Reliability, validity and responsiveness of the Step Up and Down (StUD) test for individuals with symptomatic knee osteoarthritis

Background: Stair climbing test (SCT) has been suggested as the first task affected in individuals with symptomatic knee osteoarthritis (KOA). However, there are environmental barriers for the execution of the 9- to 12step SCT. Thus, we developed a feasible one-step SCT that could be completed in 15s. Objective: To check the clinimetric properties of the 15s Step Up and Down (StUD) test in individuals with KOA. Design: Prospective validity study. Method: Eighty-two individuals with KOA participated in this study. The test-retest reliability of the StUD test was measured with a 1-week interval. The construct validity and responsiveness were assessed by testing predefined hypotheses. For this, the 30s Chair Stand Test (30CS), Timed Up and Go Test (TUG), quadriceps strength, Knee Injury and Osteoarthritis Outcome Score (KOOS), and Lequesne Algofunctional Index were used as comparator instruments. Results: The StUD test presented good test-retest reliability (ICC = 0.87; 95% CI = 0.79-0.91) and showed a moderate to good correlation with the 30CS (r = 0.65), TUG (r = -0.56), and quadriceps strength (r = 0.41). We found a higher correlation between the StUD test and the performance-based tests than the patient-reported outcome measures. The StUD test was responsive, with five out of the six (83.3%) hypotheses confirmed. Conclusion: StUD test showed good reliability, adequate validity and responsiveness. Our findings suggest that StUD is a useful performance-based test for individuals with KOA.