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

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Open Access
Automated 3D ultrasound imaging for investigating skeletal muscle in static and dynamic conditions
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Continuum Biomechanics and Mechanobiology Research Group, University of Stuttgart, 2024) Sahrmann, Annika; Röhrle, Oliver (Prof., PhD)
Open Access
Bioelectromagnetic fields for studying neuromuscular physiology : in silico investigations of EMG and MMG
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2023) Klotz, Thomas; Röhrle, Oliver (Prof., PhD)
Skeletal muscles generate bioelectromagnetic fields that contain information about the neural control of motions and the function of the muscle. One distinguishes between electromyography (EMG), the measurement of the muscle-induced electric potential field, and magnetomyography (MMG), the recording of muscle-induced magnetic fields. EMG is a well-established methodology, and its limitations have been extensively discussed in the scientific literature. In contrast, MMG is an emerging methodology with the potential to overcome some of the inherent limitations of EMG. To unlock the full potential of MMG, it is essential to support empirical observations from experiments with a solid theoretical understanding of muscle-induced bioelectromagnetic fields. Therefore, this thesis derives a novel multiscale skeletal muscle model that can predict realistic EMG and MMG signals. This model is used to conduct the first systematic comparison between surface EMG and non-invasive MMG. By using simulations, all system parameters can be controlled precisely. This would not be possible experimentally. The fundamental properties of EMG and MMG are systematically explored using simulations comparable to electrically or reflex-evoked contractions. Notably, it is shown that non-invasive MMG data is spatially more selective than comparable high-density EMG data. This property, for example, is advantageous for decomposing signals of voluntary contractions into individual motor unit spike trains. Using a novel in silico trial framework, it is demonstrated that non-invasive MMG-based motor unit decomposition is superior to the well-established surface EMG-based motor unit decomposition.
Open Access
Chemo-electro-mechanical modelling of the neuromuscular system
(2015) Heidlauf, Thomas; Röhrle, Oliver (Prof., PhD)
Body movement is the result of cascades of complex chemical, electrical, and mechanical processes taking place at different length and time scales. This thesis deals with the biophysical modelling of these processes. In detail, the generation of electrical signals in spinal motor neurons is investigated based on the Hodgkin-Huxley formalism. Next, the complex signaling pathway leading from electrical excitation to contraction and force generation of the muscle fibres is modelled. Based on a structural model of the muscle and the bidomain equations, a method is proposed to predict electromyographic signals, which are frequently recorded in the clinic and result from the propagation of electrical signals along the muscle fibres to induce the contraction. Extending this model by a continuum-mechanical approach, a multiscale model of the neuromuscular system is obtained that considers chemical, electrical, and mechanical properties and allows to predict force generation, muscle deformation, and the EMG signal during fixed-length and non-isometric contractions. The proposed framework can potentially be used as an in-silico laboratory to investigate changes in the behaviour resulting from pathological conditions or drug treatment.
Open Access
Continuum-mechanical modelling across scales : homogenisation methods and their application to microstructurally-based skeletal muscle modelling
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2021) Bleiler, Christian; Röhrle, Oliver (Prof., PhD)
A variety of materials, such as biological soft tissues, exhibit large inter- and intra-subject microstructural variations that cannot be captured with individual material tests on the macroscopic observation scale. In such scenarios, multiscale modelling approaches are used instead, which explicitly incorporate the microstructure and provide macroscopic quantities through suitable homogenisation methods. This enables the description of biological soft tissue behaviour arising from microstructural changes, for example, those caused by disease. This thesis, therefore, deals with the multiscale continuum-mechanical modelling of materials and the particular application of such methods to the description of skeletal muscle tissue. Besides a general introduction to the subject, novel analytical estimates for the effective macroscopic potential of two-phase, hyperelastic, incompressible solids are presented. These are based on the so-called tangent second-order homogenisation method and are applicable for highly nonlinear, anisotropic material behaviour at large strains. Subsequently, a novel multiscale model for skeletal muscle is presented, which describes the macroscopic behaviour of soft tissue as a direct consequence of properties at smaller scales, such as the stiffness and arrangement of individual collagen fibres. The methods and models presented in this thesis are discussed by means of representative examples, thus demonstrating their merits in comparison to alternative approaches.
Open Access
Data-driven modelling of neuromechanical adaptation in skeletal muscles in response to isometric exercise
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2022) Altan, Neriman Ekin; Röhrle, Oliver (Prof., PhD)
This study aims to model the changes in the behaviour of motor neurons of the vastus lateralis in response to unilateral isometric knee extension exercise (UIKEE). For this, the phenomenological motor control model by Fuglevand et al. (1993) has been used. Input parameters for this model have been calibrated against data from experimental studies available in literature by using Bayesian updating. The pre-exercise state of the motor neuron pool of the muscle describing the recruitment behaviour as well as the contractile properties of the motor neurons have been constructed. Data collected from a systematic review on the change in isometric strength due to UIKEE has been modelled using Bayesian lonigutidinal model-based meta-analysis. Using the model of the change in isometric strength, increase in the average motor neuron discharge rate following UIKEE has been quantified.
Open Access
Development and implementation of next-generation Retina-on-Chip platforms
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair for Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2024) Chuchuy, Johanna; Röhrle, Oliver (Prof., PhD)
Open Access
Generation, probing, and biophysical stimulation of human microtissues in microfluidic Organ-on-Chip platforms
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2022) Schneider, Oliver; Röhrle, Oliver (Prof., PhD)
Over the last decade Organ-on-Chip (OoC) emerged as disruptive technology combining aspects of microfluidics and tissue engineering. OoCs culture human tissues in tailored microenvironments under microfluidic perfusion, yielding an unprecedented recapitulation of human physiology. So far, most systems predominantly focus on physiological tissue generation. However, it is crucial to integrate stimulation and readout capabilities, leveraging OoCs from bare tissue generation tools to advanced integrated experimental platforms. This thesis focuses on the development and characterization of novel microphysiological systems to probe and actuate tissues on the microscale. We present two Heart-on-Chip platforms enabling the generation of aligned cardiac muscle fibers and investigate the integration of force and O2 sensing as well as electrical stimulation capabilities. Furthermore, we introduce and characterize two OoCs enabling the precise delivery of biomechanical stretch and compression stimuli. All in all, the systems developed in the framework of this thesis provide a flexible toolkit amenable for disease modeling or personalized medicine, offering advanced experimental capabilities for manipulating and interrogating integrated tissues.
Open Access
Image-based analysis of biological network structures using machine learning and continuum mechanics
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2020) Asgharzadeh, Pouyan; Röhrle, Oliver (Prof., PhD)
Open Access
Insights into human alpha-motoneuron discharge properties during stretch reflexes : an in-silico approach
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2024) Schmid, Laura; Röhrle, Oliver (Prof., PhD)
Open Access
Mechanobiological approach for skeletal muscle adaptation
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2022) Villota Narváez, Yesid Alexis; Röhrle, Oliver (Prof., PhD)
Skeletal muscle adaptation includes changes in shape and size, changes at the organelle function and distribution inside muscle cells, and changes at the molecular scale. Prolonged repetition of loading conditions (physical activity and exercise) promote the activation or inhibition of certain biochemical species related to protein content (muscle fiber size) and protein isoform differentiation (muscle fiber type). Current muscle-function models are not suitable to describe muscle adaptation for three main reasons: first, current models typically focus on muscle contraction, which occurs in the time scale of seconds; second, the mechanical description uses fixed parameters or parameters that are not controlled by biological aspects; and third, the mechanics of growth considers only sustained boundary conditions and overlooks biological aspects. The aim of this research is to mathematically model muscle adaptation by considering two biological aspects of the adaptation process (protein synthesis and gene-program switch), and a continuum mechanical model whose parameters evolve according to the biological part of the model. This thesis presents new models for the adaptation of skeletal muscle protein content, cross sectional area (CSA), maximum voluntary contraction (MVC) and fiber distribution.
Open Access
Modelling the functional heterogeneity of skeletal muscles : enriching continuum-mechanical models on a motor-unit level
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2021) Saini, Harnoor Deep Singh; Röhrle, Oliver (Prof., PhD)
Biomechanical computer models provide novel insights into musculoskeletal function by overcoming technical and ethical barriers faced by experimental techniques. Current macroscopic, continuum-mechanical skeletal muscle models, however, neglect certain aspects of motor-unit physiology and thus oversimplify muscle function. This Ph.D. thesis deals with methods to enrich such models with microstructurally derived motor-unit information. By doing so, contraction dynamics and joint-kinematics can be predicted, for the first time, as a combination of individual motor-unit -activity, -properties, and (three-dimensional) -anatomy. Such a model uncovers unique relationships between neuromuscular physiology and muscle function, for example, the role of motor-unit remodelling (typically occurring during ageing and neuromuscular disorders) on joint-function. This integrated neuro-musculoskeletal modelling approach can be applied to better understand phenomena such as fatigue and be used to inform medical interventions by predicting surgery outcomes or aiding movement rehabilitation protocols related to trauma, neuromuscular disorders, or ageing.
Open Access
A modelling-simulation-analysis workflow for investigating socket-stump interaction
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2019) Ramasamy, Ellankavi; Röhrle, Oliver (Prof., PhD)
In this thesis, a novel subject-specific modelling-simulation-analysis workflow is developed, which generates detailed stump models for analysis in a continuum-mechanical framework. With minimal human intervention, detailed stump models are generated from medical images, and are used in Finite Element (FE) simulations to study dynamic stump-socket interactions. Herein, the stump is composed of bone, individual muscles and fat, as opposed to the state-of-the-art models with fused muscles. An additional model representing the state-of-the-art stump geometry is generated for comparison with the proposed model. To showcase the necessity of detailed stump models, the state-of-the-art model is compared with the detailed model in a bipedal stance simulation. For this purpose, a nonlinear hyperelastic, transversely isotropic skeletal muscle constitutive law containing a deep tissue injury model, using continuum damage mechanics, is implemented in LS-DYNA. Internal strains and deep tissue injury during dynamic socket-stump interaction are analysed with bipedal stance and gait simulations. Further, the potential of forward dynamics with active stump models, and the possibility of realistic liner donning simulations are presented. Finally, the possibilities of using the proposed workflow in the context of determining socket fit and comfort are discussed.
Open Access
Multilevel convergence analysis : parallel-in-time integration for fluid-structure interaction problems with applications in cardiac flow modeling
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2020) Hessenthaler, Andreas; Röhrle, Oliver (Prof., PhD)
In this Ph.D. Thesis, multigrid-reduction-in-time (MGRIT) is considered as means to reduce the time-to-solution for numerical algorithms concerned with the solution of time-dependent partial differential equations (PDEs) arising in the field of fluid-structure interaction (FSI) modeling. As a parallel-in-time integration method, the MGRIT algorithm significantly increases the potential for parallel speedup by employing modern computer architectures, ranging from small-scale clusters to massively parallel high-performance computing platforms. In this work, the MGRIT algorithm is considered as a true multilevel method that can exhibit optimal scaling. Convergence of MGRIT is studied for the solution of linear and nonlinear (systems of) PDEs: from single- to multiphysics applications relevant to FSI problems in two and three dimensions. A multilevel convergence framework for MGRIT is derived that establishes a priori upper bounds and approximate convergence factors for a variety of cycling strategies (e.g., V- and F-cycles), relaxation schemes and parameter settings. The convergence framework is applied to a number of test problems relevant to FSI modeling, both linear and nonlinear as well as parabolic and hyperbolic in nature. An MGRIT variant is further proposed that exploits the time-periodicity that is present in many biomedical engineering applications, e.g., cyclic blood flow in the human heart. The time-periodic MGRIT algorithm proves capable of consistently reducing the time-to-solution of an existing simulation model with significant observed speedups.
Open Access
Simulating vertebroplasty using a multiphase continuum-mechanical approach : rheological characterization, numerical simulations, and experimental validation
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2024) Trivedi, Zubin; Röhrle, Oliver (Prof., PhD)
Percutaneous vertebroplasty is a surgical procedure for treating vertebral fractures involving injection of a so-called "bone cement'' into the vertebra. This Ph.D. thesis aimed to develop a computational model for simulating vertebroplasty, and thereby help practitioners determine the best operating parameters specific to each patient. The computational model employs a multiphase continuum-mechanical approach based on the Theory of Porous Media, along with discretization and upscaling methods specifically chosen and modified to suit the application. Apart from this, experiments were carried out to understand the behaviour of the bone cement in the context of vertebroplasty so that its behaviour can be correctly modelled. The developed computational model is validated using experiments done using a simple benchmark experiment. The simulations shed light on some crucial mechanical aspects of vertebroplasty that could determine the success or failure of the procedure.
Open Access
Towards a fast and stable dynamic skeletal muscle model
(Stuttgart : Institute for Modelling and Simulation of Biomechanical Systems, Chair of Continuum Biomechanics and Mechanobiology, University of Stuttgart, 2020) Mordhorst, Mylena; Röhrle, Oliver (Prof., PhD)
This thesis investigates the possibility to reduce the computational effort of a dynamic skeletal muscle model making use of model order reduction methods. For that purpose, a three-dimensional, nonlinear, dynamic skeletal muscle model based on the theory of incompressible finite hyperelasticity is introduced. After discretisation in space and time, using the mixed Taylor-Hood finite elements and the implicit Euler scheme, respectively, the obtained complex and high-dimensional differential algebraic equation system describing the three fields position, velocity and pressure, is investigated from a theoretical as well as computational point of view. Furthermore, the stability issues, encountered with a reduced-order model, built by projecting each field of the high-dimensional model onto a reduced subspace, are demonstrated. The reason for these problems is additionally investigated and confirmed from the theoretical perspective. In order to propose a suitable approach for obtaining a stable reduced order skeletal muscle model, the well-established technique of combining the reduced basis approximation with the proper orthogonal decomposition needs to be customised. The performance with respect to stability, effciency and accuracy of different reduced-order models, built from various combinations and sizes of subspaces, each of them again constructed from differently calculated POD bases, with and without enrichment by approximate supremizer solutions, is compared.