Financial incentives have been shown to promote exercise and may be an effective strategy for improving public health [1]. Unlike other gamification elements commonly embedded in physical activity apps—such as points and levels—financial incentives provide tangible, extrinsic rewards that can independently motivate behavior change. We previously developed a smartphone app that sets cadence-based walking goals and provides financial incentives upon goal completion, focusing on intensity rather than simple step counts [2]. This approach increased moderate-to-vigorous physical activity (MVPA), which is expected to improve health outcomes that step-count targets alone might not effectively achieve. The ultimate goal of this app is to demonstrate that consistent usage can help effectively manage chronic diseases such as hypertension and diabetes. Therefore, high app adherence is crucial; however, the optimal method for delivering incentives to maintain such adherence remains unclear. Prior studies comparing fixed versus lottery incentives have yielded mixed results [3,4]. Therefore, the key aim of this study was to evaluate whether fixed or lottery incentives more effectively promote engagement with our app. Analogous to “Phase 1” in drug development, this study tested the effect of these incentive structures on app engagement in a cohort of relatively healthy volunteers. The findings of this study are intended for digital health application developers who are seeking empirical data on the optimal delivery of financial incentives.

From September 2023 to March 2024, 20 participants were enrolled. This sample size provided 80% power to detect a difference of 3.3 challenges per week. Two participants dropped out during the first sequence but were included in the ITT analysis. Their baseline characteristics were as follows: 55% of participants were female, with a median age of 26 (range 19-49) years and a mean BMI of 21.5 (SD 2.2).

Regarding the primary end point, there was no significant difference in successful weekly challenges between the fixed- and lottery-incentive apps (3.4 times per wk vs 3.7 times per wk; P=.75; Table 1 and Figure 1A). Likewise, no significant differences were observed in daily walking distance (5806 m vs 5622 m; P=.72; Table 1 and Figure 1B) or daily step counts (8495 steps vs 7917 steps; P=.59; Table 1 and Figure 1C).

In this randomized crossover trial, fixed and lottery incentives produced similar levels of engagement with our app. Challenge success—our measure of engagement—requires sustained physical effort and is distinct from passive app usage. The overall challenge success rate was relatively low (3.4 to 3.7 successes per wk), suggesting that while financial incentives prompt participation, the magnitude of the reward, or the strict 15-minute time constraint, may have limited broader engagement.

Our findings contrast with those of studies highlighting the behavioral superiority of lottery models or loss-framed incentives [4], indicating that the most effective incentive structure may depend on specific app mechanics, goal difficulty, target demographics, or cultural contexts. Since neither approach showed clear superiority, we adopted the fixed-incentive model for subsequent development due to its administrative simplicity. A larger clinical trial is ongoing to evaluate whether this approach can improve long-term health outcomes.