The effect of changing the length of the components of the muscle-tendon unit of the knee joint on its isometric force and torque

Abstract

Background. A knee contracture developed due to immobilization after extra-articular injuries leads to lameness, impaired walking, and the appearance of pain in other joints of the lower limb. Long-term immobilization of the joint leads to weakening and increasing stiffness of muscles, changes in the length of tendons and myofibrils, loss of cushioning properties of cartilage, inhibition of blood circulation processes in the periarticular tissues. Adequate rehabilitation and recovery of the knee joint function after immobilization is a medical and social problem in terms of reducing disability. The goal of the work: to study the influence of changes in strength, length of muscles and tendons of the knee joint on torque. Materials and methods. The OpenSim Leg39.osim base model was used for simulation. The parameters of the muscles and tendons were changed in accordance with the conditions of immobilization for 45 days at the angle of preferential position of the limb — 15°. Three models were analyzed: a basic model without changes in muscle parameters, a model with a change in the length of the muscle-tendon unit, and a model with an additional change in muscle strength (according to calculations). Results. Changing the length of the components of the muscle-tendon unit of the flexors has little effect on both the isometric strength of the muscles and the knee joint torques produced by these muscles. A decrease in the maximum isometric force of the flexor muscles leads to a decrease in torque. When the length of the extensors changes, there is a sharp increase in the isometric force and torques of the knee joint and a change in the direction of the active torque trajectory. A decrease in muscle strength leads to a decrease in the level of torques approaching normal, but the direction remains unchanged. Conclusions. Mathematical modeling of the effect of changes in the components of the muscle-tendon unit of the muscles responsible for the movement of the knee joint showed that minor alterations in their length lead to noticeable violations of both the torque of the joint and changes in the isometric strength of the muscle during movements. Predicting the change in knee joint torque can be useful in studying specific clinical situations with contractures caused by a long-term immobilization after extra-articular injuries. But it cannot be fully transferred into practice due to the significant conditionality of model parameters. The used modeling method can show trends in the change of muscle functioning parameters when changing their geometry, and can be extended to study of other muscle and joint parameters.