Biomechanical studies are expanding across a variety of fields, from biomedicine to biomedical engineering. From the molecular to the system level, mechanical stimuli are crucial regulators of the development of organs and tissues, their growth and related processes such as remodelling, regeneration or disease. When dealing with cell mechanics, various experimental techniques have been developed to analyse the passive response of cells; however, cell variability and the extraction process, complex experimental procedures and different models and assumptions may affect the resulting mechanical properties. For these purposes, this review was aimed at collecting the available literature focused on experimental chondrocyte and chondron biomechanics with direct connection to their biochemical functions and activities, in order to point out important information regarding the planning of an experimental test or a comparison with the available results. In particular, this review highlighted (i) the most common experimental techniques used, (ii) the results and models adopted by different authors, (iii) a critical perspective on features that could affect the results and finally (iv) the quantification of structural and mechanical changes due to a degenerative pathology such as osteoarthritis.

Plantar adipose tissue is a connective tissue whose structural configuration changes according to the foot region (rare or forefoot) and is related to its mechanical role, providing a damping system able to adsorb foot impact and bear the body weight. Considering this, the present work aims at fully describing the plantar adipose tissue’s behaviour and developing a proper constitutive formulation. Unconfined compression tests and indentation tests have been performed on samples harvested from human donors and cadavers. Experimental results provided the initial/final elastic modulus for each specimen and assessed the non-linear and time-dependent behaviour of the tissue. The different foot regions were investigated, and the main differences were observed when comparing the elastic moduli, especially the final elastic ones. It resulted in a higher level for the medial region (89 ± 77 MPa) compared to the others (from 51 ± 29 MPa for the heel pad to 11 ± 7 for the metatarsal). Finally, results have been used to define a visco-hyperelastic constitutive model, whose hyperelastic component, which describes tissue non-linear behaviour, was described using an Ogden formulation. The identified and validated tissue constitutive parameters could serve, in the early future, for the computational model of the healthy foot.

Purpose: Plantar skin is a highly specialised tissue which protects the foot from injuries and adapts to external stresses. However, it can be subjected to diabetic plantar ulcers, which are among the most difficult and costly wounds to treat. Although this is a crucial topic, few studies have focused on the mechanical properties of foot skin and how disease alters them. In this context, this work aims to fully describe the mechanical behavior of plantar skin through experiments and constitutive analysis. Methods: Different experimental tests (failure tensile tests, unconfined compression at different strain rates, stress relaxation tests) were conducted on human plantar skin samples cut along the posterior-anterior (PA), lateral-medial (LM), and cranial-caudal (CC) directions. Then, experimental results were used to identify, through an inverse analysis, the parameters of the anisotropic visco-hyperelastic constitutive model adopted to describe the skin's mechanical response. Results: Plantar skin’s non-linear, anisotropic, and time-dependent behavior, with differences between the anterior and posterior foot’s regions. In addition, the constitutive model adopted is able to capture the mechanical behavior of the plantar skin Failure tensile tests showed that PA directions exhibited higher elastic modulus than LM directions in both posterior (22.05 vs 12.91 MPa) and anterior (17.39 vs 12.82 MPa) regions, while the unconfined compression tests revealed that compressive elastic moduli in the posterior region increased with increasing strain rates. Conclusion: The proposed model provides new insights into the mechanics of plantar skin, being a valuable tool for applications such as diagnosing skin diseases and developing skin substitutes.

The human plantar fascia is a soft connective tissue that extends from the calcaneal tuberosity to the metatarsal region, providing static support to the foot and functioning as a shock absorber. Its roles and pathology are closely linked to its collagen fiber composition. Therefore, in-depth investigation of the mechanics of this tissue, an area with limited prior research, is crucial, especially for developing accurate in silico foot models for clinical and surgical purposes. This study aims to address this gap by combining experimental procedures, microscopic analysis of collagen fibers, and constitutive modeling to accurately describe the behavior of the plantar fascia. Uniaxial extension tests, conducted with a multi-step stress relaxation protocol until failure, show behavioral differences between samples cut along the proximal-distal and lateral-medial directions. This is supported by histological analysis, which shows that collagen fibers predominantly align along the proximal-distal axis. The experimental data, together with histological findings, are used to calibrate the parameters of a nonlinear viscoelastic, fiber-reinforced model through an inverse analysis process. This model captures the mechanical response of the plantar fascia, with the fiber component described using a generalized structural tensor formulation. The new insights gained provide a valuable understanding of the mechanical behavior of the human plantar fascia and pave the way for improvements in existing computational models. Statement of Significance: The human plantar fascia has received limited attention, with few studies partially addressing its mechanical behavior. This work provides a comprehensive characterization of the tissue by integrating mechanical testing, histology, and constitutive modeling. The histological assessment of samples collected along different tissue directions enables the quantification of collagen fiber orientation and dispersion, offering new insights into the anisotropy of the tissue. Mechanical experiments are employed to develop and calibrate an anisotropic visco-hyperelastic model, capable of capturing the full complexity of the tissue's mechanics. Completed by a validation of the proposed model, this study delivers the essential components needed for the development of accurate in silico foot models, with potential applications in clinical and surgical evaluation of the plantar fascia.