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• This is the first osteoarthritis clinical trial to utilise activity tracker-recorded outcomes. • Activity trackers are a feasible measurement tool in a clinical trial context. • Behavioural and physical activity change was observed via step count and load rate. • Activity tracker-recorded outcomes did not show significant responsiveness or validity. In osteoarthritis (OA) clinical trials, reliable and responsive outcome measures to document physical and functional improvements are limited. This study aimed to assess whether the use of an activity tracker in an OA clinical trial is a responsive measurement tool. Secondary objectives assessed feasibility and validity. We recruited 65 participants in a prospective cohort study nested in a placebo-controlled clinical trial of platelet-rich plasma injection in knee OA. Participants wore an activity tracker (Fitbit Flex 2), and a smartphone was preloaded with a mobile application (OApp) designed to monitor load rates as a surrogate of knee loading. Participants used the systems for 7 days at baseline and for 7 days before the 2-month follow-up assessment. Effect size (ES) and standardised response mean (SRM) were calculated for change in step count and knee loading rate and regularly used knee OA outcome measures. Correlation coefficients (r) were calculated to examine the strength of the association between outcome measures.. Step count showed a trivial ES and SRM and mean knee loading rate measurements a moderate ES and SRM. We found a weak but significant correlation between change in mean steps per day and global improvement overall (r = 0.28) and Western Ontario and McMaster Universities Osteoarthritis Index function (r = -0.28). Compliance was high with the activity trackers. Despite good feasibility, this study did not show significant responsiveness or validity of the activity trackers as compared with currently recommended outcome measures in OA clinical trials. The main challenge is the lack of a gold standard outcome measure to validate against, and because of the complex interplay between pain and measured function, a lack of correlation does not necessarily represent a failed validation in this context. ACTRN12617000853347. This trial is a substudy of the "Platelet-rich plasma as a symptom-and disease-modifying treatment for knee osteoarthritis – the RESTORE trial". [ABSTRACT FROM AUTHOR]