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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Inactivity of the cat soleus muscle was induced via spinal cord isolation (SI), and the cats were maintained for 4 months. The soleus was electrically stimulated while lengthening (SI-L) or shortening (SI-S) during a simulated step cycle or during isometric (SI-I) contractions. For the SI, SI-S, SI-L, and SI-I groups, the soleus weights were 33, 55, 55, and 64% of the control, respectively, and the maximum tetanic tensions were 15, 30, 36, and 44% of the control, respectively. The specific tension was lower in all SI groups than in the control. Absolute forces at stimulation frequencies of 5-200 Hz were smaller in all SI groups than in the control. The SI-I group tended to have higher values for all force-related parameters than the other SI groups. Fatigue resistance was similar among all groups. The isometric twitch time-to-peak tension was shorter, and the frequency of the stimulation-tension response was shifted to the right in all SI groups with respect to the control. Maximum shortening velocities were 70, 59, and 73% faster for the SI, SI-S, and SI-L groups and similar to the control for the SI-I group. Inactivity resulted in an increased percentage of faster myosin heavy chains (MHCs) that was blunted in the SI-I and SI-L groups but not in the SI-S group. Pure type I MHC fibers atrophied by 80, 59, 58, and 47% in the SI, SI-S, SI-L, and SI-I groups. The data from the SI group quantify the contribution of activity-independent factors in maintaining the mechanical and phenotypic properties of the cat soleus. Relative to a fast-fatigable muscle, these results suggest that only 25% of the slowness (type I MHC) and none of the resistance to fatigue of the soleus muscle are dependent on activity-related factors. Short, daily bouts of electromechanical activation ameliorated several of these adaptations, with the isometric contractions being the most effective countermeasure. The clinical implications of these findings for rehabilitation strategies are discussed.
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PMID:Influences of electromechanical events in defining skeletal muscle properties. 1221 Mar 89

We have generated an animal model for mitochondrial myopathy by disrupting the gene for mitochondrial transcription factor A (Tfam) in skeletal muscle of the mouse. The knockout animals developed a myopathy with ragged-red muscle fibers, accumulation of abnormally appearing mitochondria, and progressively deteriorating respiratory chain function in skeletal muscle. Enzyme histochemistry, electron micrographs, and citrate synthase activity revealed a substantial increase in mitochondrial mass in skeletal muscle of the myopathy mice. Biochemical assays demonstrated that the increased mitochondrial mass partly compensated for the reduced function of the respiratory chain by maintaining overall ATP production in skeletal muscle. The increased mitochondrial mass thus was induced by the respiratory chain deficiency and may be beneficial by improving the energy homeostasis in the affected tissue. Surprisingly, in vitro experiments to assess muscle function demonstrated that fatigue development did not occur more rapidly in myopathy mice, suggesting that overall ATP production is sufficient. However, there were lower absolute muscle forces in the myopathy mice, especially at low stimulation frequencies. This reduction in muscle force is likely caused by deficient formation of force-generating actin-myosin cross bridges and/or disregulation of Ca(2+) homeostasis. Thus, both biochemical measurements of ATP-production rate and in vitro physiological studies suggest that reduced mitochondrial ATP production might not be as critical for the pathophysiology of mitochondrial myopathy as thought previously.
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PMID:Increased mitochondrial mass in mitochondrial myopathy mice. 1241 46

Fatigue of voluntary muscular effort is a complex phenomenon, influenced by peripheral and central nervous system factors. Peripheral mechanisms of fatigue include impairment in neurotransmission and impulse propagation down the sarcolemna, dysfunction within the sarcoplasmic reticulum involving calcium release and uptake, impairment in the actin-myosin cross-bridge interactions and substrate depletion or accumulation of metabolites. The central fatigue is associated with reduction of motor cortical excitability, failure to drive the motoneurons adequately, changes in neurotransmitter activity, etc. Is fatigue an unexplained disabling symptom or a protective mechanism to prevent muscle fiber degeneration?
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PMID:[Neurologic and muscular mechanisms of fatigue]. 1242 48

Muscle has an intrinsic ability to adapt to different types of work by changing fibre type and muscle mass. This process involves quantitative and qualitative changes in gene expression including those of the myosin heavy chain (MyHC) isogenes that encode different types of molecular motors. Increased expression of slow MyHC and of metabolic genes result in increased fatigue resistance. Recently, there has been some insight into how oxidative metabolism, as well as slow myosin expression, is regulated and the role of calcium in initiating switches in gene expression. In relation to muscle mass and power output it has been appreciated that local as well as systemic factors are important. Our group have cloned three types of IGF-I in human muscle which are derived from the IGF-I gene by alternative splicing. The expression of one of these that appears to be an autocrine/paracrine splice variant is only detectable after mechanical stimulation (MGF) and a systemic type (IGF-IEa) that is produced by the liver and other tissue including muscle. As the result of a reading frame shift, the MGF peptide has a different C terminal sequence to IGF-IEa. Interestingly, the MGF C terminal peptide has been found to act as a separate growth factor and to initially activate mononluceated myoblasts (satellite cells). MGF also responds to different signals and has different expression kinetics to IGF-IEa. The mechanotransduction mechanism for this signalling may directly or indirectly involve the dystrophin complex as dystrophic muscle, unlike normal muscle, is unable to express MGF in response to overload. Also the ability to express MGF has been found to decline markedly during ageing. The deficiency in expressing MGF and activating satellite cells in dystrophic and aged muscles may explain why muscle mass is not maintained in these situations. However, in normal muscle MGF appears to initiate local muscle repair with its over expression resulting in hypertrophy.
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PMID:Gene expression in muscle in response to exercise. 1460 23

Muscle strength and, to a greater extent, power inexorably decline with ageing. Quantitative loss of muscle mass, referred to as "sarcopenia", is the most important factor underlying this phenomenon. However, qualitative changes of muscle fibres and tendons, such as selective atrophy of fast-twitch fibres and reduced tendon stiffness, and neural changes, such as lower activation of the agonist muscles and higher coactivation of the antagonist muscles, also account for the age-related decline in muscle function. The selective atrophy of fast-twitch fibres has been ascribed to the progressive loss of motoneurons in the spinal cord with initial denervation of fast-twitch fibres, which is often accompanied by reinnervation of these fibres by axonal sprouting from adjacent slow-twitch motor units (MUs). In addition, single fibres of older muscles containing myosin heavy chains of both type I and II show lower tension and shortening velocity with respect to the fibres of young muscles. Changes in central activation capacity are still controversial. At the peripheral level, the rate of decline in parameters of the surface-electromyogram power spectrum and in the action-potential conduction velocity has been shown to be lower in older muscle. Therefore, the older muscle seems to be more resistant to isometric fatigue (fatigue-paradox), which can be ascribed to the selective atrophy of fast-twitch fibres, slowing in the contractile properties and lower MU firing rates. Finally, specific training programmes can dramatically improve the muscle strength, power and functional abilities of older individuals, which will be examined in the second part of this review.
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PMID:Muscle strength, power and adaptations to resistance training in older people. 1463 81

The mechanical effects of the intermediate filament protein desmin was examined in desmin deficient mice (Des-/-) and their wild type control (Des+/+). Active force generation was determined in intact soleus muscles and in skinned single fibres from soleus and psoas. A decreased force generation of skinned muscle fibres from Des-/- mice and a tendency towards decreased active force in intact soleus muscle were detected. Concentrations of the contractile protein actin and myosin were not altered in Des-/- muscles. Ca(2+)-sensitivity of skinned single fibres in Des-/- muscles was unchanged compared to Des+/+. Using a protocol with repeated short tetani an increased fatigue resistance was found in the intact soleus muscles from Des-/- mice. In conclusion, desmin intermediate filaments are required for optimal generation or transmission of active force in skeletal muscle. Although other studies have shown that the desmin intermediate filaments appear to influence Ca(2+)-handling, the Ca(2+)-sensitivity of the contractile filaments is not altered in skeletal muscle of Des-/- mice. Previous studies have reported a switch towards slower myosin isoforms in slow skeletal muscle of Des-/- mice. The increased fatigue resistance show that this change is reflected in the physiological function of the muscle.
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PMID:Lower active force generation and improved fatigue resistance in skeletal muscle from desmin deficient mice. 1467 48

The basis for all biological movement is the conversion of chemical energy to mechanical energy by different classes of motor proteins. In skeletal muscle this motor protein is myosin II, a thick filament-based molecule that harnesses the free energy furnished by ATP hydrolysis to perform mechanical work against actin proteins of the thin filament. The cyclic attachment and detachment of myosin with actin that generates muscle force and shortening is Ca2+ regulated. Intense muscle activity may lead to metabolically induced inhibitions to the function of these myofibrillar proteins when Ca2+ regulation is normal, a phenomenon referred to as myofibrillar fatigue. Studies using single muscle fibers at room temperature or lower have shown that myosin motor function is inhibited by the accumulation of the ATP-hydrolysis products ADP, Pi, and H+ as well as by excess generation of reactive oxygen species (ROS). These metabolically induced impairments to myosin motor function reduce muscle work and power output by impairing maximal Ca2+ activated force, the Ca2+ sensitivity of force, and/or unloaded shortening velocity. Based on uncertainties about their inhibitory effect on muscle function at more physiological temperatures, the influence of ATP-hydrolysis product and ROS accumulation on myofibrillar protein function of human skeletal muscle remains to be clarified.
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PMID:The myofibrillar complex and fatigue: a review. 1519 30

Exercise intolerance is a condition commonly experienced by both the healthy and those with disease. Yet we have only a limited understanding of the underlying mechanisms and, consequently, the management of this condition. In this Symposium, a major objective was to address the role of the muscle cell in weakness and fatigue. We have focused on addressing the advances made in characterizing the basis of muscle cell contractility with particular respect to the processes and proteins involved in excitation and contraction, and how these processes can be modified during repetitive activity. Three reviews are provided on this subject. Each addresses a specific link in the cascade of events from neural activation of the muscle to the generation of force. In the first review the processes involved in signal transduction in the sarcolemma and T-tubule, and which regulate membrane excitability, are examined. The second review analyzes the sarcoplasmic reticulum regulation of the intracellular messenger that controls the myofibrillar complex, namely free calcium. The final review in this series deals with the events regulating actin-myosin behaviour and the mechanical response. All reviews place special emphasis on how different sites can be modified by repetitive activity and, as a consequence, how they can represent a potential source of fatigue. Since it is important to understand the nature, manifestations, and measurement of weakness and fatigue, a comprehensive review on these topics is also provided.
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PMID:Mechanisms and management of fatigue in health and disease: symposium introduction. 1519 31

Effects of wide-range fatigue were studied on transient and steady force components of eccentric contraction of cat gastrocnemius. Muscle was activated by distributed stimulation of dissected ventral root L7 with rate of 10/s per filament. Active muscle was stretched through trapezoidal ramp with different velocities (1.25, 2.5, 10 mm/s). Fatiguing sessions consisted of standard test repetitions every 2 minutes. Normalized transient and steady components of extra force (fresh-state value = 1) were respectively 0.77+/-0.04 and 0.78+/-0.02 at 30% muscle tension drop, 0.27+/-0.04 and 0.55+/-0.02 at 60% muscle tension drop. Both transient and steady components have equal shares in extra force of eccentric contraction during moderate fatigue (up to 30% muscle tension drop); further pronouncing of fatigue entailed diminution of transient component share and corresponding augmentation of steady component one. Falling phase of transient component was subjected to fatigue and velocity of preceding lengthening in similar manner. We are speculated that steady component as reflection of actin-myosin affinity improvement during muscle stretching is less exposed to fatigue than cross-bridge rate.
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PMID:[Fatigue-related changes in dynamic and static components of eccentric contraction of the gastrocnemius muscle in the narcotized cat]. 1532 Apr 36

Although it is well established that patients suffering from malaria experience skeletal muscle problems (contracture, aches, fatigue, weakness), detailed studies have not been performed to investigate changes in the contractile function and biochemical properties of intact and skinned skeletal muscles of mammals infected with malaria. To this end, we investigated such features in the extensor digitorium longus (EDL, fast-twitch, glyocolytic) and in the soleus (SOL, slow-twitch, oxidative) muscles from mice infected with Plasmodium berghei. We first studied maximal tetanic force (T(max)) produced by intact control and malaria-infected muscles before, during and after fatigue. Triton-skinned muscle fibres were isolated from these muscles and used to determine isometric contractile features as well as a basic biochemical profile as analysed by silver-enhanced SDS-PAGE. We found that the T(max) of intact muscles and the maximal Ca2+-activated force (F(max)) of Triton-skinned muscle fibres were reduced by approximately 50% in malarial muscles. In addition, the contractile proteins of Triton-skinned muscle fibres from malarial muscles were significantly less sensitive to Ca2+. Biochemical analysis revealed that there was a significant loss of essential contractile proteins (e.g. troponins and myosin) in Triton-skinned muscle fibres from malarial muscles as compared to controls. The biochemical alterations (i.e., reduction of essential contractile proteins) seem to explain well the functional modifications resolved in both intact muscles and Triton-skinned muscle fibres and may provide a suitable paradigm for the aetiology of muscle symptoms associated with malaria.
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PMID:Functional and biochemical modifications in skeletal muscles from malarial mice. 1572 39


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