BACKGROUND: Physical activity (PA), including regular exercise, is essential for the successful management of axial spondyloarthritis (axSpA). To promote a physically active lifestyle, a digital training diary (Trainingslog) was developed in an user-centered approach by the Swiss Ankylosing Spondylitis Association (SVMB). A training diary promotes PA through feedback, goal setting and self-monitoring, which can also be used for PA counselling by physiotherapists (PT). Usability, feasibility and acceptance are essential for the successful implementation of a mobile Health Intervention such as the Trainingslog. The study objective is to evaluate the usability, feasibility and acceptance of the Trainingslog for individuals with axSpA and PTs. METHODS: A mixed-methods design was performed among potential end-users of the Trainingslog. Quantitative data was collected by use of questionnaires (System Usability Scale (SUS, 0-100 scale), user version of the Mobile App Rating Scale (uMARS, 5 point scale)) and number of training entries. Subsequently, qualitative data was gathered through semi-structured online focus groups or individual interviews. RESULTS: 11 PTs (9 women, mean age 52.5 [SD 15.6]) and 10 individuals with axSpA (6 women, mean age 48 [SD 13.4]) participated. The quantitative data showed mean SUS scores for usability of 82.5 [SD 21.76] for PTs and 77.0 [SD 9.34] for individuals with axSpA. The mean uMARS sector B scores for feasibility were 4.2 [SD 0.49] for PTs and 4.1 [SD 0.38] for individuals with axSpA. Acceptance, as indicated by the uMARS results (mean score >
3 in Sectors E and F for both groups), was given. But there was a lower-than-expected agreement in the training entries, with 59.86% of entries matching between the Trainingslog and the paper diary. The qualitative analysis unveiled that while usability and feasibility were good, acceptance was lower, primarily due to the use of a web-based link instead of an app version. CONCLUSION: The Trainingslog showed a good usability and feasibility, while the acceptance was lower than expected. Acceptance could be increased by offering the Trainingslog as an app-based version, along with implementing additional recommendations for enhancement. Consequently, the Trainingslog has the potential to be applied in PA counselling by PTs or as a self-monitoring tool for individuals with axSpA.