Background: The six-minute walk test (6MWT) measures exercise capacity in cardiorespiratory, neurological and musculoskeletal conditions. It consists in observing how far a patient can walk in 6 minutes and is usually performed in a corridor in a clinic. During the COVID 19 pandemic, as healthcare systems cancelled nonurgent outpatient appointments, many tests were done online. At the Oxford University Hospitals, cardiac patients were asked to use the open-source Timed Walk app top perform 6MWT in their community, as a substitute for the regular tests in clinic. Objective: (1) To assess participation and user acceptance of the Timed Walk app, (2) to assess the clinical usefulness of the app within the context of the pandemic, and (3) to validate and improve the algorithms that compute the walked distance from the sensors data collected by the phone. Methods: Consented cardiac patients were invited to perform a 6MWT, outdoor, using the app, at least once a month, and report the results at periodic telephone calls and visits. Any clinical decision taken based on the results of the app was registered. Patients were also sent a #usability and acceptance questionnaire and 10 of the respondents were selected for interviews. A group of 12 volunteers also provided sensors data collected by the app and a trundle wheel to measure reference distance for 10 tests, 5 of which were intentionally performed without following instructions to walk over straight paths. Results: the study run between 2021-09-29 and 2022-12-30. 55 participants consented (25 female, age: 44.80 ± 17.49) 1) Twenty-four patients performed one or more tests per month, average number of 6MWTs per month per patient was 1.14 ± 1.20. #usability was rated high on all dimensions; acceptance was high except intention to use the app beyond the study. Thematic analysis of the interviews provides useful insights on 3 themes: 2) 741 events were logged. 24% of 51 medical decisions involving 23% of 48 patients who performed at least 1 test, were influenced by the app-based 6MWT. Between 2018 and 2023 a cohort of 49 patients conducted 63 6MWT in the clinic (18 in 2021), whereas the same patients performed 605 tests using the app only in 2021. 3) Sensor data was sent for 107 tests, 52 not following instructions. Difference between reference distance and app distance was within minimal clinically significant difference for tests performed following instructions (limits of agreement: -27m, 34m). Anonymized data has been made publicly available. Conclusions: The use of the Timed Walk app for remote 6MWT allowed clinicians to obtain objective indications of the status of the patient during the pandemic. The distance estimated by the app is accurate when patients follow instructions. Motivation to use the app can vary depending on internal factors such as attitudes and health status, and external factors such as weather, fit into everyday life, how the data is used by clinicians and forgetfulness. Clinical Trial: ClinicalTrials.gov NCT05096819