A review of the biomechanics of synovial joints with emphasize to static stretching exercise

St. Stoytchev, I. Ivanov, S. Ranchev, I. Iotov

Abstract: Stretching is defined as movement applied by an external and/or internal force to increase muscle flexibility and/or range of motion of the joint. Numerous studies have found that static stretching affects both the mechanical and neurological properties of the muscle-tendon block, leading to increased musculoskeletal flexibility, rigidity, Yung’s modulus, and relaxation of stress in the system. The use of MRI in research has made it possible to clarify in vivo some processes in the joint capsule, such as reducing the thickness of cartilage when the knee is loaded with a force of up to 150% of body weight. However, what happens inside the joint during active isometric stretching, as far as we know, has not been studied and clarified. Recently, Ranchev et al. (On the biomechanical processes in human knee joint during active isometric stretching. Series on Biomechanics, Vol. 33, No. 3, 56-61, 2019) reported a preliminary study for changes in the volume of the knee joint capsule during active isometric stretching. The results showed a change in the distance between the cartilage surface of the femur and the corresponding end of the tibia during active isometric 2-minute stretching. This change of the distance between the femur and the tibia bones might be attributed to certain processes inside the joint cavity and, therefore, requires more detailed analysis of the biomechanical processes inside the joint cavity. This review aims to clarify some of them, to formulate new boundary-value problems and to design mechanical methods for evaluating the muscle forces during the stretching exercise.
The review includes rheological behavior of synovial fluid, composition of articular cartilage, mechanical behavior of articular cartilage, permeability of articular cartilage, constitutive modeling of articular cartilage, effects of stretching exercises on skeletal muscles, and dynamic interaction problems inside articular cartilage. These problems involve the utilization of classical fluid mechanics of viscoelastic, elastic, biphasic porous mixture, and filtration. Finally, we outline some unresolved problems in the experimental verification and modeling of articular cartilage and synovial fluid in their dynamic interactions.

Series on Biomechanics, Vol.35 No.2 (2021), 3-20

Keywords: Biomechanics of synovial joint; diffusional drag; filtration; poro-visco-elastic material; stretching exercise

Date published: 2021-07-02