Pressure ulcers (PUs), also known as pressure sores, are localized damage to the skin and underlying tissues, usually occurring over a bony prominence and caused by sitting or lying in one position for long time. PUs are a detriment to the well-being of people who lose their mobility either permanently or temporarily, and high morbidity and mortality are associated with PUs. Although the initiating mechanism of PUs is still unclear, it is commonly accepted that internal normal and shear stresses, due to the presence of unrelieved external loads, play a central role in the formation and development of these wounds. Despite the significance of internal stresses in PUs formation, interfacial pressures, which are a surface measure of stress, are the indicators commonly used to develop practices and protocols to minimize loading on the soft tissue. However, no direct correlation exists between interfacial pressure and internal stresses of soft tissue. Therefore, tools and methods that can show internal distributions of soft tissue's stresses and strains as a response to external loading are needed.The ability of finite element (FE) models to accurately represent the anatomical structure of the leg and buttocks area and to estimate the localized stress/strain fields within highly deformable media, makes them powerful tools to investigate soft tissue response to external loadings. Despite the significant advancement previous studies have achieved, there are still important aspects in human thigh-buttock soft tissue modeling area that need to be improved. Two challenges are identified in this dissertation: 1) Microstructurally motivated skin modeling for an individual skin layer in finite element model. 2) Parameters estimation associated with large deformations.To address the first challenge, a microstructurally based constitutive model is proposed to describe the mechanical behavior of skin. The constitutive model incorporated the distribution of collagen fiber bundle orientations and relative collagen content measured from histology, and shows good agreement with the tensile test data.To address the second challenge, an optimization procedure that is able to match nonlinear behaviors between FE simulation and in vivo experimental data is developed. The difference between 3D and semi-3D model is quantified, and the accuracy of four commonly used constitutive model representing soft tissue nonlinear mechanical behavior is compared.Finally, a thigh FE model that has detailed anatomical representation of different soft tissue types, i.e., skin, fat, and muscle, is developed. The subject-specific in vivo experimental data are used to inform the optimization procedure to obtain best-fit constitutive parameters for different soft tissue types. The research in this dissertation provides an approach to describe the in vivo mechanical behavior of soft tissues in thigh-buttock area accurately through FE modeling. The constitutive parameters informed by in vivo data in this dissertation are valuable to facilitate future FE modeling studies to achieve accurate internal stress/strain distribution of soft tissues in thigh-buttock area.
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