Reproducibility of the DrGoniometer app for field-based assessment of the break-point angle in Nordic Hamstring exercise

Smartphone-based measurement could represent a portable and cheap solution to assess the Nordic Hamstring break-point angle (BPA). We investigated the following: (1) the reproducibility of the DrGoniometer app, (2) its agreement with a computerized video analysis software used as a reference method, and (3) the effects of raters' experience. Fifty recordings were processed by four independent raters. Intraclass correlation coefficient (ICC), SEM, and minimum detectable change (MDC) were calculated. Bland-Altman plots verified the limits of agreement (LoA) with the reference method. The effect of raters' clinical experience was investigated by unpaired t-tests. The app showed excellent intra-rater (ICC = 0.89, 95% CI, 0.84-0.92) and inter-rater reliability (ICC = 0.82, 95% CI, 0.74-0.89), low error (SEM = 2°; MDC = 5.5°), and substantial agreement with video analysis [95% LoA = ±6.7°; mean difference = 1.0°]. Rater experience did not affect results (P > 0.05). The smartphone-based app offers a reliable, valid, quick, and portable method for measuring the BPA in Nordic Hamstring exercise.