What are the validity of the single-leg-squat test and its relationship to hip-abduction strength?

Context: The Trendelenburg and single-leg-squat (SLS) tests are purported measures of hip-abduction strength that have not been previously validated. Objective: To correlate isometric hip-abduction strength to frontal-plane hip motion during an SLS and determine the criterion validity of a clinical-observation-analysis method to grade an SLS against 2-dimensional kinematic analysis. Design: Single-measure, concurrent validity. Setting: Biodynamics research laboratory. Participants: 50 uninjured participants. Main Outcome Measures: Hip-abduction strength and hip and knee kinematic data during a Trendelenburg test and an SLS. Results: A weak, positive correlation between hip-abduction strength and hip-adduction angle was found during both the Trendelenburg (r = .22, P = .13) and the SLS (r = .21, P = .14) tests. The observation-analysis method revealed a low sensitivity, .23, and a higher specificity, .86, when compared with the kinematic data. Conclusion: The usefulness of the Trendelenburg and SLS test in screening hip-abductor strength in a healthy physically active population is limited. The origin of observable deficits during SLS requires further objective assessment.