## 3 Biomechanics of Articular Cartilage
### Mechanical Properties of Articular Cartilage
[PERSON] et al. [176] in their studies focused on the properties of alginate gel as a model for AC, showing a strong dependence of G\({}_{\text{eq}}\) on both the alginate concentration and the presence of sodium ions. The ratio of the viscous to elastic components in the shear response of AC is defined by the shear loss angle (\(\delta\)). In the work of [PERSON] et al. [176], the sensitivity of this parameter to changes in the structure of the extracellular matrix, especially to the degradation of proteoglycans, was demonstrated. The values of the above-mentioned biomechanical parameters of healthy AC are presented in Table 2 based on the work of [PERSON] et al. [177]. \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Typical Value** & **Measurement Method** & **Author** \\ \hline Aggregate modulus (MPa) & 0.1–2.0 & Confined compression & [PERSON] et al. [128] \\  & & tests, indentation & [PERSON] et al. [172] \\  & & [PERSON] et al. [178] \\ \hline \end{tabular}
\end{table} 
Table 2: Biomechanical parameters of healthy AC. Various measurement methods were used to determine the parameters described above. The confined compression test is a classic method for testing the mechanical properties of AC, in which a cylindrical tissue sample is axially compressed in a sealed chamber, limiting its lateral expansion [51,128]. This measurement method is one of the key tools used in AC biomechanics to determine material properties that play a key role in the diagnosis of degenerative changes, as well as in the design of biomaterials and numerical models [45,51,128,172,173,178]. In this arrangement, only axial outflow of interstitial fluid is possible through the porous endplates, enabling a detailed study of the time-dependent response (creep and relaxation) of the tissue under controlled hydraulic conditions [51,128]. [PERSON] et al. [128] used this method to experimentally verify the biphasic theory, which describes AC as a poroelastic system composed of a solid phase, which is a collagen-proteoglycan matrix, and a fluid phase in the form of water and dissolved substances. The use of this measurement method made it possible to determine key parameters for understanding mechanotransduction in AC, such as the elastic modulus of the matrix and the hydraulic permeability coefficient. In the next work of [PERSON] et al. [51], the confined compression test was used to discuss the influence of mechano-electrochemical phenomena on the mechanical response of AC. It was proven that this measurement method is particularly useful for investigating the inhomogeneity and anisotropy of AC properties with respect to different depth zones. [PERSON] et al. [172] used this measurement method to evaluate Poisson's ratio values as well as stress modulation within the tissue by examining the mechanical compression between axial strain and lateral displacements of the material. [PERSON] et al. [178] described this measurement method, comparing it to the indentation test. They emphasised its advantage of better control over boundary conditions and fluid flow, which has a beneficial effect on the theoretical analysis. The confined compression test was used by [PERSON] et al. [173] as a reference point to assess the accuracy of the developed numerical algorithm for the indentation test. A significant convergence of the model results