Kendall / Modified Thomas Test for Rectus Femoris Length The modified Thomas test is probably the most commonly used test to measure any hip extension defitits due to muscle shortening. But how reliable and valid is this test? Peeler et al. (2013) showed an almost perfect intra- and inter-rater reliability for the visual and goniometer methods (0.92-0.98). Vigotsky et al. (2016) evaluate the criterion reference validity of the Thomas test to measure peak hip extension and hip extension deficits.