Hip flexion range of motion (ROM), hip extensor strength (torque production), and hip pain are important correlates of physical function in individuals with hip osteoarthritis (OA). However, the relationships among these variables remain unclear. The purpose of this study was to examine whether hip extensor strength and hip pain mediate the association between hip flexion ROM and physical function.
Participants were 100 adults (mean age 62 years) with radiographically confirmed hip OA. Passive hip flexion ROM was measured using a digital inclinometer, and isometric hip extensor strength was measured using a force transducer. Self-report measures of physical function and pain were assessed by the Short Form 36 (SF-36) questionnaire, and physical performance was assessed by the gait speed, step test, and stair climb test.
Multiple mediation analyses, adjusted for demographic and anthropometric measures, revealed that the association between hip flexion ROM and SF-36 physical function scores was fully mediated by SF-36 bodily pain scores and hip extensor strength. Hip extensor strength mediated the hip flexion effects on gait speed and step test performance, while the SF-36 bodily pain scores mediated the hip flexion stair performance association.
In individuals with symptomatic hip OA, hip pain and hip extensor strength mediated the influence of hip flexion ROM on physical function. These results suggest that interventions to improve the range of hip motion may be clinically indicated.