Limitations and future directions
Based on the present findings, it is not entirely clear which micro interventions are most effective in changing self-control and physical activity. Future research is needed to disentangle the specific active ingredients of the intervention. The results on increasing self-control and physical activity in the comparison group, which received only daily app-based reminders and feedback on the Fitbit device, suggest the effectiveness of these micro interventions for physical activity promotion. Second, although the allocation into the two conditions was computer generated to ensure that the conditions were fully randomized with respect to participants’ baseline characteristics, the two conditions differed in some of the baseline characteristics. In our analyses, we controlled for these characteristics to statistically adjust those group differences. Third, the 1-week baseline before the beginning of the intervention might have been too short to track participants’ usual activity behavior. During the first week, participants might be highly motivated to be more active than they were previously just because of the new Fitbit device. In the future, a longer timespan to track usual pre-intervention activity behavior may be worthwhile before testing the effects of the intervention. Fourth, the sample size of this study is relatively small which limits generalizability of our findings. Future research is needed to replicate the findings in larger samples that would allow for further analyses (e.g. to identify for which participants the intervention is most effective). Finally, the 4-week follow-up period after the end of the intervention was relatively short. Future research is needed to replicate the current findings as well as to include longer follow-up periods to test the ideal timing of such an intervention.