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Background: Physical inactivity remains a public health concern, with 42% (around 1 in 2) of women and 34% (around 1 in 3) of men in the United Kingdom, for example, failing to meet moderate-to-vigorous physical activity guidelines. To promote physical activity (PA) at scale, smartphone-based mHealth (mobile health) software apps offer a promising solution. Objective: This study aims to evaluate the feasibility of implementing an mHealth app offering very small ("micro") financial incentives for PA in Leeds, United Kingdom. Methods: A 5-week single-arm proof-of-concept study was conducted with rolling recruitment among Caterpillar Health app users between September 12 and December 12, 2022 (Obesity-Related Behavioral Intervention Trial model, phase IIa). Users earned microincentives in the form of "points, " redeemable for consumer rewards (eg, movie tickets and gym passes), for meeting personalized daily step goals (US 0. 13 per goal achieved; set using data from a 5-day baseline) and completing educational quizzes (US 0. 33 per quiz). Descriptive statistics assessed feasibility outcomes (ie, reach, recruitment, retention, engagement, and acceptability) and preliminary effectiveness. Paired-samples t tests (P<. 05) examined changes in weekly mean daily step count (from baseline) and step goal achievement over 5 weeks. Results: Of 285 app downloads, 46 users consented to participate (recruitment rate: 16. 1%). Participants (mean age: 39. 9, SD 11. 1 y; 71. 1%, 33/46 woman) had a baseline step count of 5598 (SD 2664) steps/day. A total of 25 participants remained engaged (ie, completed at least 1 quiz) at study week 5 (retention rate: 54. 3%). Acceptability was high, with 75% of respondents (12/16) indicating they would recommend the app. Weekly mean daily step count did not significantly increase from baseline (mean difference 317, SD 2273, P=. 53). Weekly daily step goal achievement rate (%) decreased from study week 1 to 5 (-23. 23, SD 22. 85, P=. 02). Conclusions: Despite lower-than-expected recruitment and no statistically significant PA increase, relatively high engagement and acceptability suggest future pilot testing (Obesity-Related Behavioral Intervention Trial model, phase IIb) of a refined intervention (eg, wider selection of loyalty reward partners) and modified study protocol (eg, simplified consent process) is warranted.
Salmani et al. (2026) studied this question.