Recently, it has been postulated that fatigue is a mechanism to protect the muscle fiber from deleterious ATP depletion and cell death. The ATP-sensitive potassium (KATP) channel is believed to play a major role in this mechanism. Under metabolic stress, the channels open, reducing membrane excitability, Ca2+ release and force production. This alleviates energy demand within the fiber, as activation of the channel reduces ATP consumption from cellular ATPases. Loss of KATP channel activity during fatigue results in excessive intracellular Ca2+ ([Ca2+]i) levels, likely entering the fiber through L-type Ca2+ channels. It has been demonstrated that when mouse muscle lacking functional KATP channels are stimulated to fatigue, ATP levels become significantly lower than wild type levels. Thus, it was hypothesized that a lack of KATP channel activity impairs energy metabolism, resulting in insufficient ATP production. The focus of work for this M.Sc. project was to test this hypothesis. Fatigue was elicited in Kir6.2-/- FDB muscles for three min followed by 15 min recovery. After 60 sec, a 2.6-fold greater glycogen breakdown was observed in Kir6.2-/- FDB compared to wild type FDB. However, this effect disappeared thereafter, as there were no longer any differences between wild type and Kir6.2-/- FDB in glycogen breakdown by 180 sec. Glucose oxidation after 60 sec was also greater in Kir6.2-/- FDB compared to wild type FDB. However, levels of oxidation failed to increase in Kir6.2-/- FDB from 60 to 180 sec. Calculated ATP production during the fatigue period was 2.7-times greater in Kir6.2-/- FDB, yet measured ATP levels during fatigue are much lower in Kir6.2-/- FDB compared to wild type FDB. Taken together, it appears that muscle energy metabolism is impaired in the absence KATP channel activity.

This thesis investigates the differential effects of muscle fatigue on center of pressure (COP) sway and associated central processes (attentional demands and sensory re-weighting) in older compared with young adults. More specifically, we first sought to determine whether the effect of muscle fatigue on unipedal stance was greater during a dual-task in older versus young adults, and second, to determine whether the effect of muscle fatigue on bipedal stance was greater in a condition with less reliable proprioceptive information in older versus young adults. Our main results show that with different muscle groups fatigued (ankle or hip) and postural tasks with varying difficulty (unipedal stance or bipedal stance on compliant surface), young adults increased their COP sway displacement and velocity with muscle fatigue, but not the associated attentional demands. When the central nervous system needed to increase the weight of the vestibular inputs due to sensory information being less reliable at the ankle joints from standing on a compliant surface (peripheral somatosensory information), COP sway displacement and velocity in young adults were greater with ankle muscle fatigue. We also found that healthy older adults were able to compensate for muscle fatigue just as well as young adults when visual information was available during a unipedal stance or when visual information was not available during a bipedal stance on a firm surface. However, when standing on a compliant surface, older adults showed a greater increase in COP sway displacement compared to young adults and increased attentional demands when visual information was not available during a bipedal stance. Our results suggest that healthy young and older adults are able to compensate for ankle muscle fatigue to limit postural control alterations during quiet standing under different conditions, and that the extent of postural control alterations largely depends on the tasks performed. The compensation strategies may be less efficient for older adults with less reliable proprioceptive information and without vision. Thus, a frailer group of older adults, with already reduced proprioception and/or reduced vision could possibly have more difficulty to efficiently use the same compensation strategies, and may be more at risk of falling when fatigued.

Work-related musculoskeletal disorders (WMSDs) remain a substantial problem in the workplace. Rotation, in which workers are rotated between tasks, is widely used as an administrative control, as it is considered to reduce WMSD risk through reducing physical exposures and increasing exposure variation. However, despite its widespread use, there is limited evidence that rotating between tasks is effective in reducing the risk of WMSDs. Inconsistencies in measured outcomes of rotation may be attributed to the variety of parameters involved in determining rotation schedules, including which tasks to include in a schedule, the rate at which workers rotate, and the order in which tasks are performed. This research assessed the effects of rotation, specifically focusing on rotation frequency and task order, on muscle fatigue and performance when included tasks loaded the same muscle group. Twelve participants completed six experimental sessions in each of three studies, during which repetitive tasks were performed for one hour either with or without rotation. Each study simulated a different task, including static shoulder abduction, box lifting, and a light assembly task. Rotation occurred between lower and higher exertion levels, and each rotation schedule varied in both rotation frequency (rotating every 15 minutes vs. 30 minutes) and task order (starting with the lower vs. higher intensity task). Muscle fatigue was assessed through several measures, including electromyography, and ratings of perceived discomfort. Performance was assessed through the accuracy of shoulder moment output, the accuracy of box placement, or the speed of assembly completion. As expected, rotation was effective in reducing fatigue compared to higher intensity tasks with no rotation, although it increased fatigue compared to the lower intensity with no rotation. While effects of rotation frequency and task order were seen on some measures, results across all three studies did not indicate consistent effects of either rotation frequency or task order on fatigue or performance. As such, the practical relevance of these rotation parameters and the likely impacts of rotation are not yet clear, and further assessments are needed. Such assessments should ideally involve longer durations, field studies, and/or more direct measures of injury or injury risk. / Ph. D.

Falls in the workplace are a major cause of injuries and fatalities. Muscle fatigue is one important factor that has been linked to a decrement in postural control and a potential increased falling risk. However, potential interactive effects of muscle fatigue with other risk factors remain unclear, and practical interventions are needed to mitigate the adverse effects of muscle fatigue. The current work was conducted to address these research needs through three experimental studies. The first study investigated how muscle fatigue affects postural control during quiet standing on inclined surfaces. Inclined surfaces compromised postural control, with the most deleterious effects found while standing in a lateral direction. Fatigue did not result in further decrements in postural control during standing on inclined surfaces. The second study investigated the effects of muscle fatigue on postural control while lifting unexpected loads. Lifting an object with unexpected mass compromised postural control, with a more substantial effect found in the unexpectedly light load condition. Fatigue-related effects were not consistent, though there was evidence that lumbar muscle fatigue did not compound the adverse effects of lifting an object in unexpected mass conditions. The last study evaluated the efficacy of three interventions (two auditory stimulations and periodic rest breaks) at mitigating the adverse effects of muscle fatigue on postural control. Allowance of rest breaks did not improve postural control during the fatiguing work, though it was indicated that benefits may be present for some individuals. Both a static pure tone and moving conversation appeared to offset fatigue-induced postural instability. The current research provides a more comprehensive understanding of the contribution of muscle fatigue to fall risks during occupationally relevant tasks and assessed the efficacy of practical interventions to reduce the risk of falls. These findings may facilitate the development of strategies to prevent occupational falls related to muscle fatigue, inclined surfaces, and manual material handling tasks. / Ph. D.

In the field of ergonomics, biomechanics, sports and rehabilitation muscle fatigue is regarded as an important aspect since muscle fatigue is considered to be one of the main reasons for musculoskeletal disorders. Classical signal processing techniques used to understand muscle behavior are mainly based on spectral based parameters estimation, and mostly applied during static contraction and the signal must be stationary within the analysis window; otherwise, the resulting spectrum will make little physical sense. Furthermore, the shape and size of the analysis window also directly affect the spectral estimation. But fatigue analysis in dynamic conditions is of utmost requirement because of its daily life applicability. It is really difficult to consistently find the muscle fatigue during dynamic contraction due to the inherent non-stationary nature and associated noise in the signal along with complex physiological changes in muscles. Nowadays, in addition to linear signal processing, different non-linear signal processing techniques are adopted to find out the consistent and robust indicator for muscle fatigue under dynamic condition considering the high degree of non-linearity (caused by functional interference between different muscles, changes of signal sources and paths to recording electrodes, variable electrode interface etc.) in the signal. In this work, various linear and nonlinear-non-stationary signal processing methods, applied on surface EMG signal for muscular fatigue analysis under dynamic contraction are studied. In present study, surface EMG (sEMG) signals are recorded from Biceps Brachii muscles from eight (N=8) physically active college students during dynamic lifting 7 kg load at the rate of 20 lifts/min till they become fatigue. EMG data is processed in two ways -1. taking the whole EMG response and 2. breaking into three ranges of contraction (0-45)o, (45-90)o and >90o, to study better response region. It is observed that in spectral estimation techniques auto-regressive (AR) based spectral estimation technique gives better frequency resolution than periodogram for small epochs, as AR is based on parametric estimation. Both the previous methods provide only the frequency information in the signal. In order to estimate the time varying nature of frequency content in a signal various time-frequency signal processing techniques are used like – Short Time-Fourier Transform (STFT), Smoothed pseudo Wigner-Ville (SPWD), Choi-William distribution (CWD), Continuous Wavelet Transform (CWT), Huang-Hilbert Transform (HHT) and Recurrence Quantification Analysis (RQA) are used. The last two techniques are used by considering the EMG signal as non-linear and non-stationary signals. Among these techniques, STFT is the simplest time-frequency analysis technique. But tradeoff between time and frequency resolution is the major constraint in STFT, therefore, a window length of 256 samples are considered in this study. In order to tackle time-frequency resolution problem different Cohen-class distribution techniques are used like SPWD and CWD, where the result is severely affected by the presence of interference terms which make its interpretation really difficult. Different adaptive filters are used in SPWD and CWD to suppress these interference terms during analysis. Among these time-frequency analysis techniques continuous wavelet transform provides the most accurate results in comparison to other time-frequency analysis techniques. Similar result is obtained in present study. This fatigue response is further improved using non-linear and non-stationary techniques like HHT and RQA. HHT shows less variation in frequency response than CWT analysis result. Percentage of determinism calculated using recurrence quantification analysis method is found to be more sensitive than mean frequency estimation. Therefore, non-linear and non-stationary signal processing techniques are to be better indicator of muscle fatigue during dynamic contraction.

Repetitive stress injuries are common in the workplace where workers perform repetitive tasks continuously throughout the day. Muscle fatigue may lead to injury either directly through muscle damage or indirectly through changes in coordination, development of muscle imbalances, kinematic and muscle activation variability, and/or movement instability. To better understand the role of muscle fatigue in changes in movement parameters, we studied how muscle fatigue and muscle imbalances affected the control of movement timing, variability, and stability during a repetitive upper extremity sawing task. Since muscle fatigue leads to delayed muscle and cognitive response times, we might expect the ability to maintain movement timing would decline with muscle fatigue. We compared timing errors pre- and post-fatigue as subjects performed this repetitive sawing task synchronized with a metronome using standard techniques and a goalequivalent manifold (GEM) approach. No differences in basic performance parameters were found. Significant decreases in the temporal correlations of the timing errors and velocities indicated that subjects made more frequent corrections to their movements post-fatigue. Muscle fatigue may lead to movement instability through a variety of mechanisms including delayed muscle response times and muscle imbalances. To measure movement stability, we must first define a state space that describes the movement. We compared a variety of different state space definitions and found that state spaces composed of angles and velocities with little redundant information provide the most consistent results. We then studied the affect of fatigue on the shoulder flexor muscles and general fatigue of the arm on movement stability. Subjects were able to maintain stability in spite of muscle fatigue, shoulder strength imbalance and decreased muscle cocontraction. Little is known about the time course for adaptations in response to fatigue. We studied the effect of muscle fatigue on movement coordination, kinematic variability and movement stability while subjects performed the same sawing task at two work heights. Increasing the height of the task caused subjects to make more adjustments to their movement patterns in response to muscle fatigue. Subjects also exhibited some increases in kinematic variability at the shoulder but no changes in movement stability. These findings suggest that people alter their kinematic patterns in response to fatigue possibly to maintain stability at the expense of increased variability. / text

Introdução: Pacientes fibromiálgicos freqüentemente referem fadiga e estudos apontam esse sintoma como o segundo mais intenso, podendo limitar as atividades de vida diária, aumentar o estresse a até mesmo a dor. Não há um consenso se as queixas de fadiga correspondem a padrões alterados nos mecanismos de fadiga muscular, já que predominam as avaliações com questionários e escalas. O objetivo deste estudo foi avaliar a fadiga muscular em fibromiálgicos por meio da eletromiografia de superfície durante o teste de sentar e levantar. Métodos: Participaram do estudo 49 sujeitos divididos em dois grupos: Grupo Fibromiálgico (n=34) e Grupo Controle (n=15). Os padrões de fadiga muscular foram avaliados pela análise da freqüência mediana (MDF) do sinal eletromiográfico do terço distal do músculo vasto lateral durante o teste de sentar e levantar. A dor pela Escala Visual Analógica (EVA), os sintomas da fibromialgia pelo Questionário do Impacto da Fibromialgia (QIF) e o desempenho no teste de sentar e levantar pelo tempo total utilizado e número de repetições. Todos os participantes foram instruídos a realizar o teste numa velocidade confortável até a exaustão. A coleta da eletromiografia foi feita em três momentos, simultaneamente ao uso da escala de Borg para avaliação do nível de esforço percebido: início (T1), após um minuto (T2) e na exaustão (T3). Resultados: O grupo fibromiálgico apresentou MDF reduzida em T3 (p=0,04) e relato de esforço percebido mais intenso que o grupo controle em T2 (p=0,00). Foi observada correlação moderada entre MDF em T3 com as variáveis: intensidade da dor (-0,40; p=0,00) e tempo total em segundos (0,43; p=0,00). Conclusão: o grupo fibromiálgico apresentou sinais de fadiga precocemente com diminuição da freqüência mediana e percepção de esforço mais intenso comparado ao grupo controle. / Introduction: Fibromyalgia patients usually report fatigue and this symptom is the second more intense, limiting daily life activities, increasing stress and pain. There is no sense if fatigue complains are related to muscle fatigue altered patterns, since most studies performed questionnaires and scales to quantify this symptom. The aim of this study was to evaluate muscle fatigue patterns by surface electromyography during the sit to stand test. Methods: Participated 49 subjects in two groups: Fibromyalgia Group (n=34) and Control Group (n=15). Electromyography muscle fatigue patterns were evaluated by median frequency analyses (MDF) of the distal third of the vastus lateralis muscle during sit to stand test, pain by Visual Analog Scale (VAS), fibromyalgia symptoms by Fibromyalgia Impact Questionnaire (FIQ) and the test performance by total time spent in test and by number of movements. All subjects performed the sit to stand test in a comfortable velocity until exhaustion. Electromyography recording was performed in tree moments, simultaneously at Borg scale perceived effort evaluation: initial time (T1), after one minute (T2) and at exhaustion (T3). Results: Fibromyalgia group present MDF decrease at T3 (p=0,04) and more intense perceived effort at T2 (p=0,00). There was a moderate correlation for MDF in T3 with pain intensity (-0,40; p=0,00) and total time in sit to stand test in seconds (0,43; p=0,00). Conclusion: Fibromyalgia patients showed early muscle fatigue signs with MDF decrease and more intense perceived effort.

This research examined the time-course of muscle damage in the squat, bench press, and deadlift. Ten resistance-trained males performed four sets to failure with 80% of one-repetition maximum (1RM) for each exercise on three separate weeks. Swelling, range of motion (ROM), delayed onset muscle soreness (DOMS), lactate dehydrogenase (LDH), creatine kinase (CK), and average concentric velocity (ACV) were assessed pretraining and at five timepoints post-training: -0, 24, 48, 72, and 96 hours. Swelling (p<0.01) increased immediately post-training, and DOMS (p<0.01) increased at 24 hours post-training in the bench press condition. Additionally, DOMS increased at 48 hours in both squat and deadlift conditions (p<0.01). Squat and deadlift elevated CK immediately post-training (p<0.01), but LDH only increased in the squat post-training. Immediately post in the bench press ACV was decreased (p<0.01) along with in the squat for up to 72 hours (p<0.01), however, ACV did not change following the deadlift (p>0.05). / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2018. / FAU Electronic Theses and Dissertations Collection

Low back problems are among the most common nonfatal occupational injuries reported in the United States, and account for substantial healthcare expenditures (e.g., medical care costs) and losses to worker productivity. A strong association has been well-documented between occupational exposure to repetitive trunk motion and low back problems, particularly among workers performing manual material handling (i.e., lifting) activities. A feature of repetitive motion believed important to the development of work-related musculoskeletal disorders (MSDs), including low back problems, is a lack of within-individual, between-cycle variation of physical exposure summary measures, e.g., when observed visually, the cycle-to-cycle motion pattern appears consistent. An active literature has emerged using concepts of motor control to improve ergonomists’ understanding of physical exposure variation (i.e., motor variability) arising from individual-level mechanisms during repetitive work. Fundamentally, for any particular individual, the onset of exposure to a repetitive physical activity (i.e., task training) involves a learning process during which motor control strategies are developed to accomplish the task effectively. The cycle-to-cycle variability of motor learning metrics, such as postural and task performance summary measures, has been observed to exponentially decay during task training. From an ergonomics perspective, a temporal reduction in postural variability may lead to greater cumulative loading and physiological fatiguing of the underlying muscle tissues (due to more consistent cycle-to-cycle movements), thus increasing MSD risk over time. However, it is not known if, or to what extent, physical task characteristics (e.g., work pace) modify the temporal behavior of motor variability during training of a repetitive occupational activity. Moreover, the relationships between motor variability, task performance, and muscle fatigue during occupational task training are not well understood. The goal of this dissertation was to present new information concerning occupationally relevant metrics of motor learning during training of a laboratory-simulated, repetitive lifting activity. In this study, participants performed 100 repetitions (i.e., cycles) of the lifting task in each of four experimental sessions (i.e., visits) at different combinations of box load (low or high) and work pace (slow or fast). Three main observations were discussed in this dissertation: (i) participants exhibited a greater temporal reduction in the cycle-to-cycle variability of trunk postural summary measures during training of a heavier-weighted and faster-paced lifting activity (Chapter 3), which may have facilitated increases in the efficiency and repeatability of box movements (Chapter 4), (ii) the cycle-to-cycle variability of the erector spinae (back) muscle activity summary measures increased, but the variability of the multifidus muscle activity summary measures decreased, over time during faster-paced lifting (Chapter 3), and (iii) a greater temporal increase in trunk postural variability (i.e., a more “flexible” trunk movement strategy) was generally associated with lesser electromyographic back muscle fatigue during training of the lifting task (Chapter 5). Collectively, these research findings may open pathways to the development of new task design criteria and ergonomic guidelines to promote motor variability in the workplace and, ultimately, improve workers’ musculoskeletal health.

Muscle fatigue is universally experienced in daily life, from recreational physical activity to the workplace. However, our ability to estimate fatigue is limited. Several attempts have been made to mathematically model the effects of fatigue, such as how long a muscle contraction may be sustained, known as `endurance time.' However, these simple models of endurance time are limited to static contractions when the body is not moving, but muscles are contracted. This research aims to advance a previously proposed analytical model of muscle fatigue to represent complex tasks such as with rest intervals and dynamic contractions. Multiple methodologies were employed to assemble data to examine the model prediction accuracy, including 1) compiling previously published data involving intermittent rest intervals (i.e., meta-analysis); 2) experimentally collecting data on intermittent fatigue for shoulder flexion as it is not well represented in the literature; and 3) experimentally collecting data on fatigue during a dynamic task for elbow flexion as dynamic tasks have been virtually ignored in fatigue literature. The results of these investigations indicate that a mathematical model of fatigue is reasonably accurate in predicting an average fatigue response across multiple subjects for both intermittent and dynamic tasks, but does not currently reflect the often wide variation in muscle fatigue development that is observed between individuals. Accordingly, this type of modeling approach may have value for general assessments of fatigue accumulation, but will need further development and modification to better represent individual characteristics.