BACKGROUND: Recent advances in wearable technologies make it possible to accurately quantify real-world mobility performance through technically validated digital mobility outcomes (DMOs). The aim of the present study was to evaluate the construct validity (convergent, divergent, and known-groups validity) of 24 DMOs quantifying walking activity (amount and pattern) and gait (pace, rhythm and bout-to-bout variability) in people with COPD. METHODS: Part of the Mobilise-D observational cohort study, people with COPD, recruited from seven European sites, wore an activity monitor for 7 days during daily life. Functional capacity, health status, dyspnoea, lung function, quadriceps torque and experience of difficulty with physical activity were used as constructs for convergent validity testing (Pearson/Spearman correlation coefficients). Diastolic blood pressure was used as an unrelated construct for divergent validity (criterion: |r|<0.2). Known-groups validity was evaluated across Global Initiative for Chronic Obstructive Lung Disease (GOLD) stages (I-IV), GOLD ABE and modified Medical Research Council dyspnoea grades (linear models with p-for-trend). RESULTS: 549 participants (37% females), had mean+/-sd age of 68+/-8 years, post-bronchodilator forced expiratory volume in 1 s (FEV(1)) 54+/-20% predicted and 6-min walk distance 416+/-119 m. Convergent validity was supported for the majority of DMOs (17 out of 24) with correlation coefficients meeting or exceeding the a priori hypotheses by clinical experts. All DMOs supported divergent validity. 22 out of 24 DMOs distinguished between disease severity groups successfully. Expert consensus supported construct validity of 17 DMOs. CONCLUSIONS: Construct validity was supported for all walking activity (amount and pattern) DMOs, and most of the gait (pace, rhythm, and bout-to-bout variability) DMOs, indicating the clinical utility of these measures.
