Comparing the reliability of the single leg squat test using two, three, and four category ordinal rating scales

Background The single-leg squat test (SLST) is supported by evidence for reliability and validity across various visual rating methods, but the optimal number of ordinal categories for assessing movement quality remains unclear. The study aims to determine the most informative and reliable number of ordinal scoring categories for visually rating the SLST. Methods A total of 58 subjects with lower extremity injuries participated. A single therapist rated the SLST with a 1-week interval to establish intra-rater reliability. Two therapists independently rated the SLST at the same time point to establish inter-rater reliability. Two-, three-, and four-category ordinal scales were simultaneously used to evaluate SLST performance in rating the components of trunk deviation, hip adduction, and lower extremity internal rotation. Reliability was assessed using unweighted kappa (κ) values. Results The overall intra-rater reliability was κ = 0.60, 0.35, and 0.20, with inter-rater reliability being κ = 0.60, 0.61, and 0.33 for the two-, three-, and four-category scales, respectively. When specifically looking at the components of trunk deviation, hip adduction, and lower extremity internal rotation, intra-rater reliability was moderate to good for two-category scales (κ = 0.47–0.65), fair to good for three-category scales (κ = 0.3–0.7), and fair to good for four-category scales (κ = 0.36–0.65). Inter-rater reliability for the three components was good to excellent (κ = 0.65–0.86) for the two-category, good to excellent (κ = 0.69–0.86) for the three-category, and fair to excellent (κ = 0.55–0.76) for the four-category. Conclusion When applied to specific movement components, the three-category ordinal scale demonstrated the best balance between detailed assessment and reliability for visually rating trunk deviation, hip adduction, and lower extremity internal rotation during the SLST in patients with lower extremity injuries.