Intra and Interobserver Reliability of the Angular Kinematic Analysis of the Hip during the Sit-and-reach Test for Measuring the Length of Hamstring Muscles in University Students

Background: The sit-and-reach test (SRT) is used to measure the flexibility of the lumbar spine and hamstring muscles and is better when the hip joint angle (HJA) is measured concomitantly. Objective: To assess the intra and interobserver reliability of the SRT for measuring hamstring muscle length through angular kinematic analysis. Method: Fifty university students (X= 21.5 years; SD= 1.5) without musculoskeletal abnormalities took part. A standard SRT bench with a door (to assess the action of the gastrocnemius muscles) and a digital photographic camera positioned on a tripod were used. Skin markers were placed on the anterosuperior iliac spine and greater trochanter. Two images were recorded: one with the door closed (CD) and another with the door open (OD). To test the intra and interobserver reliability, the intraclass correlation coefficient (ICC) (random effect with one factor) and the Bland and Altman concordance test (÷ = mean difference between measurements) were used, with the respective 95% confidence intervals (95% CI). Results: The intraobserver reliability of the HJA test was: ICCdc= 0.97 (95% CI [0.95;0.99]) and Bland and Altman ÷ = -0.02 (95% CI [-0.11;0.07]); ICCdo = 0.98 (95% CI [0.96;0.99]), ÷ = -0.01 (95% CI [-0.11;0.08]). For the interobserver reliability: ICCdc= 0.96 (95% CI [0.94;0.98]) and ÷ = -0.07 (95% CI [-0.19;0.03]); ICCdo= 0.96 (95% CI [0.93;0.98] and ÷ = -0.06 (95% CI [-0.19;0.52]. Conclusion: The intra and interobserver reliability tests (CD and OD) for HJA assessment using kinematic analysis showed high reliability. The technique is easy to apply (only requiring a standard SRT bench and a photographic camera) and provides a reliable method for measuring hamstring muscles using kinematic analysis in clinical practice.