The Kiplin gamified intervention was effective to improve PA during intervention and follow-up periods in comparison to baseline daily step count and to nonparticipants. Responses to the intervention significantly differed as a function of individuals' initial PA. The engagement with the service and the age of the participants positively moderated the intervention effect. This study confirms the effectiveness of gamified interventions to promote PA in real-life.

AC, CF, and MD declare that they have no competing in- terests. AM PhD grant is funded by the French National Association for Research and Technology (ANRT) and Kiplin. GH is employed by Kiplin.

The work of AM is supported by an ANRT grant (Cifre PhD Thesis) and by the company Kiplin. The funders had no input in the design of the study and no influence on the interpretation or publication of the study results.

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The study was approved by local Ethics Committee (IRB00013412, CHU de Clermont Ferrand IRB #1, IRB number 2022-CF063) with compliance to the French policy of individual data protection.

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I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.

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The data and code for the statistical analyses used in the present study are available on Open Science Framework (https://osf.io/scnu7/).

https://osf.io/scnu7/