BackgroundAcute lateral ankle sprains (LAS) account for 4–5% of all Emergency Department (ED) visits. Few patients receive the recommended care of exercise rehabilitation. A simple solution is an exercise app for mobile devices, which can deliver tailored and real-time adaptive exercise programs.PurposeThe purpose of this pilot study was to investigate the use and preliminary effect of an app-based exercise program in patients with LAS seen in the Emergency Department at a public hospital.Materials and methodsWe used an app that delivers evidence-based exercise rehabilitation for LAS using algorithm-controlled progression. Participants were recruited from the ED and followed for four months. Data on app-use and preliminary effect were collected continuously through the exercise app and weekly text-messages. Baseline and follow-up data were collected though an online questionnaire. Semi-structured interviews were performed after participants stopped using the app. Results: Health care professionals provided 485 patients with study information and exercise equipment. Of those, 60 participants chose to enroll in the study and 43 became active users. The active users completed a median of 7 exercise sessions. Most of the active users were very satisfied or satisfied (79%-93%) with the app and 95.7% would recommend it to others. The interviews showed that ankle sprains were considered an innocuous injury that would recover by itself. Several app users expressed they felt insufficiently informed from the ED health care professionals. Only 39% felt recovered when they stopped exercising, and 33% experienced a recurrent sprain in the study period. Conclusion: In this study, only few patients with LAS became active app users after receiving information in the ED about a free app-based rehabilitation program. We speculate the reason for this could be the perception that LAS is an innocuous injury. Most of the patients starting training were satisfied with the app, although few completed enough exercise sessions to realistically impact clinical recovery. Interestingly more than half of the participants did not feel fully recovered when they stopped exercising and one third experienced a recurrent sprain.Trial-identifiershttps://clinicaltrials.gov/ct2/show/NCT03550274, preprint (open access):https://www.medrxiv.org/content/10.1101/2022.01.31.22269313v1.