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

Two metabolic features of altitude-adapted humans are the maximal O2 consumption (VO2max) paradox (higher work rates following acclimatization without increases in VO2max) and the lactate paradox (progressive reductions in muscle and blood lactate with exercise at increasing altitude). To assess underlying mechanisms, we studied six Andean Quechua Indians in La Raya, Peru (4,200 m) and at low altitude (less than 700 m) immediately upon arrival in Canada. The experimental strategy compared whole-body performance tests and single (calf) muscle work capacities in the Andeans with those in groups of sedentary, power-trained, and endurance-trained lowlanders. We used 31P nuclear magnetic resonance spectroscopy to monitor noninvasively changes in concentrations of phosphocreatine [( PCr]), [Pi], [ATP], [PCr]/[PCr] + creatine ([Cr]), [Pi]/[PCr] + [Cr], and pH in the gastrocnemius muscle of subjects exercising to fatigue. Our results indicate that the Andeans 1) are phenotypically unique with respect to measures of anaerobic and aerobic work capacity, 2) despite significantly lower anaerobic capacities, are capable of calf muscle work rates equal to those of highly trained power- and endurance-trained athletes, and 3) compared with endurance-trained athletes with significantly higher VO2max values and power-trained athletes with similar VO2max values, display, respectively, similar and reduced perturbation of all parameters related to the phosphorylation potential and to measurements of [Pi], [PCr], [ATP], and muscle pH derivable from nuclear magnetic resonance. Because the lactate paradox may be explained on the basis of tighter ATP demand-supplying coupling, we postulate that a similar mechanism may explain 1) the high calf muscle work capacities in the Andeans relative to measures of whole-body work capacity, 2) the VO2max paradox, and 3) anecdotal reports of exceptional work capacities in indigenous altitude natives.
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PMID:Skeletal muscle metabolism and work capacity: a 31P-NMR study of Andean natives and lowlanders. 186 76

Fatigue and myalgia are common in patients with acquired immunodeficiency syndrome (AIDS). To determine whether altered muscle metabolism or impaired activation of muscle might account for these symptoms, we utilized three different exercise protocols to produce fatigue in nine AIDS patients who complained of both fatigue and exercise-exacerbated myalgia. Five were taking azidothymidine (AZT), which may cause a mitochondrial myopathy. Simultaneous measures of force, EMG, and muscle metabolites (phosphocreatine, inorganic phosphate, adenosine triphosphate, and intracellular pH) using phosphorus nuclear magnetic resonance spectroscopy were performed during fatigue and recovery. There were no significant differences between patients and controls in terms of fatigability, muscle metabolism, or muscle activation. These results provide no support for the hypothesis that fatigue or myalgia in AIDS patients derives from altered muscle metabolism or that AZT produces mitochondrial myopathy.
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PMID:Fatigue and myalgia in AIDS patients. 192 2

Exercise intolerance is a major cause of morbidity in chronic heart failure (CHF) and has traditionally been attributed to skeletal muscle hypoperfusion during exercise. However, intrinsic abnormalities in skeletal muscle biochemistry and histology may also play an important role in the pathophysiology of exertional fatigue in CHF. Studies using 31P nuclear magnetic resonance (NMR) spectroscopy have demonstrated early skeletal muscle metabolic changes during exercise including excessive acidification and phosphocreatine depletion. Patients with CHF show muscle fibre atrophy with transformation of type I to II fibres accompanied by a decrease in oxidative enzyme capacity. Most of the drugs currently used to treat patients with CHF do not improve oxygen availability within exercising muscle or exercise capacity although some of them increase blood flow to skeletal muscle or alter the pattern of blood flow distribution. Physical training programmes improve exercise performance, ventilation, autonomic function and symptomatic status in CHF. Training can also increase cardiac output and reduce peripheral vascular resistance with concomitant increases in blood flow to exercising muscle and reduced arterial and venous lactate. 31P-NMR studies in patients with CHF have demonstrated significantly less acidification and phosphocreatine depletion during exercise after physical training. Animal studies suggest that the NMR changes in skeletal muscle of CHF depend on both the severity of heart failure and physical deconditioning, whereas training may reverse or prevent these alterations.
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PMID:Peripheral abnormalities in chronic heart failure. 192 83

Muscle biopsies of the descending portion of the trapezius muscle from female patients with chronic trapezius myalgia and from healthy women were analyzed with enzyme histochemical and immunohistochemical methods. Frequency, area, and capillarization of the muscle fiber types were determined. A biochemical analysis determined the lactate concentration of mixed muscle samples and the adenosine triphosphate (ATP) and phosphocreatine levels in single muscle fibers. The patients had larger type I fibers and a lower capillary:fiber area ratio for type I and type IIA fibers. The patients also exhibited lower levels of ATP and phosphocreatine in both type I and type II fibers. It is suggested that there might have been an imbalance between the capillary supply and the cross-sectional fiber area of type I and type IIA fibers in the patients. This imbalance might be of significance in the development of muscular fatigue and pain.
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PMID:Changes in muscle morphology in chronic trapezius myalgia. 194 20

The bioenergetic correlates of skeletal muscle fatigue were assessed in vivo with phosphorus-31 nuclear magnetic resonance (31P-NMR) spectroscopy. After surgical construction of latissimus dorsi muscle ventricles, seven beagles underwent 31P-NMR spectroscopy during 12-min exercise protocols at 25- and 85-Hz stimulation frequencies and during both isovolumetric and dynamic contractions. Exercise at 85 Hz was associated with significantly greater fatigue than exercise at 25 Hz. At both frequencies, the onset of exercise was associated with a marked increase in inorganic phosphate (Pi) and a decrease in phosphocreatine (PCr). As the muscle fatigued at 85 Hz but not at 25 Hz, the phosphorus spectra returned to near baseline with a decrease in Pi and increase in PCr. For a given amount of force generated, the Pi-to-PCr ratio was higher for dynamic contractions than for isovolumetric contractions. This study indicates that high-frequency fatigue is unlikely to result from the direct effects of high-energy phosphate metabolism and that contractions producing external work consume more metabolic energy than equally forceful isometric contractions.
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PMID:Skeletal muscle bioenergetics during frequency-dependent fatigue. 200 85

Inspiratory resistive loaded (IRL) breathing results in hypoventilation and diaphragmatic fatigue in the piglet. We studied the effects of 6 h of IRL on ten 1-mo-old piglets. The load was adjusted to increase spontaneously generated transdiaphragmatic pressure five to six times baseline. Six 1-mo-old piglets acted as controls and were identically instrumented but were not subjected to IRL. Measurements of ventilation, blood gases and pH, diaphragmatic electromyogram, force-frequency curve, blood flow, and end-expiratory lung volume were obtained hourly. Diaphragmatic muscle samples were obtained after 6 h for determination of ATP, phosphocreatine, lactate, and glycogen levels. No changes occurred in the control animals. IRL resulted in a significant decrease in ventilation, an increase in diaphragmatic EMG, onset of abdominal expiratory muscle activity, and a fall in end-expiratory lung volume by 1 h. The force-frequency curve adjusted for lung volume change fell by 20% at all frequencies of stimulation at 1 h and by 40% at 6 h. Blood flow to the costal and crural diaphragm increased by 51 and 141%, respectively. No differences were noted in ATP, phosphocreatine, lactate, or glycogen between control and IRL animals. It is concluded that submaximal spontaneous contractions of the piglet diaphragm over a 6-h period cause a substantial decrease in its maximal force-generating capacity that is not related to substrate depletion.
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PMID:Diaphragmatic force and substrate response to resistive loaded breathing in the piglet. 201 Apr 11

Potential mechanisms of fatigue (metabolic factors) and potentiation (phosphate incorporation by myosin phosphorylatable light chains) were investigated during recovery from a 60-s maximal voluntary isometric contraction (MVC) in the quadriceps muscle of 12 subjects. On separate days before and for 2 h after the 60-s MVC, either a 1-s MVC or electrically stimulated contractions were used as indexes to test muscle performance. Torque at the end of the 60-s MVC was 57% of the initial level, whereas torques from a 1-s MVC and 50-Hz stimulation were most depressed in the immediate recovery period. At this time, muscle biopsy analyses revealed significant decreases in ATP and phosphocreatine and a 19-fold increase in muscle lactate. Conversely, isometric twitch torque and torque from a 10-Hz stimulus were the least depressed of six contractile indexes and demonstrated potentiation of 25 and 34%, respectively, by 4 min of recovery (P less than 0.05). At this time, muscle lactate concentration was still 16 times greater than at rest. An increased phosphate content of the myosin phosphorylatable light chains (P less than 0.05) was also evident both immediately and 4 min after the 60-s MVC. We conclude that the 60-s MVC produced marked force decreases likely due to metabolic displacement, while the limited decline in the twitch and 10-Hz torques and their significant potentiation suggested that myosin phosphorylation may provide a mechanism to enhance contractile force under conditions of submaximal activation during fatigue.
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PMID:Simultaneous potentiation and fatigue in quadriceps after a 60-second maximal voluntary isometric contraction. 202 65

In this paper we develop a theory for calculating the chemical energy liberation and heat production of a skeletal muscle subjected to an arbitrary history of stimulation, loading, and length variation. This theory is based on and complements the distribution-moment (DM) model of muscle [Zahalak and Ma, J. biomech. Engng 112, 52-62 (1990)]. The DM model is a mathematical approximation of the A. F. Huxley cross-bridge theory and represents a muscle in terms of five (normalized) state variables: A, the muscle length, c, the sarcoplasmic free calcium concentration, and Q0, Q1, Q2, the first three moments of the actin-myosin bond-distribution function (which, respectively, have macroscopic interpretations as the muscle stiffness, force, and elastic energy stored in the contractile tissue). From this model are derived two equations which predict the chemical energy liberation and heat production rates in terms of the five DM state variables, and which take account of the following factors: (1) phosphocreatine hydrolysis associated with cross-bridge cycling; (2) phosphocreatine hydrolysis associated with sarcoplasmic-reticulum pumping of calcium; (3) passive calcium flux across the sarcoplasmic-reticulum membrane; (4) calcium-troponin bonding; (5) cross-bridge bonding at zero strain; (6) cross-bridge strain energy; (7) tendon strain energy; and (8) external work. Using estimated parameters appropriate for a frog sartorius at 0 degree C, the energy rates are calculated for several experiments reported in the literature, and reasonable agreement is found between our model and the measurements. (The selected experiments are confined to the plateau of the isometric length-tension curve, although our theory admits arbitrary length variations.) The two most important contributions to the energy rates are phosphocreatine hydrolysis associated with cross-bridge cycling and with sarcoplasmic-reticulum calcium pumping, and these two contributions are approximately equal under tetanic, isometric, steady-state conditions. The contribution of the calcium flux across the electrochemical potential gradient at the sarcoplasmic-reticulum membrane was found to be small under all conditions examined, and can be neglected. Long-term fatigue and oxidative recovery effects are not included in this theory. Also not included is the so-called 'unexplained energy' presumably associated with reactions which have not yet been identified. Within these limitations our model defines clear quantitative interrelations between the activation, mechanics, and energetics in muscle, and permits rational estimates of the energy production to be calculated for arbitrary programs of muscular work.
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PMID:A distribution-moment model of energetics in skeletal muscle. 849 84

Uremic patients often complain of fatigue and muscle weakness. In order to elucidate the abnormalities of energy metabolism in the muscles of such patients, we measured the concentrations of phosphocreatine (PCr), inorganic phosphate (Pi) and adenosine triphosphate (ATP) as well as the intracellular pH in skeletal muscles by 31P-NMR at rest, during aerobic and anaerobic exercise and during recovery in 15 uremic patients (7 non-dialyzed patients and 8 dialyzed patients) and 6 control subjects. At rest, there was no difference in intracellular pH between the uremic patients and controls, but the concentrations of PCr and ATP in the skeletal muscle were lower in the uremic patients. However, during aerobic exercise, the uremic patients showed a rapid decrease in intracellular pH and a delay in its recovery. They also revealed an increased PCr utilization during aerobic exercise and its delayed resynthesis during recovery. During anaerobic exercise, the uremic patients, especially non-dialyzed patients, displayed a slower decrease in pH than the controls and a delay in its recovery. An increased PCr utilization during anaerobic exercise and a delayed resynthesis during recovery were also demonstrated. These findings suggest that the aerobic and anaerobic energy productions in uremic patients are impaired and that the energy production of the muscle depends on anaerobic glycolysis during exercise. Hemodialysis apparently facilitates recovery of the inhibited enzyme activities of oxidative phosphorylation and glycolysis in uremic patients.
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PMID:Impaired muscle energy metabolism in uremia as monitored by 31P-NMR. 203 34

Force and relaxation were measured during electrical stimulation of the quadriceps muscle of 14 volunteers. Stimulation produced 51.2 s of intermittent ischaemic contractions either as 16 3.2-s tetani or as 64 0.8-s tetani. Changes during recovery were followed for 180 s. On 8 subjects muscle biopsies were taken during work and after the rest period for determination of ATP, phosphocreatine and intermediates in glucolysis. The stimulation using 0.8-s contractions gave more pronounced fatigue and slowing of relaxation. There was a good correlation between force and relaxation during work but this relation changed during recovery, indicating that no general relation exists between these two contraction characteristics. In the 0.8-s stimulation more ATP was utilized and there were more profound changes in metabolite levels. We found a correlation between estimated [H2PO4-] and relaxation covering both work and recovery and hypothesize that inorganic phosphate and its removal by phosphocreatine resynthesis during recovery might be important. Since stimulation patterns differ in force and relaxation even after the recovery period we suggest that additional factors, such as pH, are of importance in this work model.
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PMID:Relaxation and force during fatigue and recovery of the human quadriceps muscle: relations to metabolite changes. 204 18


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