Relationship between Hip Flexion Contracture and Hip-Joint Contact Force in Standing Posture: A Computer Simulation Study

Mechanical stress on articular cartilage and long-duration standing postures are risk factors for hip osteoarthritis progression. This study aims to examine the relationship between hip flexion contracture and the hip-joint contact force in standing postures using computer simulation. A musculoskeletal model composed of seven segments (Head, Arms, and Trunk (HAT) and thighs, shanks, and two feet) was created. Various standing postures (708 variations) were generated, and five hip flexion contracture conditions were set: zero contracture and flexions of 0°, 10°, 20°, and 30°. A standing posture satisfying the hip flexion contracture condition with the minimum sum of the muscle activations was obtained as the optimal standing posture, and the hip-joint contact force in the optimal standing posture was calculated. A sensitivity analysis was conducted by varying four parameters (the objective function, physiological cross-sectional area, force-length relation, and muscle moment arm length). The hip-joint contact force and hip extensor muscle forces (i.e., those of the gluteus maximus, semitendinosus, semimembranosus, and biceps femoris long head) during standing increased with the development of hip flexion contracture. The hip-joint contact force for the standing posture with a 30° hip flexion contracture was almost twice that for the no-contracture condition (8.7 and 3.7 N/kg, respectively). The sensitivity analysis showed that variation of the four parameters did not affect our main finding. The main finding of this study is that hip-joint contact force during standing increases with the development of hip flexion contracture. The findings of this study may help to prevent hip osteoarthritis progression.