Concurrent validity and reliability of a smartphone app for assessing weight-bearing ankle dorsiflexion using 2D video analysis

Introduction Reduced ankle dorsiflexion range of motion (ADROM) is a risk factor for lower extremity injuries, and weight-bearing measurements are known to better reflect functional movement. This study aimed to determine the concurrent validity and the intra- and inter-rater reliability of a smartphone application (SA, The Angle Meter app) for measuring ADROM during the weight-bearing lunge test (WBLT). Methods A reliability and validity study was conducted involving 57 participants. Three raters (two experienced and one novice) assessed ADROM using the SA in two separate sessions, in a randomized order. All measurements were concurrently recorded on video, and a two-dimensional video analysis (Kinovea software) was used as the reference standard. A total of 342 measurements were analyzed for concurrent validity, 114 for inter-rater reliability, and 57 for intra-rater reliability. Pearson's correlation coefficient (r), intraclass correlation coefficient (ICC 2,1), standard error of the estimate (SEE), Bland-Altman analysis, standard error of measurement (SEM), and minimum detectable change at 95% confidence (MDC 95 ) were used for analysis. Results The SA demonstrated excellent concurrent validity (r = 0.98; SEE = 0.92°; ICC = 0.977; 95% CI: 0.966–0.983), with limits of agreement from −1.80° to 1.47°. Intra-rater reliability was high across all raters (ICC range: 0.83–0.86), including the novice. Inter-rater reliability was also good (ICC = 0.846; SEM = 1.65°; MDC 95 = 4.57°). Conclusion The Angle Meter app is a valid, reliable, and practical tool for assessing ADROM during WBLT, even when used by novice raters.