The Nordic Hamstring Exercise is widely used to prevent muscle strain injuries. Although isokinetic dynamometry is considered a gold standard for assessment, it is often impractical in many settings, due to several reasons. A cost-effective alternative for physical evaluations of the same muscle could be Smartphone-based, which have greatly increased in recent years due to technological advancements. This study aims to assess the concurrent validity and inter-rater reliability of the My Jump Lab smartphone application in measuring the break point angle (BPA) during the Nordic Hamstring Exercise. Twenty-four healthy participants were recruited for this study and performed five repetitions of the nordic hamstring exercise, while being recorded via smartphone for BPA estimation. The same exercise was compared with kinematic analysis, as reference method. Concurrent validity was assessed by Pearson`s product-moment correlation coefficient and Bland-Altman Plots, while inter-rater reliability was analysed by intraclass correlation coefficient (ICC), coefficient of variation (CV) and standard error of measurement (SEM). Results revealed a very high and significant level of correlation (r > 0.9, p< 0.001) for break point angle measurements, with small proportional bias. The app showed excellent inter-rater reliability (ICC >0.9), with very small variations (CV ˜ 1%) and SEM lower that 1.3 degrees. These outcomes suggest that My Jump Lab is a valid and reliable tool for BPA assessment, that can be used confidently to assess hamstring muscle strain in a simple, portable and practical manner.
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