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

The purpose of this study was to compare the intramuscular and the intravascular events in relation to energy metabolism during progressive arm exercise. Twelve healthy untrained Japanese males participated in this study as subjects. They performed wrist flexion in a ramp incremental load of 0.14 W/min until exhaustion. 31P-MR spectra were obtained from wrist flexor muscle before and throughout the exercise. Venous blood was also sampled from antecubital vein with one minute interval during the exercise, and a change in plasma lactate concentration (La) was observed. Intracellular pH (pH) was calculated from a chemical shift between phosphocreatine (PCr) and inorganic phosphate (Pi) of the 31P-MR spectra. Change in pH showed a threshold behavior during exercise. Threshold points at decline in pH (pHT), increase in Pi/PCr (PT), and increase in La (LT) were determined by piecewise linear regression analysis of minutes-by-minutes data. Mean values of pHT, PT and LT were 43.0, 42.5, and 24.8% of maximal work rate, respectively. LT was significantly smaller than pHT and PT. This result suggests that lactic acidosis has already existed when pH is kept at resting level, and pHT reflects the capacity of remaining intracellular biochemical homeostasis, which might be one of the physiological indices of muscle fatigue.
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PMID:Thresholds for decrease in intracellular pH and increase in blood lactate during progressive exercise: 31P-MRS study. 147 66

Muscle contractions at lengths below the optimum for force development were previously found to cause less fatigue than contractions at the optimum length (Lo). Decreased fatigability was suggested to arise from fewer cross-bridge interactions in shortened sarcomeres. In the present study, this suggestion was tested by monitoring energy use of human ankle dorsiflexor muscles during and after contractions at Lo and shortened lengths (Ls) with phosphorus nuclear magnetic resonance spectroscopy. The nuclear magnetic resonance spectra indicated similar rates of ATP use during contractions at Lo and Ls. Phosphocreatine, at an initial concentration of 37 mM, was reduced to an equivalent extent by 2 min of ischemic exercise at Lo (to 2.3 mM) and Ls (to 4.7 mM). Changes in pH (indicating glycolytic ATP production) were also equivalent at Lo and Ls. Exercise caused pH to fall from an initial level of 7.07 to 6.5 at Lo and to 6.53 at Ls. In relation to previous experiments performed under similar conditions on human ankle dorsiflexor muscles, the present experiments suggest that in shortened muscle the decreased force found in this and previous studies and the decreased fatigability that was previously found may not be simply due to fewer cross-bridge interactions in shortened sarcomeres. Examination of the relationships between developed force and levels of metabolites suggests that changes of force during fatigue and recovery correlate better with intracellular [Pi] and H2PO4- than with [H+].
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PMID:Influence of human muscle length on energy transduction studied by 31P-NMR. 150 63

To differentiate the effects of high energy phosphates, pH, and [H2PO4-] on skeletal muscle fatigue, intracellular acidosis during handgrip exercise was attenuated by prolonged submaximal exercise. Healthy human subjects (n = 6) performed 5-min bouts of maximal rhythmic handgrip (RHG) before (CONTROL) and after prolonged (60-min) handgrip exercise (ATTEN-EX) designed to attenuate lactic acidosis in active muscle by partially depleting muscle glycogen. Concentrations of free intracellular phosphocreatine ([PCr]), adenosine triphosphate ([ATP]), and orthophosphate ([P(i)]) and pH were measured by 31P nuclear magnetic resonance spectroscopy and used to calculate adenosine diphosphate [ADP], [H2PO4-], and [HPO4(2-)]. Handgrip force output was measured with a dynamometer, and fatigue was determined by loss of maximal contractile force. After ATTEN-EX, the normal exercise-induced muscle acidosis was reduced. At peak CONTROL RHG, pH fell to 6.3 +/- 0.1 (SE) and muscle fatigue was correlated with [PCr] (r = 0.83), [P(i)] (r = 0.82), and [H2PO4-] (r = 0.81); [ADP] was 22.0 +/- 5.7 mumol/kg. At peak RHG after ATTEN-EX, pH was 6.9 +/- 0.1 and [ADP] was 116.1 +/- 18.2 mumol/kg, although [PCr] and [P(i)] were not different from CONTROL RHG (P greater than 0.05). After ATTEN-EX, fatigue correlated most closely with [ADP] (r = 0.84). The data indicate that skeletal muscle fatigue 1) is multifactorial, 2) can occur without decreased pH or increased [H2PO4-], and 3) is correlated with [ADP] after exercise-induced glycogen depletion.
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PMID:Human muscle fatigue after glycogen depletion: a 31P magnetic resonance study. 150 2

Endurance muscle performance is highly dependent on ATP production from mitochondrial oxidative phosphorylation. To study the role of the mitochondrial oxidative enzymes in muscle fatigue, we analyzed the relationship between the concentrations of substrates associated with ATP synthesis and the muscle performance of electrically stimulated rabbit muscle under CO2-induced acidosis. Two different conditions of pacing-induced muscle performance were produced in the gastrocnemius and soleus muscle groups in anesthetized rabbits by stimulating the sciatic nerve submaximally at two frequencies. Phosphorus nuclear magnetic resonance was used to measure ATP, phosphocreatine, and Pi and to provide data for a calculation of intracellular pH and free ADP. To induce acidosis, the animal was ventilated with 20% CO2. The administration of CO2 effectively reduced the intracellular pH from 6.9 to 6.7 and reduced the isometric tension-time integral (TTI) to below half the value measured in normocapnia at the low pacing frequency. A twofold increase in the pacing frequency resulted in a doubling of the TTI in normocapnia and a tripling of TTI in hypercapnia. The increases in TTI corresponded with increases in free ADP and Pi concentrations. Under the various conditions, all free ADP values were near the in vitro Michaelis-Menten constant (Km) of ADP. The Michaelis-Menten relationship of the oxidative phosphorylative enzymes was applied to the change in substrate concentrations with respect to TTI. From this relationship we observed that the in vivo Km of free ADP was 26 microM, which is close to the in nitro Km, and that Km and maximal reaction velocity did not change under hypercapnia and increased pacing frequency.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Substrate regulation of mitochondrial oxidative phosphorylation in hypercapnic rabbit muscle. 155 27

The supply of energy is of fundamental importance for the ability to sustain exercise. The maximal duration of exercise is negatively related to the relative intensity both during dynamic and static exercise. Since exercise intensity is linearly related to the rate of energy utilisation this suggests that energetic deficiency plays a major role in the aetiology of muscle fatigue. Characteristic metabolic changes in the muscle are generally observed at fatigue--the pattern being different after short term exercise (lactate accumulation and phosphocreatine depletion) from after prolonged exercise at moderate intensity (glycogen depletion). A common metabolic denominator at fatigue during these and many other conditions is a reduced capacity to generate ATP and is expressed by an increased catabolism of the adenine nucleotide pool in the muscle fibre. Transient increases in ADP are suggested to occur during energetic deficiency and may be the cause of fatigue. Experimental evidence from human studies demonstrate that near maximal power output can be attained during acidotic conditions. Decreases in muscle pH is therefore unlikely to affect the contractile machinery by a direct effect. However, acidosis may interfere with the energy supply possibly by reducing the glycolytic rate, and could by this mechanism be related to muscle fatigue.
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PMID:Metabolic factors in fatigue. 156 13

Repetitive static exercise (RSE) is a repetitive condition of partial ischaemia/reperfusion and may therefore be connected to the formation of oxygen-derived free radicals and tissue damage. Seven subjects performed two-legged intermittent knee extension exercise repeating at 10 s on and 10 s off at a target force corresponding to about 30% of the maximal voluntary contraction force. The RSE was continued for 80 min (n = 4) or to fatigue (n = 3). Four of the subjects also performed submaximal dynamic exercise (DE) at an intensity of about 60% maximal oxygen uptake (VO2max) for the same period. Whole body oxygen uptake (VO2) increased gradually with time during RSE (P less than 0.05), indicating a decreased mechanical efficiency. This was further supported by a slow increase in leg blood flow (P less than 0.05) and leg oxygen utilization (n.s.) during RSE. In contrast, prolonged RSE had no effect on VO2 during submaximal cycling. Maximal force (measured in six additional subjects) declined gradually during RSE and was not completely restored after 60 min of recovery. After 20 and 80 min (or at fatigue) RSE phosphocreatine (PC) dropped to 74% and 60% of the initial value, respectively. A similar decrease in PC occurred during DE. Muscle and arterial lactate concentrations remained low during both RSE and DE. The three subjects who were unable to continue RSE for 80 min showed no signs of a more severe energy imbalance than the other subjects. A continuous release of K+ occurred during both RSE and DE.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Repetitive static muscle contractions in humans--a trigger of metabolic and oxidative stress? 156 68

1. Peak isometric force of single fast (type 1) and slow (type 3) muscle fibres of Xenopus decreased when fibres were stimulated intermittently above their predicted sustainable duty cycle at 20 degrees C. Type 1 fibres could be fatigued to zero force. In most type 3 fibres force did not decrease below 50% of the original (P0) before activation failure, as indicated by irregular contractions. 2. Fibres were rapidly frozen at different force levels and analysed by high-performance liquid chromatography (HPLC) for ATP, IMP, phosphocreatine (PCr) and creatine (Cr). Lactate was determined enzymatically in type 1 fibres only. The relationships between force and PCr, and between force and ATP during fatigue were, apart from the range of values obtained, the same for both fibre types. When force had fallen to about 60-80% of original, PCr was fully reduced. At lower force levels, the ATP content-decreased, and a concomitant rise of IMP content was found. At zero force, ATP had fallen to about 25% of its value in rested type 1 fibres, and up to 200 mumol lactate (g dry weight)-1 had accumulated. 3. Recovery from fatigue was studied in fibres where force had fallen to 0.6 P0 (both fibre types) and 0.2 P0 (type 1 only). After 1 h of recovery ATP had in all cases returned to the level measured in rested fibres. In fibres fatigued to 0.6 P0, force almost returned to its original value. However, in type 1 fibres fatigued to 0.2 P0, it returned to only 0.3 P0. After 1 h of recovery the PCr/Cr ratio in type 1 fibres was lower (probability, P less than 0.05) than in control fibres, whereas in type 3 fibres it was not significantly different from controls. 4. The relationship between peak force and stimulus frequency, which had a sigmoid shape in fully rested fibres, was drastically changed by fatiguing stimulation. Immediately after fatiguing stimulation of type 1 fibres, force hardly increased with stimulus frequency, corresponding to the observation that calcium efflux from the sarcoplasmic reticulum was decreased at high stimulus frequencies. The force-frequency relationship of type 3 fibres was the same before and after intermittent stimulation.
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PMID:Metabolic changes with fatigue in different types of single muscle fibres of Xenopus laevis. 159 75

Dynamic changes in intracellular phosphocreatine (PCr), inorganic phosphate (Pi), and pH in human forearm muscle were studied from rest through heavy exercise by means of a ramp exercise protocol and 31P nuclear magnetic resonance spectroscopy. Eighteen healthy volunteers performed an isotonic wrist flexion exercise of repeated contractions at a frequency of 0.5 Hz. Work rate was increased continuously (ramped) at approximately 0.13 W each minute from 0.34 to 1.5 W or until fatigue. Pi/PCr was used as an estimate of the cellular phosphorylation potential of the muscle. Exercise caused a progressive increase in Pi/PCr with an initial slow and later fast component. The transition between these components was distinct and corresponded to the onset of pH decline in all subjects. These changes in Pi/PCr and pH were best fit (P less than 0.05) by a piecewise linear regression model with a break point or threshold. Repeated ramp testing of six subjects showed that the threshold was reproducible (r = 0.98). The results of this study demonstrate the existence of an intracellular metabolic threshold and suggest that indirect threshold measures (lactate and ventilatory thresholds) may reflect events at the cellular level.
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PMID:Coincident thresholds in intracellular phosphorylation potential and pH during progressive exercise. 175 3

Adenosine triphosphate (ATP) is the sole fuel for muscle contraction. During near maximal intense exercise the muscle store of ATP will be depleted in < 1s, therefore, to maintain normal contractile function ATP must be continually resynthesized. During intense exercise (from approximately 75% VO2 max to near maximal workloads) this is achieved principally by the oxidation of carbohydrate and the anaerobic utilisation of phosphocreatine (PCr) and carbohydrate. The relative contribution of carbohydrate oxidation to total energy provision decreases, while that from anaerobic utilization increases. During prolonged intense exercise (approximately 75% VO2 max), the oxidation of glucose derived from skeletal muscle and liver glycogen stores is the primary pathway for ATP resynthesis. It is widely accepted that the availability of carbohydrate limits performance during this type of exercise as the point of exhaustion has been shown to be closely related to the depletion of muscle and liver glycogen stores. It is probable that carbohydrate depletion results in the inability of skeletal muscle to maintain the required rate of ATP resynthesis and therefore, the work intensity must be reduced for exercise to continue. During short lasting near maximal exercise (0-30 s), the anaerobic utilization of muscle PCr and glycogen will fuel muscle contraction. Evidence is available to indicate that fatigue during this type of exercise is related to the inability of type II fibres to maintain the required very high rate of ATP resynthesis. This has been suggested to result from a rapid depletion of type II fibre PCr stores and an insufficiency of the glycogenolytic rate to compensate for the fall in ATP production when the PCr store is depleted. In this situation the force generation has to decrease due to insufficient energy supplies.
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PMID:Skeletal muscle energy metabolism and fatigue during intense exercise in man. 184 55

Fibromyalgia (FM) is a syndrome characterized by widespread musculoskeletal pain and tenderness at specified sites, fatigue, and unrefreshing sleep. The American College of Rheumatology (ACR) has recently accepted diagnostic criteria for FM. The ACR criteria have high diagnostic specificity, sensitivity, and accuracy. Laboratory investigations show a disturbed microcirculation in painful muscles, a decrease in adenosine triphosphate and phosphocreatine, and a reduced relaxation rate. Pain analyses indicate that the pain is nociceptive. A characteristic physiological sleep disturbance has been described that is correlated to the symptoms. The etiology of FM is not known. The etiology may be different in different patients. FM is a clinical entity, but should be regarded as a syndrome rather than a disease.
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PMID:Fibromyalgia--a clinical entity? 186 19


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