Neuromuscular fatigue, even that caused by light submaximal exercise, impairs motor performance and alters motor planning. This impairment is evident in muscle reaction time, force production capacity and joint position sense as well as in more complex tasks such as postural stability. When an individual is fatigued their postural sway increases and they are less able to recover from unexpected postural perturbations. Although a large number of work-related falls are caused by fatigue every year, the mechanisms behind the instability are not well understood. Since postural control does not require a large amount of muscular strength it is unclear whether the post-fatigue changes in posture are due to impairment within the muscle fibers or are a central modification of the motor plan used to execute the movement task. In order to better understand neuromuscular fatigue researchers have labeled fatigue occurring within the muscles ‘peripheral fatigue’ and that occurring within the central nervous system ‘central fatigue’. At the onset of a muscular contraction peripheral and central fatigue develop simultaneously, making it difficult to clearly articulate the role that they each play in the decreased motor performance found post-fatigue. Techniques such as transcranial magnetic and electrical nerve stimulation quantify the contribution of central fatigue to the decreased maximal force production but the impact on motor planning is still not well understood. Therefore, the primary aim of this doctoral dissertation was to isolate central fatigue from peripheral muscle fatigue and to compare the influence that it may have on dynamic postural control to the changes caused by general fatigue of the local postural muscles. This overarching research goal was accomplished through five separate studies. The first study in this dissertation determined that at least seven postural trials needed to be performed to ensure that the participants had fully adapted to the postural task before the fatigue protocol was implemented. Experiment 2 characterized the fatigue produced by bilateral, isometric ankle muscle contractions and examined the recovery of the central and peripheral changes throughout a ten-minute post-fatigue recovery period. The results demonstrated that the alternating maximal ankle plantar and dorsiflexor contractions created central and peripheral fatigue. Central fatigue recovered within the first two minutes post-fatigue while peripheral fatigue lasted throughout the ten-minute post-fatigue period. Experiment 3 analyzed the impact of this ankle muscle fatigue protocol on the postural response to a continual, externally driven, sinusoidal oscillation of the support platform. In this study the fatigued participants were able to stabilize their center of mass displacement using two different anticipatory postural responses to the backwards perturbation whereas all of the participants used the same anticipatory response to the forwards perturbation. All three postural responses became progressively more conservative throughout the ten-minute post-fatigue period, despite the rapid recovery of the ankle force production capacity. The final two studies characterized the fatigue produced during a continuous, isometric forearm contraction and assessed the impact on ankle motor performance (Experiment 4) and on postural control (Experiment 5). Peripheral fatigue created in the forearm muscles during this contraction remained throughout the post-fatigue testing session. Central fatigue and a decreased maximal force production capacity were quantified in both the forearm and ankle plantarflexor muscles immediately after the forearm contraction, indicating that central fatigue created during the forearm exercise crossed over to the distal and unrelated ankle plantarflexor muscles. The influence of the central fatigue created during the forearm contraction affected the anticipatory postural response in a similar way to the fatigue created by the ankle fatigue protocol. The post-fatigue modification of the postural response dissipated as the central fatigue recovered. Taken together, these five studies extend the current understanding of how exercise induced neuromuscular fatigue modifies the central nervous system’s control of complex motor tasks.

Submitted in fulfilment of the requirements for the degree of Master of Engineering: Mechanical Engineering, Department of Mechanical Engineering, Faculty of Engineering and the Built Environment, Durban University of Technology, Durban, South Africa. 2017. / In this study, the fatigue life of fiber reinforced composite (FRC) materials system was investigated. A nano-filler was used to increase the service life of the composite structures under cyclical loading since such structures require improved structural integrity and longer service life. Behaviour of glass fiber reinforced composite (GFRC) enhanced with various weight percentages (1 to 5 wt. %) of Cloisite 30B montmorillonite (MMT) clay was studied under static and fatigue loading. Epoxy clay nanocomposite (ECN) and hybrid nanoclay/GFRC laminates were characterised using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The mechanical properties of neat GFRC and hybrid nanoclay/GFRC laminates were evaluated. Fatigue study of the composite laminates was conducted and presented using the following parameter; matrix crack initiation and propagation, interfacial debonding, delamination and S–N relationship. Residual strength of the materials was evaluated using DMA to determine the reliability of the hybrid nanoclay/GFRC laminates. The results showed that ECN and hybrid nanoclay/GFRC laminates exhibited substantial improvement in most tests when compared to composite without nanoclay. The toughening mechanism of the nanoclay in the GFRC up to 3 wt. % gave 17%, 24% and 56% improvement in tensile, flexural and impact properties respectively. In the fatigue performance, less crack propagations was found in the hybrid nanoclay/GFRC laminates. Fatigue life of hybrid nanoclay/GFRC laminate was increased by 625% at the nanoclay addition up to 3 wt. % when compared to neat GFRC laminate. The residual strength of the composite materials revealed that hybrid nanoclay/GFRC showed less storage modulus reduction after fatigue. Likewise, a positive shift toward the right was found in the tan delta glass transition temperature (Tg) of 3 wt. % nanoclay/GFRC laminate after fatigue. It was concluded that the application of nanoclay in the GFRC improved the performance of the material. The hybrid nanoclay/GFRC material can therefore be recommended mechanically and thermally for longer usage in structural application. / M

Background: Leukemia is the most common form of cancer in children, with eight out of ten children surviving the disease. Children who develop leukemia are usually between two and six years old. Fatigue is one of the most common side effects in children undergoing treatment for leukemia. Fatigues can reduce the mental, physical and social well-being of the patient. Aim: The purpose of this study was to investigate how fatigue affects children with leukaemia during care and changes in fatigue over time.  Method: A literature review that includes ten scientific articles that have been quality reviewed according to the Swedish Agency for Medical and Social Evaluation (SBU) review template and according to Willman, Stoltz och Bahtsevani (2011) template. Result: The results from 10 articles are reported in two different categories. These were; impact on quality of life during care and sleep. The results show that children with leukaemia rated their quality of life low during the care period. Fatigue also had negative effects on children's sleep. Conclusion: Sleep and quality of life in children with leukemia are negatively affected by fatigue. Changes in the degree of fatigue could be demonstrated in several articles. / SAMMANFATTNING Bakgrund: Leukemi är den vanligaste cancerformen hos barn där åtta av tio barn överlever sjukdomen. Barn som insjuknar i leukemi är oftast mellan två och sex år gamla. Fatigue är en av de vanligaste biverkningarna som förekommer hos barn som genomgår behandling av leukemi. Fatigues kan minska det psykiska, fysiska och social välbefinnandet hos patienten. Syfte: Syftet med denna studie var att undersöka hur fatigue påverkar barn med leukemi under vårdtiden samt förändringar av fatigue över tid.  Metod: En litteraturöversikt som har en deskriptiv design och de vetenskapliga artiklarna har kvalitetsgranskats efter Statens beredning för medicinsk och social utvärderings (SBU) granskningsmall samt Willman, Stoltz och Bahtsevani (2011) granskningsmall.  Resultat: Resultatet från tio artiklar redovisas i två olika kategorier. Dessa var; påverkan på livskvalitén under vårdtiden och sömn. Det framkom i resultatet att fatigue hade en negativ inverkan på livskvalitén hos barn med leukemi där barnen skattade sin livskvalité som låg under vårdtiden. Fatigue hade även negativa effekter på barns sömn.  Slutsats: Livskvalitén och sömnen hos barn med leukemi påverkades negativt av fatigue. Förändringar av graden fatigue kunde påvisas i ett flertal artiklar.

Ph. D.

Bakgrund Fatigue är ett begrepp som beskriver trötthet som påverkar den drabbades dagliga liv negativt. I samband med en cancerbehandling drabbas i stort sett alla patienter av fatigue i olika grad. När sedan behandlingen är slutförd kvarstår problemet med trötthet, och behov finns av rehabilitering som syftar till att patienten ska återgå till en normal nivå av trötthet. Antalet patienter som överlever sin cancerbehandling ökar och då även deras behov av rehabilitering. Därför är det viktigt att utvärdera den cancerrehabilitering som bedrivs idag. Syfte Att beskriva hur fatigue förändras hos patienter som genomgått cancerrehabilitering. Metod Patienter som deltagit i ett program för cancerrehabilitering har fått fylla i ett formulär som mäter nivån av fatigue. Det formulär som använts är Shirom-Melamed Burnout Questionnare (SMBQ). De har fått detta formulär innan behandlingens start, 3 månader och 6 månader efter behandlingens slut. Resultat Sammanfattningsvis pekar resultatet mot att fysisk och mental fatigue minskade hos patientgruppen 3 månader efter sin vistelse. Denna effekt avtog dock efter 6 månader och de patienter som beskrivs som palliativa visar en kraftig försämring. Slutsats Den behandling som patienterna genomgick hade önskad effekt på fatigue. På längre sikt hade dock inte behandlingen samma effekt, då trötthetsvärdet återgick till samma nivå som innan rehabilitering. De palliativa patienterna har alltså ett behov av ytterligare behandling då deras fatigue förvärrades över tid. Klinisk betydelse Behov finns av en metod som ger mätbara och varaktiga resultat, detta kan då motivera att fler patienter remitteras för onkologisk rehabilitering diskuteras. / Background Fatigue is hard to define but can be described as an expression of tiredness affecting the patients daily life in a negative way. In concert with a treatment for cancer practically all patients are affected by varying degrees of fatigue. When the treatment is over this fatigue problem remains, and the need for rehabilitation to put the patient back to normal levels of fatigue. As the number of patients who survive their cancer treatment increase, so also will the need for rehabilitation. It’s therefore important to evaluate the cancer rehabilitation care being used today. Purpose This study aims to describe the change in fatigue with patients having been through cancer rehabilitation. Method Participating patients have filled out a form measuring levels of fatigue. The form being used is the Shirom-Melamed burnout questionnare (SMBQ). The patients have received this form before start of treatment, 3 months and 6 months after end of treatment. Results In summary physical and mental fatigue levels are down in patients 3 months after their stay. This effect receded after 6 months and patients described as palliative show substantially worse results. Conclusion The treatment patients have gone through had the desired effect on fatigue. In a longer perspective the treatment did not have the same effect, values related to fatigue returned to levels similar to before treatment. The palliative patients have a need for additional treatment as their fatigue worsened over time. Clinical importance There is a need for a method to give measurable and long term results, this can then motivate more patients being put through a oncologic rehabilitation program.

No description available.

The purpose of this thesis was to improve our understanding of the progression of fatigue and recovery during repetitive work and to examine selected methods for predicting fatigue. In Chapter 2, a psychophysical methodology was used to validate the Maximum Acceptable Effort (MAE) equation of Potvin (2012) at duty cycles of greater than 0.5. The results from that study were used to evaluate the MAE equation in the higher duty cycle range. In Chapter 3, the fatigue process during complex MVC-relative force profiles was examined in a repetitive handgrip task. In Chapter 4, I examined the effect of manipulating the order of presentation of various MVC-relative force levels for a repetitive thumb flexion task. Additionally, the influence of post-activation potentiation was examined by stimulating the flexor pollicis longus (FPL) at specific time points during the complex profile. In Chapter 5, Xia and Frey Law’s (2008) three-compartment model (3CMXFL) of muscle fatigue was modified to more accurately reflect physiological processes. The model, with physiological modifications (3CMGMU), as well as the original 3CM optimized for complex tasks (3CMOPT), was optimized to predict the fatigue levels from the experiments described in Chapters 3 and 4, as well as 4 other similar experimental protocols. The predicted fatigue from the 3CMXFL was also compared to the experimental data. The 3CMOPT and 3CMGMU were compared against known endurance times. The 3CMGMU is proposed as an ergonomic tool for evaluating fatigue in repetitive tasks, and the future directions for fatigue modelling and using the MAE equation for complex force-time histories are addressed. This thesis provides the first studies of fatigue accumulation during complex MVC-relative time histories. The findings from this thesis can be applied to the workplace to reduce the risk of injury as a result of muscle fatigue. / Thesis / Doctor of Philosophy (PhD)

Premature failures of vital gas turbine components, such as blades and vanes, have been the result of increasing demands of power generation facilities. As power needs fluctuate throughout the day, operators are quickly firing up gas turbines as a means of providing instant power. Traditionally, these engines run at constant operating conditions; however, contemporary operating conditions call for these engines to be applied on an “as necessary” basis. The result of the cyclic startup and shutdown of gas turbines has led to a phenomenon known as creep-fatigue (CF). A coupling of two primary failure mechanisms in gas turbines, CF conditions exacerbate the mechanisms of creep and fatigue, ultimately leading to a premature failure of components. Traditionally, independent creep and fatigue analyses are conducted to determine the limiting life factor of gas turbines. Recently, fracture mechanics approaches have been successfully used in extending the traditional analyses to include fatigue- and creep-crack growth analyses. Founded on existing approaches to creep-fatigue crack growth analyses, including experimental elastic and plastic fracture mechanics approaches, a coupled creep-fatigue crack initiation and propagation model is developed. To bring these models to fruition, the current study utilizes the development of an experimental setup capable of subjecting a modified fracture specimen to creep-fatigue conditions. With two test temperatures key to turbine components, a blunt notch compact tension specimen was subjected to trapezoidal load waveforms with various lengths of holds at maximum load. A developed direct current potential drop (DCPD) system was used to monitor crack initiation and crack lengths throughout the duration of tests. Numerical simulations on a representative specimen were conducted, to correlate and predict key fracture mechanics parameters used in the development of creep-fatigue crack initiation and propagation models. iv Metallurgical analysis of specimens was conducted, implementing both optical and scanning electron microscopy. From the experimental and numerical studies, a model for both the initiation and propagation of cracks on a single specimen is furnished. Through the use of elastic-plastic fracture mechanics parameters, the proposed models are observed to predict crack initiation and replicate crack propagation rates based on the experimental conditions. Assisting in the implementation of the proposed models, intended uses and applications for the models are provided, simplifying the life prediction analyses for components expected to fail due to creepfatigue service conditions.

No description available.

In this work, understanding the microstructural effects of monotonic and cyclic failure of wrought 7075-T651 and cast A356 aluminum alloys were examined. In particular, the structure-property relations were quantified for the plasticity/damage model and two fatigue crack models. Several types of experiments were employed to adapt an internal state variable plasticity and damage model to the wrought alloy. The damage model was originally developed for cast alloys and thus, the model was modified to account for void nucleation, growth, and coalescence for a wrought alloy. In addition, fatigue experiments were employed to determine structure-property relations for the cast alloy. Based on microstructural analysis of the fracture surfaces, modifications to the microstructurally-based MultiStage fatigue model were implemented. Additionally, experimental fatigue crack results were used to calibrate FASTRAN, a fatigue life prediction code, to small fatigue-crack-growth behavior. Lastly, a set of experiments were employed to explore the damage history effect associated with cast and wrought alloys and to provide motivation for monotonic and fatigue modeling efforts.