Hip Muscle Strength Is Associated With Symptom Severity in People With Femoroacetabular Impingement Syndrome, but Only in People With Pre-Existing Hip Muscle Weakness: A Secondary Analysis of Baseline Data From a Randomized Controlled Trial

OBJECTIVE: To explore associations between hip muscle strength and symptom severity in individuals with femoroacetabular impingement (FAI) syndrome. DESIGN: Cross-sectional study. METHODS: Baseline data from 150 participants with FAI syndrome who participated in a clinical trial were analysed. Hip abduction, adduction, flexion, extension, internal rotation, and external rotation muscle strength assessments were performed using a hand-held dynamometer. Symptom severity was assessed using the International Hip Outcome Tool-33 Symptoms subscale (iHOT-Symptoms). Linear and non-linear associations were explored using generalized linear models. RESULTS: Non-linear associations (spline) between hip muscle strength values and iHOT-Symptoms scores were observed. Lower hip flexion (Average Marginal Effect (AME) = 0.25 (95%CI, 0.12 to 0.39)), abduction (AME = 0.25 (95%CI, 0.11 to 0.40)), and adduction (AME = 0.29 (95%CI, 0.11 to 0.47)) muscle strength values were associated with worse iHOT-Symptoms scores. Greater hip muscle strength was not associated with better iHOT-Symptoms scores. Hip external rotation (AME = 0.10 (95%CI, −0.01 to 0.20)), internal rotation (AME = 0.10 (95%CI, 0.01 to 0.19)), and extension (AME = 0.02 (95%CI, −0.02 to 0.07)) muscle strength values were not meaningfully associated with iHOT-Symptoms scores. CONCLUSION: Reduced hip flexion, abduction, and adduction muscle strength was associated with worse symptoms in patients with FAI syndrome, but only in individuals with hip muscle weakness. Hip external rotation, internal rotation and extension muscle strength were not associated with symptom severity.