Muscle contribution to hip contact force during walking is lower in individuals with femoroacetabular impingement syndrome, compared with controls.

Altered hip loading and biomechanics in individuals with femoracetabular impingement syndrome (FAIS) may affect the joint's habitual mechanical environment, potentially increasing the risk of osteoarthritis . Examining differences in contributions of muscle and external loads (i.e., gravitational and intersegmental-inertial forces) to hip contact forces, compared with controls, may aid our understanding of FAIS pathomechanics and assist with the development of more effective treatments. Whole-body motion and electromyograms of 14 lower limb muscles were acquired from 41 participants with FAIS and 24 healthy controls whilst walking overground at self-selected speed. Contributions made by muscle and external (gravitational and intersegmental-inertial) forces to hip contact force during the stance phase of walking were estimated using an electromyogram-assisted neuromusculoskeletal model and compared between-groups using statistical parametric mapping. Throughout stance, muscle contributed ∼80% of hip contact force for both participants with FAIS and controls. Compared with controls, participants with FAIS generated ∼20% lower total muscle force (mean difference: -0.75 N·BW-1, 95% CI -1.13 to - 0.35, p < 0.001) primarily due to lower adductor (-0.27 N·BW-1, 95% CI -0.48 to - 0.06, p = 0.001), extensor (-0.40 N·BW-1, 95% CI - 0.65 to -0.16, p < 0.001) and flexor (-0.71 N·BW-1, 95% CI -1.07 to -0.35, p < 0.001) muscle group forces at different stages of stance. Compared with controls, lower hip contact force in participants with FAIS during the stance phase of walking were the result of lower flexor, extensor and adductor muscle forces and could be targeted in non-operative interventions (e.g., physiotherapy).