Context: Goniometric assessment of hip-extension range of motion is a standard practice in clinical rehabilitation settings. A weakness of goniometric measures is that small errors in landmarking may result in substantial measurement error. A less commonly used protocol for measuring hip range of motion involves applying trigonometric principles to the length and vertical displacement of the upper part of the lower extremity to determine hip angle; however, the reliability of this measure has never been assessed using the modified Thomas test.Objective:To compare the intrarater and interrater reliability of goniometric (GON) and trigonometric (TRIG) techniques for assessing hip-extension range of motion during the modified Thomas test.Design: Controlled laboratory study. Setting: Institutional athletic therapy facility. Patients or Other Participants: A total of 22 individuals (12 men, 10 women; age range, 18-36 years) with no pathologic knee or back conditions.Main Outcome Measure(s): Hip-extension range of motion of each participant during a modified Thomas test was assessed by 2 examiners with both GON and TRIG techniques in a randomly selected order on 2 separate days.Results: The intraclass correlation coefficient (ICC) revealed that the reliability of the GON technique was low for both the intrarater (ICC ¼ 0.51, 0.54) and interrater (ICC ¼ 0.30, 0.65) comparisons, but the reliability of the TRIG technique was high for both intrarater (ICC ¼ 0.90, 0.95) and interrater (ICC ¼ 0.91, 0.94) comparisons. Single-factorial repeated-measures analyses of variance revealed no mean differences in scoring within or between examiners for either measurement protocol, whereas a difference was observed when comparing the TRIG and GON tests due to the differences in procedures used to identify landmarks.Conclusions: Using the TRIG technique to measure hipextension range of motion during the modified Thomas test results in superior intrarater and interrater reliability when compared with the GON technique.Key Words: assessment, musculoskeletal testing, flexibility Key PointsFor measuring hip-extension range of motion, reliability of the trigonometric technique was high, but reliability of the goniometric technique was poor to moderate. Both intrarater and interrater reliability were higher when examiners used the trigonometric technique than when they used the goniometric technique to assess hip-extension range of motion during the modified Thomas test.