Muscle Fatigue in Musculoskeletal Numerical Models

The investigation of the musculoskeletal system is a challenging task, since comprehensive knowledge of muscle and joint forces within the human body is required. Therefore, in recent years numerical models have been developed for a better understanding of the musculoskeletal system. Especially for the investigation of long-term effects, the issue of muscle fatigue needs to be taken into consideration in these models. The objectives of this thesis was to develop a novel EMG based muscle fatigue algorithm and the implementation into a state-of-the-art musculoskeletal modelling system. This included the investigation of the progress of muscle fatigue of single muscles, as well as the behaviour of muscle recruitment pattern when experiencing fatigue. Therefore, two experimental studies were conducted in the course of this thesis, in order to analyse the progress of muscle fatigue of single muscles in correlation with relative muscle loadings and to study the behaviour of muscle recruitment pattern of thorax muscles when experiencing fatigue. Based on the results of the first study a fatigue algorithm was developed and implemented to the AnyBody Modeling SystemTM (AMS). Both experimental studies were simulated in the altered AMS to validate the fatigue algorithm and to analyse the behaviour of the muscle recruitment solver of the modified system. The results show a good correlation between the simulated muscle fatigue and the experimental data. Furthermore, it revealed a reduction of maximum force capacity of the muscles of about 10-15% compared to the non-fatigued condition. The analysis of the muscle recruitment pattern indicated an additional activation of muscles in the upper back as well as the abdomen. The numerical simulation of these exercises in the AMS revealed a shift of muscle activity to the upper back

Penning-trap mass spectrometry of highly charged, neutron-rich Rb and Sr isotopes in the vicinity of A≈100A\approx100

The neutron-rich mass region around $A\approx100$ presents challenges for modeling the astrophysical $r$-process because of rapid shape transitions. We report on mass measurements using the TITAN Penning trap at TRIUMF-ISAC to attain more reliable theoretical predictions of $r$-process nucleosynthesis paths in this region. A new approach using highly charged ($q=15+$) ions has been applied which considerably saves measurement time and preserves accuracy. New mass measurements of neutron-rich $^{94,97,98}$Rb and $^{94,97-99}$Sr have uncertainties of less than 4 keV and show deviations of up to 11$\sigma$ to previous measurements. An analysis using a parameterized $r$-process model is performed and shows that mass uncertainties for the A=90 abundance region are eliminated

Dual Mode NOx Sensor: Measuring Both the Accumulated Amount and Instantaneous Level at Low Concentrations

The accumulating-type (or integrating-type) NOx sensor principle offers two operation modes to measure low levels of NOx: The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NOx. Experimental results are presented demonstrating the sensor’s integrating properties for the total amount detection and its sensitivity to both NO and to NO2. We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NOx in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed

A third crystal form of Wolinella succinogenes quinol:fumarate reductase reveals domain closure at the site of fumarate reduction

Quinol:fumarate reductase (QFR) is a membrane protein complex that couples the reduction of fumarate to succinate to the oxidation of quinol to quinone. Previously, the crystal structure of QFR from Wolinella succinogenes was determined based on two different crystal forms, and the site of fumarate binding in the flavoprotein subunit A of the enzyme was located between the FAD‐binding domain and the capping domain [Lancaster, C.R.D., Kröger, A., Auer, M., & Michel, H. (1999) Nature402, 377–385]. Here we describe the structure of W. succinogenes QFR based on a third crystal form and refined at 3.1 Å resolution. Compared with the previous crystal forms, the capping domain is rotated in this structure by approximately 14° relative to the FAD‐binding domain. As a consequence, the topology of the dicarboxylate binding site is much more similar to those of membrane‐bound and soluble fumarate reductase enzymes from other organisms than to that found in the previous crystal forms of W. succinogenes QFR. This and the effects of the replacement of Arg A301 by Glu or Lys by site‐directed mutagenesis strongly support a common mechanism for fumarate reduction in this superfamily of enzymes

Essential role of Glu-C66 for menaquinol oxidation indicates transmembrane electrochemical potential generation by Wolinella succinogenes fumarate reductase

Quinol:fumarate reductase (QFR) is a membrane protein complex that couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, in a reaction opposite to that catalyzed by the related enzyme succinate:quinone reductase (succinate dehydrogenase). In the previously determined structure of QFR from Wolinella succinogenes, the site of fumarate reduction in the flavoprotein subunit A of the enzyme was identified, but the site of menaquinol oxidation was not. In the crystal structure, the acidic residue Glu-66 of the membrane spanning, diheme-containing subunit C lines a cavity that could be occupied by the substrate menaquinol. Here we describe that, after replacement of Glu-C66 with Gln by site-directed mutagenesis, the resulting mutant is unable to grow on fumarate and the purified enzyme lacks quinol oxidation activity. X-ray crystal structure analysis of the Glu-C66 → Gln variant enzyme at 3.1-Å resolution rules out any major structural changes compared with the wild-type enzyme. The oxidation-reduction potentials of the heme groups are not significantly affected. We conclude that Glu-C66 is an essential constituent of the menaquinol oxidation site. Because Glu-C66 is oriented toward a cavity leading to the periplasm, the release of two protons on menaquinol oxidation is expected to occur to the periplasm, whereas the uptake of two protons on fumarate reduction occurs from the cytoplasm. Thus our results indicate that the reaction catalyzed by W. succinogenes QFR generates a transmembrane electrochemical potential

Experimental support for the "E pathway hypothesis" of coupled transmembrane e^- and H⁺ transfer in dihemic quinol:fumarate reductase

Reconciliation of apparently contradictory experimental results obtained on the quinol:fumarate reductase, a diheme-containing respiratory membrane protein complex from Wolinella succinogenes, was previously obtained by the proposal of the so-called “E pathway hypothesis.” According to this hypothesis, transmembrane electron transfer via the heme groups is strictly coupled to cotransfer of protons via a transiently established pathway thought to contain the side chain of residue Glu-C180 as the most prominent component. Here we demonstrate that, after replacement of Glu-C180 with Gln or Ile by site-directed mutagenesis, the resulting mutants are unable to grow on fumarate, and the membrane-bound variant enzymes lack quinol oxidation activity. Upon solubilization, however, the purified enzymes display ≈1/10 of the specific quinol oxidation activity of the wild-type enzyme and unchanged quinol Michaelis constants, K m. The refined x-ray crystal structures at 2.19 Å and 2.76 Å resolution, respectively, rule out major structural changes to account for these experimental observations. Changes in the oxidation–reduction heme midpoint potential allow the conclusion that deprotonation of Glu-C180 in the wild-type enzyme facilitates the reoxidation of the reduced high-potential heme. Comparison of solvent isotope effects indicates that a rate-limiting proton transfer step in the wild-type enzyme is lost in the Glu-C180 → Gln variant. The results provide experimental evidence for the validity of the E pathway hypothesis and for a crucial functional role of Glu-C180. anaerobic respiration; atomic model; bioenergetics; membrane protein; succinate:quinone oxidoreductas