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Hip muscle weakness and altered hip biomechanics during walking are often observed in people with femoroacetabular impingement syndrome, although little is known about biomechanics during higher impact tasks. The aim of our study was to explore relationships between hip muscle strength and hip biomechanics during running in people with femoroacetabular impingement syndrome, including exploring sex as an effect-modifier of this relationship. Forty-two adults with unilateral femoroacetabular impingement syndrome (20 females; age 18-50 years; alpha angle ≥60°) completed assessments of hip muscle strength and hip biomechanics during running. Strength was assessed using a hand-held dynamometer for the hip flexors, extensors, abductors, adductors, internal rotators, and external rotators. Hip biomechanics were assessed during overground running (3–3.5 m/s) using three-dimensional motion capture and a force plate. Linear models assessed the relationships between hip strength and hip biomechanics of the symptomatic limb, controlling for body mass and running velocity along with an interaction term (strength*sex). A significant negative relationship was observed between hip external rotator strength and hip frontal plane range of motion (i.e., excursion), independent of sex (estimate = −0.039, 95%CI −0.071 to −0.008, P = 0.02). Four sex-specific interactions were observed, with a significant positive relationship between hip external rotator strength and peak hip extension moment in women (estimate = −0.413, 95%CI −0.713 to −0.114, P = 0.01) but not in men. We found significant relationships between hip external rotator strength and stance phase running biomechanics, providing further understanding on two impaired physical measures that may inform exercise-based management strategies in femoroacetabular impingement syndrome. • Hip weakness and altered movement exist with femoroacetabular impingement syndrome. • Associations between muscle strength and running biomechanics were explored. • Stronger hip external rotators were associated with less hip frontal plane motion. • Significant sex-specific interactions were observed in some associations. • Our results may help to inform exercise-based management strategies. [ABSTRACT FROM AUTHOR]