Background The modified Thomas test is the most used and most reliable test to assess the hip flexors` flexibility. However it does not evaluate the interaction of both legs. The objective of this study is to propose a new assessment tool for hip flexor flexibility, the Reactive Hip Flexion (RHF) Test. Methods An observational, intra-test, and test-retest study with repeated measures was carried out with the aim of assessing the validity and reliability of the RHF Test. The participants were males and females aged between 18 and 35 years old who had a training schedule of at least 2 days a week, without a musculoskeletal lower limb or lumbopelvic pathology. The reliability of the test was examined using the intraclass correlation coefficients (ICCs) by a two-way random model to establish inter-rater reliability and a two-way mixed model to assess intra-rater reliability. The precision was measured by the standard error of measurement (SEM). In addition, the minimum detectable change (MDC95%) was calculated. Results Twenty-six participants (52 hips) (47% female) completed the study. No correlations were observed between anthropometric variables and RHF peak force or active knee extension (AKE) measurements. There was an excellent intra-rater and inter-rater ICC in the hip flexors` reactive peak force and the AKE measurements, with a low SEM. Conclusions This study demonstrated that the proposed RHF test technique is valid and reliable when used in healthy youth population.
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