Single leg squat ratings by clinicians are reliable and predict excessive hip internal rotation moment

Background: Single leg squats are commonly used subjective assessments of general biomechanical function, injury risk, as a predictor for recovery and as an outcome measure of rehabilitation. While 3D motion capture is a useful tool for elite sports performance and research it is impractical for routine clinical use. Research question: This cross-sectional study aims to: assess reliability and validity of clinicians' subjective ratings of single leg squats compared to 3D motion capture, and to identify whether performance predicts joint moments. Methods: 22 healthy military volunteers were simultaneously recorded on video and 3D motion capture performing single leg squats. Videos were reviewed twice by 5 physiotherapists rating performance on a 0-5 scale assessing squat depth, hip adduction, pelvic obliquity, pelvic tilt and trunk flexion summated into a composite score. Results: Hip adduction and trunk flexion exhibited moderate to substantial inter-and intra-rater reliability (range kappa= 0.408-0.699) other individual criteria were mostly fair (kappa <= 0.4). Composite scores for inter-rater reliability were ICC(1,1)= 0.419 and ICC(1,kappa)= 0.783 and intra-rater reliability were ICC(1,1)= 0.672 and kappa(w)= 0.526. Validity against 3D kinematics was poor with only 6/75 individually rated criteria reaching kappa > 0.40. Correlation was found between composite scores and hip internal rotation moment (r(s)= 0.571, p= 0.009). Significance: Repeated use of single leg squats by a single practitioner is supported. Comparisons between clinicians are unreliable but improved by average measures from multiple raters. Heterogeneous reliability across scoring components suggests a qualitative description of the criteria scored is less ambiguous than using composite scores in a clinical setting. Composite scores may be more useful for analysis at a population level. Poor validity against kinematic data suggests clinicians use additional information upon which they find agreement such as estimating kinetics. Correlation between hip internal rotation moment and subjective ratings may be such an example of clinicians trying to identify excessive abnormal loading.