Simulation of double-leg stance in conditions of limited hip mobility

Hip osteoarthritis is one of the most common and disabling conditions affecting the elderly. Coxarthrosis is accompanied by impairment of the amortization properties of cartilage, its thinning and destruction, the appearance of pain syndrome, impaired motor functions due to a decrease in muscle strength and the development of stable flexion-adduction contractures, which change congenital motor programs, and, with a prolonged course of degenerative disease, lead to the formation of pathological habits. Objective: to determine the required strength of the muscles of the lower limb in conditions of limited hip mobility to support an upright posture in double-leg stance. Materials and methods. The work of the muscles of the lower extremities under conditions of restricted hip mobility was simulated using the OpenSim 4.0 software. It is based on the ToyLandingModel, which has contact geometry objects to fix the model on the support area. Four models were created: norm (without limitation of joint mobility), model 2 — adduction of 5°, model 3 — adduction of 7°, flexion of 10°, model 4 — adduction of 10°, flexion of 20°, shortening of the femur bones by 2 cm. Results. It was found that with insignificant adduction contractures of the hip joint, the work of the muscles of the lower limb changes slightly during double-leg stance. With flexion-adduction contractures, changes are observed in almost all muscles of the lower limb. There are some peculiarities in the work of muscles under contractures. All the muscles around the thigh reduce the strength necessary to maintain balance, while the lower leg muscles, on the contrary, increase the required strength several times. For example, m.medial gastrocnemius with flexion-adduction contracture and limb shortening develops10 times higher compensatory force (200 N) than in normal conditions (20 N), and although muscle resources are 1500 N, it is very demanding to maintain an upright posture. Similarly, m.tibialis posterior require an increase in strength (threefold), but the antagonist muscle m.tibialis anterior, on the contrary, reduces the force of contraction by an average of 100 N. Conclusions. According to the data of the conducted modeling of double-leg stance with limited hip mobility, it was proved that an increase in limited movements changes the nature of muscle contraction of the entire lower limb and pelvis. The analysis of the obtained results showed that restriction of movements reduces the required force of muscle stabilization around the hip joint, and increases the required force of contraction of the leg muscles. That is, there is an imbalance in the muscles.