University Koblenz

Koblenz, Germany

University Koblenz

Koblenz, Germany
SEARCH FILTERS
Time filter
Source Type

Rockenfeller R.,University Koblenz | Gunther M.,University of Stuttgart | Gunther M.,Friedrich - Schiller University of Jena
Computer Methods in Applied Mechanics and Engineering | Year: 2017

The active isometric force–length relation (FLR) of mammalian skeletal muscle is one of the most investigated characteristics throughout biomechanics. Numerous experiments have been conducted that reveal insights on the mechanisms of muscle contraction. However, the entity of molecular processes is yet not fully understood. Modelers thus rely on a rather descriptive characterization of experimental findings. Starting with the well-known, piece-wise linear formulation by A. Gordon, A. Huxley and F. Julian in 1966, a variety of structurally distinguishable FLR models have been developed. Five decades later, the original idea was taken up to derive the first purely physiological FLR formulation, based on sliding filament and cross-bridge theory. This derivation offers us the opportunity to contrast a broad variety of 19 distinct FLR models. By comparing their ability to fit experimental data, we deduce qualitative as well as quantitative acceptance criteria such as symmetry, normalization, complexity, and physiological interpretability. Resultant, different models comprise different advantages. The new piece-wise linear model is the overall most favorable, a further piece-wise exponential model is mathematically more robust, a polynomial model of fourth order has the best optimization properties, and a certain purely exponential model is the computationally cheapest. This work gives a detailed overview, as well as a mathematical/physiological assessment of existing FLR models, and serves as a guideline for modelers to choose a proper formulation based on individual requirements. © 2016 Elsevier B.V.


Foesel B.U.,Ludwig Maximilians University of Munich | Foesel B.U.,Leibniz Institute DSMZ Deutsche Sammlung von Mikroorganismen und Zellkulturen | Nagele V.,Ludwig Maximilians University of Munich | Naether A.,University of Bremen | And 14 more authors.
Environmental Microbiology | Year: 2014

16S rRNA genes and transcripts of Acidobacteria were investigated in 57 grassland and forest soils of three different geographic regions. Acidobacteria contributed 9-31% of bacterial 16S rRNA genes whereas the relative abundances of the respective transcripts were 4-16%. The specific cellular 16S rRNA content (determined as molar ratio of rRNA:rRNA genes) ranged between 3 and 80, indicating a low in situ growth rate. Correlations with flagellate numbers, vascular plant diversity and soil respiration suggest that biotic interactions are important determinants of Acidobacteria 16S rRNA transcript abundances in soils. While the phylogenetic composition of Acidobacteria differed significantly between grassland and forest soils, high throughput denaturing gradient gel electrophoresis and terminal restriction fragment length polymorphism fingerprinting detected 16S rRNA transcripts of most phylotypes in situ. Partial least squares regression suggested that chemical soil conditions such as pH, total nitrogen, C:N ratio, ammonia concentrations and total phosphorus affect the composition of this active fraction of Acidobacteria. Transcript abundance for individual Acidobacteria phylotypes was found to correlate with particular physicochemical (pH, temperature, nitrogen or phosphorus) and, most notably, biological parameters (respiration rates, abundances of ciliates or amoebae, vascular plant diversity), providing culture-independent evidence for a distinct niche specialization of different Acidobacteria even from the same subdivision. © 2013 John Wiley & Sons Ltd and Society for Applied Microbiology.


Kern-Isberner G.,TU Dortmund | Beierle C.,University of Hagen | Finthammer M.,University of Hagen | Thimm M.,University Koblenz
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2012

The handling of uncertain information is of crucial importance for the success of expert systems. This paper gives an overview on logic-based approaches to probabilistic reasoning and goes into more details about recent developments for relational, respectively first-order, probabilistic methods like Markov logic networks, and Bayesian logic programs. In particular, we feature the maximum entropy approach as a powerful and elegant method that combines convenience with respect to knowledge representation with excellent inference properties. While comparing the different approaches is a difficult task due to the variety of the available concepts and to the absence of a common interface, we address this problem from both a conceptual and practical point of view. On a conceptual layer we propose and discuss several criteria by which first-order probabilistic methods can be distinguished, and apply these criteria to a series of approaches. On the practical layer, we briefly describe some systems for probabilistic reasoning, and go into more details on the KReator system as a versatile toolbox for various approaches to first-order probabilistic relational learning, modelling, and reasoning. Moreover, we illustrate applications of probabilistic logics in various scenarios. © 2012 Springer-Verlag.


Furbach U.,University Koblenz | Furbach F.,University of Kaiserslautern | Freksa C.,University of Bremen
CEUR Workshop Proceedings | Year: 2016

In this note, we discuss and analyse a shortest path finding approach using strong spatial cognition. It is compared with a symbolic graph-based algorithm and it is shown that both approaches are similar with respect to structure and complexity. Nevertheless, the strong spatial cognition solution is easy to understand and even pops up immediately when one has to solve the problem.


Rockenfeller R.,University Koblenz | Gunther M.,University of Stuttgart | Gunther M.,Friedrich - Schiller University of Jena
Mathematical Biosciences | Year: 2016

Determining dynamic properties of mammalian muscles, such as activation characteristics or the force–velocity relation, challenges the experimentalist. Tracking system, apparatus stiffness, load oscillation, force transducer, other sensors, and additional measuring devices may be incorporated, integrated and evaluated in an experimental set-up. In contrast, isometric contraction experiments (ICEs) are less challenging, but are generally not considered to reveal dynamic muscle properties. Yet, a sensitivity analysis of our muscle model discloses the influence of concentric, eccentric and activation parameters on isometric force development. Accordingly, we used solely experimental ICE data to identify muscle model parameters that generally describe concentric as well as eccentric muscle performance. In addition, we compared two different activation dynamics in regards to their physiological relevance to improve model-fits to ICE data. To this end, we optimized different combinations of such dynamic parameter subsets with respect to their influence on contraction solutions. Depending on muscle length in our optimized model, the contractile element reached shortening peaks during activation in the range 9–39% of its maximum contraction velocity, and about 8–25% during lengthening when deactivated. As a first result, we suggest one formulation of activation dynamics to be superior. Second, the step in slope of the force-velocity relation at isometric force was found to be the least influential among all dynamic parameters. Third, we suggest a specially designed isometric experimental set-up to estimate this transition parameter. Fourth, because of an inconsistency in literature, we developed a simple method to determine switching times of the neural stimulation and thus electro-mechanical delay (EMD) values from measuring muscle force in ICEs only. © 2016 Elsevier Inc.


Wagner C.,University Koblenz | Singer P.,Graz University of Technology | Strohmaier M.,University Koblenz | Huberman B.A.,Hewlett - Packard
WWW 2014 - Proceedings of the 23rd International Conference on World Wide Web | Year: 2014

One potential disadvantage of social tagging systems is that due to the lack of a centralized vocabulary, a crowd of users may never manage to reach a consensus on the description of resources (e.g., books, users or songs) on the Web. Yet, previous research has provided interesting evidence that the tag distributions of resources may become semantically stable over time as more and more users tag them. At the same time, previous work has raised an array of new questions such as: (i) How can we assess the semantic stability of social tagging systems in a robust and methodical way? (ii) Does semantic stabilization of tags vary across different social tagging systems and ultimately, (iii) what are the factors that can explain semantic stabilization in such systems? In this work we tackle these questions by (i) presenting a novel and robust method which overcomes a number of limitations in existing methods, (ii) empirically investigating semantic stabilization processes in a wide range of social tagging systems with distinct domains and properties and (iii) detecting potential causes for semantic stabilization, specifically imitation behavior, shared background knowledge and intrinsic properties of natural language. Our results show that tagging streams which are generated by a combination of imitation dynamics and shared background knowledge exhibit faster and higher semantic stability than tagging streams which are generated via imitation dynamics or natural language phenomena alone. Copyright is held by the International World Wide Web Conference Committee (IW3C2).


Frehse J.,University of Bonn | Steinhauer M.,University Koblenz | Weigant W.,University of Bonn
Archive for Rational Mechanics and Analysis | Year: 2010

We consider the Navier-Stokes equations for compressible isothermal flow in the steady two-dimensional case and show the existence of a weak solution for homogeneous Dirichlet boundary conditions. © 2010 Springer-Verlag.


Frehse J.,University of Bonn | Steinhauer M.,University Koblenz | Weigant W.,University of Bonn
Journal of Mathematical Fluid Mechanics | Year: 2011

We consider the Navier-Stokes equations for compressible isothermal flow in the steady two dimensional case and show the existence of a weak solution in the case of mixed boundary conditions. The proof is based on an analysis of the stream function, exploiting the convective term. © 2009 Birkhäuser Verlag Basel/Switzerland.


PubMed | University of Stuttgart and University Koblenz
Type: | Journal: Computational and mathematical methods in medicine | Year: 2015

We mathematically compared two models of mammalian striated muscle activation dynamics proposed by Hatze and Zajac. Both models are representative for a broad variety of biomechanical models formulated as ordinary differential equations (ODEs). These models incorporate parameters that directly represent known physiological properties. Other parameters have been introduced to reproduce empirical observations. We used sensitivity analysis to investigate the influence of model parameters on the ODE solutions. In addition, we expanded an existing approach to treating initial conditions as parameters and to calculating second-order sensitivities. Furthermore, we used a global sensitivity analysis approach to include finite ranges of parameter values. Hence, a theoretician striving for model reduction could use the method for identifying particularly low sensitivities to detect superfluous parameters. An experimenter could use it for identifying particularly high sensitivities to improve parameter estimation. Hatzes nonlinear model incorporates some parameters to which activation dynamics is clearly more sensitive than to any parameter in Zajacs linear model. Other than Zajacs model, Hatzes model can, however, reproduce measured shifts in optimal muscle length with varied muscle activity. Accordingly we extracted a specific parameter set for Hatzes model that combines best with a particular muscle force-length relation.


PubMed | University of Stuttgart and University Koblenz
Type: | Journal: Mathematical biosciences | Year: 2016

Determining dynamic properties of mammalian muscles, such as activation characteristics or the force-velocity relation, challenges the experimentalist. Tracking system, apparatus stiffness, load oscillation, force transducer, other sensors, and additional measuring devices may be incorporated, integrated and evaluated in an experimental set-up. In contrast, isometric contraction experiments (ICEs) are less challenging, but are generally not considered to reveal dynamic muscle properties. Yet, a sensitivity analysis of our muscle model discloses the influence of concentric, eccentric and activation parameters on isometric force development. Accordingly, we used solely experimental ICE data to identify muscle model parameters that generally describe concentric as well as eccentric muscle performance. In addition, we compared two different activation dynamics in regards to their physiological relevance to improve model-fits to ICE data. To this end, we optimized different combinations of such dynamic parameter subsets with respect to their influence on contraction solutions. Depending on muscle length in our optimized model, the contractile element reached shortening peaks during activation in the range 9-39% of its maximum contraction velocity, and about 8-25% during lengthening when deactivated. As a first result, we suggest one formulation of activation dynamics to be superior. Second, the step in slope of the force-velocity relation at isometric force was found to be the least influential among all dynamic parameters. Third, we suggest a specially designed isometric experimental set-up to estimate this transition parameter. Fourth, because of an inconsistency in literature, we developed a simple method to determine switching times of the neural stimulation and thus electro-mechanical delay (EMD) values from measuring muscle force in ICEs only.

Loading University Koblenz collaborators
Loading University Koblenz collaborators