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

Determinations of fatigue ratio, twitch and tetanus tension, and contraction and half-relaxation times of the isometric twitch were made in 21 single fast-twitch motor units from the rat tibialis anterior muscle. Single motor units were functionally isolated by microdissection of the ventral root, and the glycogen depletion technique was used to demonstrate the muscle fibers in the unit. Morphological and immuno- and enzyme-histochemical methods were applied to serial muscle cross sections to characterize the muscle fibers in the unit. Three of the units had muscle fibers of the IIa type according to staining both for myofibrillar adenosinetriphosphatase after acid preincubation and with the use of monoclonal antibodies specific for myosin heavy chains (MHCs), i.e., the IIa-MHC isoform. The other 18 units were of the IIb type according to enzyme-histochemistry, but immunohistochemistry showed that in six of these units the muscle fibers exhibited the novel type IIx-MHC isoform and in the other 12 units the IIb-MHC isoform. It was found that the IIx motor units have contraction and half-relaxation times similar to those of types IIa and IIb units but have morphological, physiological, and biochemical properties that distinguish them from the latter two types.
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PMID:MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type. 185 63

1. In the preceding paper we analysed the force decline in fatigue of isolated mouse muscle fibres. Using the same preparation and stimulation scheme we have now examined another aspect of muscle fatigue, namely slowing of relaxation. 2. Relaxation at the end of a tetanic contraction can usually be divided into two phases, an initial nearly linear force decline, followed by an exponential phase. We have analysed the initial phase in terms of decline rate and duration. In rested fibres the decline rate after a 350 ms tetanus was not affected by a 30% reduction of tetanic tension (obtained by decreasing the stimulation frequency). 3. Relaxation became gradually slower during fatiguing stimulation with a maximum reduction at the time when tetanic tension was reduced to about 75% of the original (end of phase 2, see preceding paper). At this time the decline rate of the linear phase had fallen to 55.2 +/- 2.9% (mean +/- S.E.M., n = 25) and its duration had increased to 151.3 +/- 14.2% of the control (unfatigued 350 ms, 70 Hz tetanus). 4. Short rest periods (duration = 10s), given at various times during fatiguing stimulation, resulted in a clear, but partial, normalization of the relaxation parameters; at the time of maximum slowing the mean decline rate increased from 50.3 to 58.7% and the duration decreased from 167.9 to 144.0% of the control (n = 14). 5. The influence of intracellular acidosis on relaxation was studied by exposing rested fibres to 30% CO2 instead of the normal 5%. This resulted in a decline rate of 43.0 +/- 2.6% and a duration of 221.1 +/- 13.1% of the control (n = 7). 6. In amphibian muscle relaxation is known to become gradually slower during a single, prolonged tetanus. The existence of such an effect also in the present preparation was studied by giving 'interrupted' tetani with a total duration of about 2 s. In rested fibres the mean rate of relaxation was found to fall from 140.9 to 71.8% (n = 11) of the control (end of 350 ms stimulation) with a time constant of about 0.5 s. Thus, a marked slowing during a long tetanus occurs also in mammalian muscle. 7. A distinct slowing of relaxation during prolonged tetani was observed also in the fatigued and in the acidified state. Under these two conditions the mean rate of relaxation fell from 87.0 to 34.0% (n = 3) and from 74.0 to 23.0% (n = 5) of the control, respectively, with time constants similar to that in the rested state.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Slowing of relaxation during fatigue in single mouse muscle fibres. 190 16

Adult cats were used to study the recovery of muscles that had become altered by long-term electrical stimulation. Chronic activation was delivered to the deafferented common peroneal nerve (no pain, no reflexes), and contractile properties were measured for peroneus longus muscle. After 4 wk of great daily amounts of treatment at moderately high pulse rates (30-40 Hz delivered during 50% of daily time), the peroneus longus became considerably weaker, demonstrated a longer time course of twitches and a slower rate of rise of tetanic force, and became less fatigable. Furthermore, its twitch-to-tetanus ratio decreased, and there was no longer any depression of electromyogram (EMG) amplitude during fatigue tests. After 4 wk of subsequent rest it was found that 1) twitch speed and maximum tetanic force had returned to nearly normal values, 2) fatigue resistance showed some return toward normal but was still significantly enhanced, and 3) no significant recovery had yet occurred of the altered twitch-to-tetanus ratio, the abolished EMG depression, or the slowed rate of rise of tetanic tension. During the poststimulation recovery period, the progressive increase of isometric twitch speed was not promoted by the administration of small daily amounts of high-rate stimulation (100-Hz bursts). The results support the conclusions that 1) the time course of recovery differs among physiological properties, 2) the EMG and force reactions that occur during a fatigue test are not strongly coupled, as demonstrated by the alterations of their relationship during poststimulation recovery, and 3) in cat's fast muscles, there is still no evidence for rate-specific effects of chronic stimulation on isometric twitch speed.
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PMID:Recovery after intense chronic stimulation: a physiological study of cat's fast muscle. 205 55

To study effects of fatigue on muscle excitability and contractility in myotonic dystrophy (MyD), we evaluated, by ulnar nerve supramaximal stimulation, both single shock and 40 Hz tetanus, M wave and force parameters from adductor pollicis. In 8 MyD patients and in 6 controls amplitude of M wave, electromechanical delay, single twitch and tetanus tension, contraction and half-relaxation times were recorded in basal condition and at different times after 75 sec. of maximal voluntary contraction. Reduction of force related to fatigue was per cent lesser in MyD compared to controls. Electro-mechanical delay, basally longer in MyD, showed after fatigue 15% increment compared to 47% in controls. Half-relaxation time increased in both groups, but in MyD recovery was faster. Peculiar alterations of excitation-contraction coupling and contractility occurring in MyD can explain the observed modifications of fatigue phenomena in this disease.
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PMID:[Changes in muscular excitability and contractility caused by fatigue in Steinert's disease]. 210 52

Ninety-seven motor units of medial gastrocnemius muscle of the rat were examined. Among the examined motor units, fast (F) and slow (S) units were distinguished on the base of the presence or absence of sag reflex in unfused tetanus, induced by stimulation at 40 Hz. The fatigue test, in turn, permitted to distinguish fast fatigable (FF) and fast resistant (FR) units. Fatigue index was calculated using two techniques, the results of which differed significantly from each other. The following basic features of motor units were examined: contraction time, half-relaxation time and twitch force. A more detailed analysis of all characteristics of the units disclosed that their classification into fast and slow units on the base of the sag test alone was unjustified. A more appropriate classification resulted when multiple properties of the units were considered. The distinguished types of the units differed significantly from each other in the examined properties. On the other hand, a significant individual variability in the properties was also shown to exist within each type of the units. Various characteristics of the motor units showed correlation with each other in the whole population of motor units and/or their individual types.
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PMID:Division of motor units in medial gastrocnemius muscle of the rat in the light of variability of their principal properties. 213 39

We examined the effects of theophylline on contractile properties and high-frequency fatigue of canine diaphragm in vitro. Eighteen diaphragm muscle bundles were obtained from 10 anesthetized dogs and equilibrated in oxygenated Krebs solution to 100, 200, or 300 mg/l theophylline. These bundles were compared with 18 matched control bundles from the contralateral hemidiaphragm. No statistically significant differences in twitch tension, tetanic tension, twitch-to-tetanus ratio, time to peak tension, or half-relaxation time were observed. Concentrations of 300 mg/l theophylline, however, significantly (P less than 0.05) increased force production at 10 Hz by 32%. A similar tendency was present at lower concentrations and exhibited a clear dose-response behavior. High-frequency fatigue was similar in control and theophylline-treated bundles. We conclude that supratherapeutic in vitro concentrations of theophylline do not increase maximal tetanic tension and do not protect against muscle fatigue but potentiate relative force production at low stimulation frequencies. This relatively small effect cannot be explained by poor diffusion of the drug in the muscle bundle, because theophylline concentrations in the muscle bath and in the muscle bundle were virtually identical. Moreover, it remains unclear whether this potentially beneficial effect can be achieved at in vivo attainable serum concentrations.
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PMID:Theophylline minimally alters contractile properties of canine diaphragm in vitro. 226 59

We have measured the distribution of intracellular calcium concentration in isolated single muscle fibres from Xenopus laevis using the fluorescent calcium indicator fura-2 with digital imaging fluorescence microscopy. Under control conditions, resting and tetanic calcium were uniform throughout a fibre. When fatigue was produced using a prolonged, high-frequency tetanus, the distribution of calcium within muscle fibres became non-uniform, with greater levels near the outer parts of a fibre than near the centre. This non-uniform distribution of calcium was rapidly abolished by lowering the stimulation frequency. When fatigue was produced using a series of repeated intermittent tetani, tetanic calcium showed an initial small increase, followed by a decrease as stimulation was continued. The distribution of calcium remained uniform under these conditions. Calcium distribution was also uniform during recovery from intermittent tetanic stimulation. Although fibres varied considerably in their fatigue resistance, the time for tension to fall to 50% was correlated with the reduction in tetanic calcium seen at this time. These results indicate that there are at least two patterns of reduced calcium release that can contribute to the development of fatigue. The appearance of a calcium gradient is consistent with impaired t-tubular conduction, while a uniform reduction of calcium is likely to be due to the action of metabolic factors on systems controlling calcium homeostasis within the cell.
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PMID:Spatial gradients of intracellular calcium in skeletal muscle during fatigue. 233 50

1. The purpose of this study was to examine the time course of tetanic force during a standard fatigue test and to distinguish between the appearance of potentiation and fatigue among the four motor-unit types of a cat hindlimb muscle. 2. Motor units of the tibialis posterior muscle in the adult cat were assigned to four categories (i.e. types S, FR, FI, FF) based on conventional criteria (Burke, Levine, Tsairis & Zajac, 1973). The mean (+/- S.D.) time course of peak force was constructed for each motor-unit type and, within each type, for those units that potentiated (a greater than 3% increase in peak force compared to the initial value) and those that did not potentiate. 3. The average time courses of force differed between motor-unit types. There was, however, considerable variability within each motor-unit type. For the same relative force output, the forces exerted by slow-twitch units were less variable than those exerted by fast-twitch units. In addition, the variability among slow-twitch units was relatively constant during the fatigue test while variability among fast-twitch units either increased or decreased with time. 4. For a given motor-unit type, the average time course of force did not depend on whether force in each tetanus was expressed as a peak value, an average peak value, or a force-time integral. 5. Some motor units within each type exhibited potentiation. Most of the variability in the time course of the peak force for each motor-unit type could be accounted for by the potentiating units. Motor units that exhibited only force decline (i.e. fatigue), regardless of unit type, had less variable time courses of peak force. Since potentiation was transient in some unit types, it was assumed that at least two opposing processes (i.e. fatigue and potentiation) occurred simultaneously in these units (see also, Krarup, 1981; Rankin, Enoka, Volz & Stuart, 1988; Garner, Hicks & McComas, 1989). 6. It is concluded that the expression of force potentiation throughout a fatiguing regimen is variable among motor units and that this is not related to conventional motor-unit types. This dissociation suggests that the mechanisms that form the basis for the conventional distinction between motor-unit types are different from those which lead to force potentiation.
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PMID:Motor-unit force potentiation in adult cats during a standard fatigue test. 234 3

1. The purpose of this study was to investigate tetanic force development and relaxation in single motor units that were subjected to a standard fatigue test. 2. Motor units of tibialis posterior, a hindlimb muscle in the adult cat, were assigned to four categories (i.e. types S, FR, FI, FF) using conventional criteria. 3. Based on the first tetanus of the fatigue test, type S units took significantly longer to develop force and to relax than the fast-twitch units. Within the fast-twitch subpopulations, type FR and FI units were significantly slower to develop force and to relax than were type FF units, but there were no significant differences between type FR and FI units. 4. After 120 s of the fatigue test, the rates of force development were faster than initial values in type S and FR units, but were largely unchanged for the type FI and FF units. Most relaxation parameters were unaffected by stimulation in type S and FR units, but all parameters became significantly slower in type FI and FF units. 5. The average time courses of force development and relaxation showed that during 240 s of the fatigue test, type S units exhibited either a progressive increase in a parameter or no change at all. In contrast, fast-twitch units displayed profiles that included initial increases in a force development or relaxation parameter followed by variable amounts of decline that corresponded to fatigability. 6. It is concluded that repetitive activation affects the development and relaxation of tetanic force in all motor-unit types. Average changes in these parameters tended to parallel the conventional classification of motor units into four categories.
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PMID:Force development and relaxation in single motor units of adult cats during a standard fatigue test. 234 4

1. Changes in force and stiffness were recorded simultaneously during 1 s isometric (fixed ends) tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (temperature 1-3 degrees C; sarcomere length, 2.10 micron). Stiffness was measured as the change in force that occurred in response to a 4 kHz sinusoidal length oscillation of the fibre. Some experiments were performed in which stiffness was determined from a fast (0.2 ms) length step that was applied to a 'tendon-free' segment of the muscle fibre during the tetanus plateau. 2. A moderate degree of fatigue was produced by decreasing the time between tetani from 300 s (control) to 15 s. By this treatment the maximum tetanic force (Ftet) was reversibly reduced to 70-75% of the control value. Maximum tetanic stiffness (Stet) was related to Ftet according to the following regression (both variables expressed as percentage of their control values): Stet = 0.369 Ftet + 62.91 (correlation coefficient, 0.95; P less than 0.001). A 25% decrease in isometric force during fatigue was thus associated with merely 9% reduction of fibre stiffness. 3. Whereas the rate of rise of force during tetanus was markedly reduced by fatiguing stimulation, the rate of rise of stiffness was only slightly affected. 4. Intracellular acidification (produced by raised extracellular CO2 concentration) largely reproduced the contractile changes observed during fatigue. However, for a given decrease in tetanic force there was a smaller reduction in fibre stiffness during acidosis than during fatigue. 5. Caffeine (0.5 mM) added to the fibre after development of fatigue and intracellular acidosis greatly potentiated the isometric twitch but did not affect maximum tetanic force. This finding provides evidence that the contractile system was fully activated during the tetanus plateau both in the fatigued state and during acidosis. 6. The results suggest that the decrease in contractile strength after frequent tetanization (intervals between tetani, 15 s) is attributable to altered kinetics of cross-bridge function leading to reduced number of active cross-bridges and, most significantly, to reduced force output of the individual bridge. The possible role of increased intracellular H+ concentration in the development of muscle fatigue is discussed.
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PMID:Changes in force and stiffness induced by fatigue and intracellular acidification in frog muscle fibres. 239 50


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