Browsing by Author "Röhrle, Oliver (Prof. PhD)"

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A 3D continuum-mechanical model for forward-dynamics simulations of the upper limb
(Stuttgart : Institut für Mechanik (Bauwesen), Lehrstuhl für Kontinuumsmechanik, Research Group on Continuum Biomechanics and Mechanobiology, Universität Stuttgart, 2015) Sprenger, Michael; Röhrle, Oliver (Prof. PhD)
In this thesis, a suitable theoretical modelling procedure is presented, providing numerical simulations of the material behaviour of muscle-tendon complexes that are included into an articulation. In order to do that, it is necessary to introduce the anatomical and physiological fundamentals of the musculoskeletal system and in particular the upper limb. Caused by the complex microscopic property of the participating muscles, appropriate constitutive equations for the muscle, tendon, and other soft tissues are presented and embedded into the Theory of Finite Elasticity which provides a suitable framework in describing the finite deformation regime. The resulting set of coupled partial differential equations is spatially discretised using the finite element method, which has proven to provide a powerful numerical technique for finding approximate solutions to such BVP. A contact formulation is modularly included to the finite element method to consider contact between the elastic muscle-tendon complexes and rigid bones. The solution of the subject specific BVP is achieved within CMISS. The resulting system of equations was solved in a monolithic manner. While the material mechanical contribution is linearised numerically, the contact mechanical contribution is linearised analytically. The geometry of the Upper Limb Model is established from the virtual human data set. By introducing the Upper Limb Model with its static equivalent system, a continuum-mechanically based framework could be established. This enables stand-alone investigations as well as a coupling to other frameworks. Three different concepts to facilitate the muscle activation are presented in order to use the Upper Limb Model and the equivalent static system. In a first step, muscle activation is prescribed to demonstrate the feasibility of the system and investigated its convergence behaviour as a stand-alone framework. In a second step, the Upper Limb Model is linked to the forward-inverse model established by Prof. David Lloyd’s Musculoskeletal Research Group. Therefore, experimental data is acquired and processed. The results of the forward-inverse model are compared to those of the Upper Limb Model. The third step was conceptually introduced but not implemented. Yet, this concept of coupling FE simulations to MBS is very promising. Besides the well known tendon-displacement method, a second method to determine the lever arm is established by employing properties of the muscle force such as its point of action and orientation. The in silico experiments produced muscle reaction forces, muscle fibre stretch distributions, lever arms, and equilibrium positions. In addition, the impact of contact on a musculoskeletal system is investigated. These results are elaborately visualised and discussed to provide a better mechanical understanding of the examined musculoskeletal system.