Within the context of a metabolic model of fatigue, formulated from a review of the literature, a decrease in intramuscular pH was identified as a potential limiting factor to the performance of high intensity exercise (HIE). This suggested a role for skeletal muscle buffering (B) in retarding the intramuscular acidosis typically incurred during HIE. Previous studies on human muscle, were largely unable to confirm this role, partly due to the lack of a clearly defined and consistently utilised method of measurement of B. The validity of the procedures currently employed in the determination of B was therefore investigated, revealing that many of the individual differences in B, previously attributed physiological significance, may have originated from inappropriate methodology. In a cross-sectional examination of the vastus lateralis muscle of young, active individuals, low correlations (p<0.05) between the muscle carnosine concentration ([CAR]) and in the 'in vitro B' (Bvit) (r=0.30), and between either [CAR] or Bvit and the % type II fibre area were observed. An elevated Bvit was shown to be of importance in minimising the decrement in pH during HIE, but did not per se permit the accumulation of a higher muscle lactate concentration, or allow for an enhanced HIE performance; these factors were more dependent on the % type II fibre area. The muscle pH, following dynamic HIE was highly variable between individuals, and showed a significant (p<0.05) negative correlation with the % type II fibre area. 16 weeks of isokinetic training of the quadriceps resulted in significant improvements (p<0.05) in dynamic HIE performance with no concomitant improvements in [CAR] or Bvit. The data suggest that the muscle buffer value per se, is not a major limiting factor to HIE, and that rather, the glycolytic capacity and pH tolerance of the muscle may be the more important factors to consider.

Tensile static and tension-tension fatigue behaviour has been studied in coupon specimens made from continuous fibre reinforced glass/epoxy and carbon/epoxy laminates of various lay-ups, including a series of GFRP and CFRP 0,90,0 cross-ply laminates with different 90&deg; ply thicknesses and CFRP 0, 90, +/-45 laminates with different ply stacking sequences. A variety of techniques has been used to monitor the accumulation of damage; microscopy on the polished edge of coupons, penetrant-enhanced X-radiography for CFRP laminates and visual observations for the transparent GFRP laminates. Under static loading, mechanical properties and damage thresholds are established for the onset of events such as cracking in the 90' and 45&deg; plies and delamination. The experimentally determined 90&deg; ply cracking threshold strains agree with predictions based on fracture mechanics, provided residual thermal strains are taken into account. Fatigue failure data are obtained for the CFRP laminates and plotted as conventional S-logN curves. The fatigue behaviour of the CFRP laminates has also been described qualitatively using a form of fatigue life representation in terms of the predominant damage mechanisms observed during cyclic loading. A detailed study of transverse ply matrix cracking showed that the mode of crack propagation depended on the type of loading. Crack growth across the width of the ply was instantaneous under static loading and at high cyclic stresses. At low cyclic stresses, i.e. below the static cracking threshold, cracks initiated only after a number of cycles (dependent on the stress level) and then grew slowly across the width of the ply throughout the course of loading. Slow crack growth was also observed at high cyclic stresses when the density of full width cracks was high and the crack spacing was small. The crack growth rate was found to be independent of crack length and to depend on crack spacing and hence was strongly influenced by interactions between neighbouring cracks. Fatigue growth of individual cracks was modelled using an approach based on an expression for the stress intensity factor at the tip of a transverse ply crack and a Paris fatigue crack growth relationship.

La fatigue cognitive représente un phénomène auquel nous sommes tous familiers. Nous en faisons quotidiennement l'expérience, celle-ci étant associée à une réduction de productivité, une augmentation de risques professionnels et une diminution de notre qualité de vie. Malgré l’importance sociétale de ces implications, qui ont fait de l'étude de la fatigue cognitive une de plus investiguées dans le domaine des sciences cognitives, il subsiste un manque de vision commune ainsi qu'une théorie unifiée de la fatigue cognitive, déterminant son origine et ses fonctions, tandis que son caractère stochastique continue à diviser le milieu scientifique. La présente dissertation présente une approche multimodale combinant des données comportementaux et de neuroimagerie spectroscopie proche infrarouge (fNIRS) pour investiguer les facteurs à l’origine du déclenchement du phénomène ainsi que les dynamiques cérébrales associées. Nos résultats suggèrent que la fatigue cognitive ressentie associée à la chute de performance est une fonction de la charge cognitive définie par le Time Based Shared Resources Model (TBRS). Au niveau neural, nous discutons la présence des systèmes de compensation et les changements de connectivité cérébrale dans ce déclenchement. et comment les possibles mécanismes responsables de la maintenance de performance durant des demandes attentionnelles soutenues y sont associées. / Doctorat en Sciences psychologiques et de l'éducation / info:eu-repo/semantics/nonPublished

La modélisation des effets de l’entraînement sur la performance permet de mieux conceptualiser les adaptations du sportif pour aider les entraîneurs confrontés à de multiples incertitudes, avec comme alternatives possibles à une performance de haut niveau, le surmenage, le surentraînement et la blessure. La quantification quotidienne de la CE est alors la première étape de la périodisation de l’entraînement. Notre revue de la littérature montre que ces effets sont un état non définitif des interactions entre différents facteurs exogènes et endogènes (Banister et coll., 1975, Busso et coll., 1990, Morton et coll., 1990). La notion de charge d’entrainement implique un niveau d’effet se traduisant dans l’organisme sous la forme de réactions fonctionnelles qui ont une certaine intensité et une certaine durée. Cela implique de suivre quotidiennement plusieurs marqueurs perceptifs.Les progrès de l’informatique et des objets communicants nous ont permis de concevoir un système informatique ergonomique afin de récupérer, de traiter, et de visualiser un grand volume de données, mais aussi de proposer une modélisation alternative des effets de l’entraînement. Nous inspirant des mécanismes généraux de régulation de l’homéostasie et de la chronobiologie, nous avons postulé que la capacité de performance quotidienne peut être modélisée par une onde dont l’amplitude, après avoiratteint un maximum, diminue graduellement pour tendre à retourner à son niveau initial durant les jours qui suivent (oscillation harmonique amortie). Nos résultats permettent de discuter des possibilités d’un tel système sur l’analyse des pratiques des entraîneurs en termes de gestion de la charge d’entraînement aux différentes périodes de l’année. Malgré le faible nombre de sujets (six joueuses), ils montrent que l’utilisation du concept d’oscillation harmonique amortie pour modéliser les effets cumulés et différés de l’entraînement semble prometteuse. Mais avant de l’appliquer à de plus vastes types de publics et de pratiques (réathlétisation, suivi de l’activité physique d’un public âgé), de plus amples recherches sont nécessaires / Modelling the training effects on performance allow to better conceptualize athleteadaptations to training to help coaches faced with many uncertainties, as possiblealternatives to high level performance, overreaching, overtraining and injury. The dailytraining load (TL) quantification is then the first step of the training periodization. Ourlitterature review shows that these affects are non final statement of interactions betweenexogenous and endogenous factors (Banister et coll., 1975, Busso et coll., 1990, Morton et coll., 1990). The concept of TL involves an effect level resulting in the body in the form offunctional responses that have a certain intensity and a certain time. It involves followseveral perceptual markers daily.The advances made in the area of computer and communication technologies nowoffer new opportunities for designing an ergonomic computer system to collect, process,and visualize large amounts of data, but also provides an alternative modelling of thetraining effects. Inspired by the general mechanisms regulating homeostasis andchronobiology, we postulated that the performance capacity of each training day can bemodeled by a wave whose amplitude, after reaching a maximum, gradually decrease totend to return to its initial level during the following days (damped harmonic oscillation).Our results allow us to discuss the possibilities of such a system on an analysis ofthe coaches practices in terms of training load management at different times of the plan.Despite the small number of subjects (six players), they show that the use of the concept ofdamped harmonic oscillation model for the cumulative and delayed effects of trainingappears promising. But before applying it to a wide range of subjects and practices(rehabilitation, physical activity of older person), further investagtion is needed

In modern automotive engines, the vast majority of light vehicle diesel (LVD) pistons are made from gravity die cast monolithic AlSi based alloy systems. Presently, the market drivers for reduced emissions, more efficient fuel consumption and increased specific power output are providing cyclic thermal and mechanical fatigue loading above the safe life durability threshold for the current AlSi based alloy systems. Peak temperatures in the diesel piston’s fatigue critical combustion bowl region are presently 420 °C for the AlSi based alloys, which represents a homologous TH value in excess of 0.8. In combination with peak temperatures of 420 C, the pistons are subject to cylinder pressures up to 220 bar, inducing mechanical stress amplitudes 15-20% greater than the allowable component fatigue strength for 1x108 cycles, in some applications. This durability deficit naturally leads to a requirement for new material and process solutions aimed at improving thermal and mechanical fatigue resistance at temperatures in excess of 420 C.One solution to this problem is to locally reinforce the pistons combustion bowl edge with a metal matrix composite (MMC) system. In this study, an aluminium based metal matrix composite (AlMMC) has been investigated and has shown some promise with increases in iso-thermal high cycle (1x 107) fatigue strength of 50 % compared to standard monolithic piston alloys. The AlMMC consists of a premium AlSi based LVD piston alloy matrix reinforced with 0.15 Vf of an interconnected network of 2-4 mm long Fe based fibres. The AlMMC is manufactured by pressure assisted infiltration of a sintered metallic fibre preform with as cast materials having a pore density of 0.2 %. In contrast to the use of ceramic fibre reinforcement systems generally requiring high pressure infiltration techniques, preform infiltration is considered possible with a comparably inexpensive manufacturing route. The Fe based fibre preforms can be infiltrated at lower pressure due to the reactivity between the Fe based fibres and the AlSi based matrix alloy. Unfortunately, this increased reactivity, although an advantage for preform infiltration, can result in (FeAlXX)Si(+X) interfacial reaction products forming between the fibre and matrix at operating temperatures of greater than 440 °C. These interfacial reactions result in a 15-20 m interfacial intermetallic layer after prolonged periods of exposure (>500 hrs), resulting in depleted fibre Vf and void formations on the matrix side of the interface.

Dans un contexte international de réduction des émissions de CO2, le groupe VALEO, soutenu par l’ADEME, développe de nouveaux alterno-démarreur permettant d’assurer des fonctions telles que le « Start-Stop ». La géométrie complexe et les nouveaux profils de mission entrainent une sollicitation par fatigue des ventilateurs de l’alterno-démarreur, dont la durée de vie est, en l’état actuel des connaissances, difficile à prévoir. L’amélioration de la fiabilité des composants passe donc par une analyse des mécanismes d’endommagement cyclique du matériau qui constitue l’objectif de la thèse. Les travaux ont été dirigés selon trois axes. Tout d’abord le comportement de l’acier ferritique a été étudié sur éprouvette de laboratoire en fatigue oligocyclique sous différents chargements (influence de la déformation moyenne, de la vitesse de déformation, de l’introduction d’un temps de maintien). Puis, une étude microstructurale basée sur l’imagerie MET et l’EBSD a permis de comprendre et d’évaluer l’endommagement plastique cyclique du matériau sur éprouvette de laboratoire comme sur composants réels, avant et après utilisation. Cette analyse a conduit à la construction d’indicateurs d’endommagement basés sur les évolutions de la microstructure des éprouvettes après chargement cyclique et le lien avec la durée de vie a été fait. Au final, les informations récoltées ont été utilisées afin d’améliorer la modélisation du comportement du matériau utilisée dans le logiciel de calcul par éléments finis ANSYS®. Ainsi, une loi matériau mieux adaptée a été choisie, à savoir le modèle d’écrouissage mixte de Chaboche, et les paramètres associés ont été identifiés. / In the international context of CO2 emissions reduction, VALEO, supported by the French Environment and Energy Agency, develops new car alternators allowing the “Start-Stop” function. Due to their specific geometry and the complex loading conditions, alternator cooling fans are subjected to fatigue damage. Their life time, based on current knowledge, is difficult to assess. A better reliability of the component depends on the understanding of the cyclic damage mechanisms of the material and this is the aim of the PhD thesis.The work followed three axes. At first, fatigue behaviour of the ferritic steel was studied on lab specimens and by means of Low Cycle Fatigue tests. They were carried out under different loading signals, studying the influence of mean strain, of strain rate or of a holding time. Then, a TEM study on one hand, and EBSD investigations on the other hand, focused on the microstructure, allowed to understand and to estimate cyclic plastic damage of the lab specimens, as well as of the fans (before and after cycling in real using conditions). Through this study, damage criteria based on the evolutions of the microstructure have been built and a relationship with the life time has been established. At the end, fatigue information was used in order to improve the material database of the commercial Finite Elements Method (FEM) software called ANSYS®. A better representation of the fatigue behaviour of the material was proposed through the Chaboche model.

Fatigue is a major disabling symptom in patients with rheumatic disorders such as RA and GPA. This work was carried out to investigate whether macro/microstructural changes are associated with the presence and severity of fatigue in patients with RA. We obtained T1, diffusion tensor and magnetisation transfer magnetic resonance scans from a group of 60 patients with RA (30 with active disease and 30 with inactive disease) at baseline and follow-up scans in addition to 14 patients with GPA, 14 matched fatigue controls FC and 20 healthy controls HC. No significant differences in a range of volumetric qMRI, DTI or MTI measures were found between RA active and non-active patients. Volumetric data analysis revealed that subcortical GM change in volume is related to fatigue irrespective of whether patients had RA or GPA. Moreover, longitudinal findings in patients with RA whose fatigue levels improved between the two scanning sessions indicate that, firstly, white matter microstructural abnormalities occur in the neuronal circuitry connecting bilaterally the striatal and cortical regions. Secondly, there is an imbalance in limbic-thalamic-cortical circuit connecting basal ganglia with limbic system of the improver group. Finally, demyelination of either neural circuit to the facilitator afferent vagus nerve could contribute to fatigue in this patient cohort. Fatigue-related RA does not appear to be related to disease activity or sleep disturbance and might therefore be related to other factors such as pain or depression status. Reductions in the myelin integrity as detected by the MTR in RA fatigue patients overlapped with those detected with the DTI analysis, especially in the cerebellar connections, corona radiata, corpus callosum, fornix, corticospinal tract and right cingulum suggesting that brain volumes, FA and MTR measures might be potential biomarkers of fatigue-related RA.

This paper describes an investigation carried out to study the fatigue failures of a "keyed tapered-shaft assembly" as affected by the keyway. Two fatigue testing machines were built and used: one tested a keyed assembly in reversed bending, and the other tested a keyed assembly in reversed torsion or a combination of reversed bending and reversed torsion. Both sled-runner and round-ended keyways were tested and the failures were compared with a view to establishing a law of failure. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Since a significantly greater percentage of body weight is supported by the non-paretic limb following stroke, a greater amount of fatigue may be present during daily activities. This may affect the ability of these individuals to maintain a stable upright posture. The presence of falls following a stroke has been attributed in part to this asymmetrical stance post-stroke. Therefore the purpose of this study was to assess the effect of quadriceps muscle fatigue on bi-pedal posture in individuals who had a stroke and an age-matched control group. Although individuals after stroke displayed greater postural sway under the paretic limb than the non-paretic limb or control subjects, results of this study show that sustaining an isometric knee extension of the non-paretic limb induces changes in postural control for individuals after stroke, but that these changes do not markedly differ from those of healthy age-matched controls.

The purpose of this study was as follows: 1. To determine if the recovery heart rate of ice hockey players is affected by elevation of the legs; 2. To determine the subjective recovery time and to calculate the correlation between subjective and objective recovery times; 3. To determine the heart rate at four and one-half minutes after exercise. Eight of an initial twelve members of the University of British Columbia Varsity Ice Hockey Team, ranging in age from twenty to twenty-seven years, participated in the study. Age, weight, height and position played were obtained for all subjects. Through radio telemetry, in conjunction with an electrocardioscribe and a heart rate monitor, the following data was obtained: a) resting heart rate b) terminal heart rate c) recovery heart rate d) recovery half-time e) post-exercise recovery rate f) subjective recovery time Each subject was required to skate six times: three trials as a control subject, which recovered in normal sitting position, and three times as an experimental subject, which recovered in the recumbent position with the legs raised on a bench twenty inches high. Reliability measurements performed using the Pearson Product-Moment Method resulted in an r = 0.420 for resting heart rates, and an r = 0.649 for terminal heart rates. A correlation between recovery half-times and the subjective recovery times for each of the six trials was calculated. A "t" test for correlated samples was performed between the means of the trials of the control group and between the means of the trials of the experimental group recovery half-times. A "t" test was then performed to determine the significance of the difference between the mean of the means of the recovery half-times of the control and experimental groups. A t value of 2.79, significant at the .05 level of confidence was found. Finally a test of significance was performed on the post-exercise recovery heart rates of the control and experimental groups. On the basis of the statistical analysis and with respect to the small sample studied, the following conclusions appear warranted: 1. Elevation of the legs significantly reduced the recovery time at the .05 level of confidence. 2. The reliabilities of resting and terminal heart rates were too low to be of significant value. 3. The subjects were not able to predict, with any degree of accuracy, when they had recovered sufficiently to begin another skating period. 4. Elevation of the legs and normal sitting produced no significant difference in the heart rates at the end of the four and one-half minute recovery period. / Education, Faculty of / Curriculum and Pedagogy (EDCP), Department of / Graduate