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

The site of exercise-induced muscle fatigue is suggested to be the muscle membrane, which includes the sarcolemma and T-tubule membrane; the excitability of the membrane is dependent on the membrane potential. Significant potassium flux from the intracellular space of contracting muscle may decrease the membrane potential to half its resting value. This is true for isolated muscle preparations as well as for the whole body exercise in humans. Specific K+ channels have been identified, that may account for the intracellular K+ loss. Calcium-sensitive K+ channels open when intracellular Ca2+ concentrations increase, as during excitation. ATP-sensitive K+ channels may be involved but may open only at ATP concentrations well below those attained at exhaustion. However, ATP may be compartmentalized and only the membrane-bound ATP concentration may be of significance. Ca2+ accumulation and ATP depletion cause cell destruction; these changes induce an increased K+ conductance, which may inactivate the membrane and consequently prevent tension development. It is hypothesized that such a safety mechanism is identical to the fatigue mechanism.
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PMID:Role of exercise-induced potassium fluxes underlying muscle fatigue: a brief review. 205 40

Many data suggest an involvement of toxic oxygen radicals in the termination of endurance to muscle fatigue. Being reduced glutathione (GSH), an efficient intracellular physiological antioxidant, experiments have been performed to discover whether exogenous GSH modifies endurance to exhaustive swimming in mice. GSH was administered to mice as a single dose (250, 500, 750 or 1000 mg/kg i.p.) or as repeated doses (250 mg/kg i.p. once a day during 7 days) 10 min before a swimming test to exhaustion. GSH 500, 750 and 1000 mg/kg, increased endurance to swimming by respectively 102.4%, 120.0% and 140.7%. GSH 250 mg/kg did not affect endurance when injected in a single dose but increased it by 103.7% when injected once a day for 7 days.
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PMID:Exogenous glutathione increases endurance to muscle effort in mice. 206 90

Three female and three male highly trained endurance runners with mean maximal oxygen uptake (VO2max) values of 60.5 and 71.5 ml.kg-1.min-1, respectively, ran to exhaustion at 75%-80% of VO2max on two occasions after an overnight fast. One experiment was performed after a normal diet and training regimen (Norm), the other after a diet and training programme intended to increase muscle glycogen levels (Carb). Muscle glycogen concentration in the gastrocnemius muscle increased by 25% (P less than 0.05) from 581 mmol.kg-1 dry weight, SEM 50 to 722 mmol.kg-1 dry weight, SEM 34 after Carb. Running time to exhaustion, however, was not significantly different in Carb and Norm, 77 min, SEM 13 vs 70 min, SEM 8, respectively. The average glycogen concentration following exhaustive running was 553 mmol.kg-1 dry weight, SEM 70 in Carb and 434 mmol.kg-1 dry weight, SEM 57 in Norm, indicating that in both tests muscle glycogen stores were decreased by about 25%. Periodic acid-Schiff staining for semi-quantitative glycogen determination in individual fibres confirmed that none of the fibres appeared to be glycogen-empty after exhaustive running. The steady-state respiratory exchange ratio was higher in Carb than in Norm (0.92, SEM 0.01 vs 0.89, SEM 0.01; P less than 0.05). Since muscle glycogen utilization was identical in the two tests, the indication of higher utilization of total carbohydrate appears to be related to a higher utilization of liver glycogen. We have concluded that glycogen depletion of the gastrocnemius muscle is unlikely to be the cause of fatigue during exhaustive running at 75%-80% of VO2max in highly trained endurance runners. Furthermore, diet- and training-induced carbohydrate super-compensation does not appear to improve endurance capacity in such individuals.
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PMID:Carbohydrate supercompensation and muscle glycogen utilization during exhaustive running in highly trained athletes. 207 68

Ten young women tennis players (YP: 15-30 years) and 10 veteran women tennis players (VP: 40-51 years) of equivalent skill took part in this study. In the laboratory, maximal heart rate (maxHR), VO2max and blood lactate concentration (LA) at exhaustion were measured. On the field, each match was carried out as an official competition. However, in order to obtain a strenuous match, some experimental conditions were imposed (duration, hydration, skill of opponent, etc.). Heart rate (HR) was recorded throughout the match and LA was measured at rest before the match and immediately at the end of the match. While mean heart rate intensity remained relatively steady in YP it tended to increase as the game went on in VP. Due to the lower maxHR and VO2max, VP play at a higher percentage of maxHR and thus probably at a greater relative exercise intensity than YP. For the last part of the match, in some VP, who stopped playing due to exhaustion, HR intensity reached a considerable high level. No significant increase in LA was found at the end of the match in either group. If individual values were considered, no large increase in LA was found in the exhausted women. Obviously fatigue did not result from a muscle lactate accumulation. On the other hand, this moderate LA suggests that the oxygen transport was not a limiting factor of activity, although maxHR, thus probably a maximum cardiac output, was reached. Among the possible factors responsible for the exhaustion a decrease in kinetics of heart rate recovery may be considered in veteran tennis players.
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PMID:Effects of age on heart rate response during a strenuous match of tennis. 207 45

This study examines the effects of 3-5 weeks of physical rest on selected physical, physiological and psychological parameters obtained from 12 Olympic but latterly underperforming competitors and their matched control subjects. Cardiorespiratory data were directly determined from their work to volitional exhaustion on either a treadmill, cycle, or rowing ergometer. Anaerobic power and capacity were evaluated through modified Wingate tests. For psychometric assessments, the Profile of Mood States (POMS) was used. For the Olympic competitors, one-way analyses of variance (ANOVA) revealed significant increases (p less than 0.05) in body weight, maximum respiratory exchange ratio, maximum oxygen consumption, and heart rate at the anaerobic threshold, following the rest period. There was also a significant reduction in fatigue and mood profile score, and a significant increase in vigour. No significant changes were found in the matched control subjects. The present data show that resting for 3-5 weeks assists underperforming elite competitors to improve their aerobic performance.
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PMID:Rest in underperforming elite competitors. 209 24

This study examines lactate and K+ fluxes from muscle to blood during and after intense exercise. Ten men performed exhaustive dynamic exercise (mean load 65 W, mean duration 3.18 min) with the knee extensors of one leg. The mean lactate efflux was 15.5 (range 8.9-24.0) mmol min-1 at exhaustion, and it was linearly related to the lactate gradient. A linear relationship was also obtained if the H+ gradient was taken into account. Muscle pH decreased from 7.14 at rest to 6.71 (range 6.50-6.87) at exhaustion. At rest and during late recovery blood lactate was distributed across the erythrocyte membrane according to the membrane potential (intra-/extracellular ratio of 0.5), but during rapid lactate release this ratio decreased to 0.2. In-vitro experiments demonstrated a time constant of 1.2 min for lactate efflux from the erythrocytes. Approximately 70% of the K+ ions released from the muscle to the blood accumulated in the plasma; the rest were taken up by other tissues. However, erythrocytes were not involved as a dilution space. The small change in erythrocyte K+ concentration was due to cellular volume changes. During recovery the kinetics of K+ reuptake by the muscle were described by a very fast (less than 1 min) and a slow component (greater than 1 min): the magnitude of the former was equivalent to what had accumulated in the plasma. Individuals displayed a wide range of intramuscular lactate concentrations and pH values at exhaustion. Further, the pH changes were not as extreme as previously reported, suggesting that pH may not be the only factor involved in the fatigue process. A possible role for the potassium shifts as a limiting factor for muscle function is discussed.
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PMID:Lactate and potassium fluxes from human skeletal muscle during and after intense, dynamic, knee extensor exercise. 212 76

Skeletal muscle, liver and heart glycogen variations, induced by swimming in thermal water (at 35 degrees C) as a model of physical exercise for clinical use, were studied. Muscle and liver glycogen moderately decreases after a 30-min period of swimming and comes near to depletion after 60 min. Heart glycogen decreases only slightly after 60 min. Blood glucose and plasma insulin decrease only after 60 min of swimming. A 30-min swim in thermal water, cooled to 25 degrees C, depletes muscle and liver glycogen and slightly decreases heart glycogen. Under these conditions, plasma insulin decreases and hypoglycemia occurs. The results seem to indicate some advantages of swimming in hot thermal water in order to prevent glycogen store depletion as the physiological prerequisite for a physical exercise of clinical interest to obtain therapeutical benefits, avoiding premature fatigue and exhaustion.
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PMID:Effect of swimming in thermal water on skeletal muscle, liver and heart glycogen. 213 67

Metabolic fatigue is a characteristic muscle response to intense exercise that has outstripped the rate of ATP replacement. The accumulation of metabolic by-products, namely hydrogen ions and diprotonated phosphate, interferes with actin-myosin interaction, effectively preserving muscle ATP levels by preventing further ATP hydrolysis. Muscle force and metabolite concentrations return to normal in about 5 minutes. Less intense exercise causes a more subtle, non-metabolic fatigue due to a still-undefined disturbance of excitation-contraction coupling, which can last for several hours. In this type of fatigue, greater effort is required to generate submaximal forces. Endurance exercise is mainly limited by the size of muscle glycogen stores and how efficiently they are used. Endurance training permits an athlete to work aerobically at high rates, consuming a mixture of lipid and carbohydrate fuels. When muscle glycogen is used up, exercise can only continue at the relatively low rate supportable by lipid metabolism. Anaerobic exercise is also limited by subjective factors such as dyspnoea and muscle pain, which have objective determinants. Extremely prolonged exercise can lead to general collapse because of dehydration, hyperthermia, or hypoglycaemia. None of these factors explains the phenomenon of asthenia, a subjective sense of exhaustion that produces no objective impairment of physical performance. The metabolic myopathies are experiments of nature that promise to shed new light on the biochemical basis of muscle fatigue. This will require quantitative studies of the kind provided by topical magnetic resonance spectroscopy, correlating physiology and metabolism in vivo.
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PMID:Muscle metabolism during fatigue and work. 226 24

In vivo nuclear magnetic resonance (NMR) spectroscopy was used to define several intracellular high energy phosphate variables of the gastrocnemius muscle of normal subjects during rest, graded plantar flexion exercise to exhaustion, and recovery. There were nine males and eight females with an average age of 34 +/- 8 years. At rest, pH averaged 7.09 +/- 0.03 and the energy cost index (ECI)--the ratio of inorganic phosphate to phosphocreatine--averaged 0.13 +/- 0.03. At peak exercise, the ECI increased markedly to 2.71 +/- 2.0 (P less than 0.001) and pH fell precipitately to 6.76 +/- 0.17 (P less than 0.001), indicating the high intensity of the exercise. Exercise endurance averaged 12 +/- 5 mins; it was not highly correlated with sex, age (r = 0.35), rest pH (r = 0.26), rest ECI (r = 0.38), peak exercise pH (r = 0.23) or peak exercise ECI (r = 0.38), nor exercise changes in pH (r = 0.17) and ECI (r = 0.28). At 23 mins post exercise all variables were similar to rest. Rest pH was the only variable different between males (7.10 +/- 0.03) and females (7.07 +/- 0.03) (P less than 0.05). Thus, dynamic exercise of large skeletal muscles in normal subjects was characterized by marked temporal changes in high energy phosphate profiles and very low pH at exhaustion. No single metabolic variable correlated highly with exercise endurance, suggesting that the intracellular pathophysiology of exhaustive muscle exercise and clinical fatigue may be multifactorial.
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PMID:Metabolism of normal skeletal muscle during dynamic exercise to clinical fatigue: in vivo assessment by nuclear magnetic resonance spectroscopy. 227 74

1. Human soleus muscles were fatigued under ischaemic conditions by intermittent stimulation at 15 Hz. When maximal voluntary plantarflexion was then attempted, the loss of torque was found to be associated with a reduction in voluntary EMG activity. 2. The decrease in EMG activity could not have been due to 'exhaustion' of descending motor drive in the central nervous system since fatigue had been induced by electrical stimulation of peripheral nerve fibres. Similarly, the decrease could not be explained by changes at the neuromuscular junction or muscle fibre membrane, since changes in the M wave (evoked muscle compound action potential) were relatively modest. 3. When the excitability of the soleus motoneurones was tested during fatigue, using the H (Hoffmann) reflex, it was found to be significantly reduced. Control experiments with ischaemia or electrical stimulation, but without fatigue, failed to demonstrate any significant effects on reflex excitability. 4. The findings in this study favour the concept of reflex inhibition of alpha-motoneurones during fatigue.
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PMID:Reflex inhibition of human soleus muscle during fatigue. 227 45


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