Human skeletal muscle behavior in vivo: Finite element implementation, experiment, and passive mechanical characterization.

In this study, useful methods for active human skeletal muscle material parameter determination are provided. First, a straightforward approach to the implementation of a transversely isotropic hyperelastic continuum mechanical material model in an invariant formulation is presented. This procedure is found to be feasible even if the strain energy is formulated in terms of invariants other than those predetermined by the software's requirements. Next, an appropriate experimental setup for the observation of activation-dependent material behavior, corresponding data acquisition, and evaluation is given. Geometry reconstruction based on magnetic resonance imaging of different deformation states is used to generate realistic, subject-specific finite element models of the upper arm. Using the deterministic SIMPLEX optimization strategy, a convenient quasi-static passive-elastic material characterization is pursued; the results of this approach used to characterize the behavior of human biceps in vivo indicate the feasibility of the illustrated methods to identify active material parameters comprising multiple loading modes. A comparison of a contact simulation incorporating the optimized parameters to a reconstructed deformed geometry of an indented upper arm shows the validity of the obtained results regarding deformation scenarios perpendicular to the effective direction of the nonactivated biceps. However, for a valid, activatable, general-purpose material characterization, the material model needs some modifications as well as a multicriteria optimization of the force-displacement data for different loading modes.

Mechanical soft tissue property validation in tissue engineering using magnetic resonance imaging experimental research.

RATIONALE AND OBJECTIVES: To perform magnetic resonance imaging (MRI) scans regarding material parameter and model validation in computational simulations of mechanical interaction of human soft-tissue with body-supporting devices, enhanced medical prognosis in pressure sore prophylaxis, and comfort optimization in automotive and aircraft seating. MATERIALS AND METHODS: In vivo human gluteal fat and passive muscle tissue material parameters of a volunteer evaluated via combined MRI numerical method and body-supporting foam material parameters employed in finite element (FE) simulations of tissue-support interaction were verified by a defined loading scenario using MRI. MRI of the loaded configurations were performed and compared with simulation results for displacement field information. RESULTS: Deformation of gluteal skin/fat and passive muscle-tissue and support material under interacting loading using numerical simulation complied with the MRI results. Accordance was found for deformed skin surface and internal fat-muscle tissue boundaries by superimposing experimental and numerical outputs. Further evidence of established through in vivo gluteal tissue parameters was thus provided and tissue material isotropy assumption was shown for use in simulated buttock loading interactions. Additionally, a new concept of FE model validation regarding non-MRI-sensitive materials such as polyurethane foam was introduced comprising peripheral surface visualization. CONCLUSION: Imaging techniques are essential in biomechanical modeling and provide key information regarding model validation and validity assessment.