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

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.
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PMID:Physiological responses of rat plantaris motor units to overload induced by surgical removal of its synergists. 297 14

The properties of flexor digitorum longus (FDL) muscles and of individual motor units were studied in cats 30-50 wk after self-reinnervation by FDL motoneurons (FDL----FDL) or cross-reinnervation by soleus (SOL) motoneurons (SOL----FDL). Individual motor units were functionally isolated by intracellular recording and stimulation of identified SOL alpha-motoneurons. Glycogen-depletion methods permitted histochemical study of muscle fibers belonging to physiologically characterized muscle units. The observations were compared with data from normal cat FDL muscles and motor units (27). Intentionally self-reinnervated FDL muscles (FDL----FDL; n = 5) were normal in size and wet weight. FDL----FDL motor units could be classified into the same physiological categories found in normal FDL [types: fast contracting, fatigable (FF), fast contracting, fatigue resistant (FR), and slow (S); n = 24], with approximately the same proportions as normal. The histochemical muscle fiber types associated with these categories were also qualitatively normal although there was evidence of marked distortion of the normal histochemical mosaic. These data confirm other studies of self-reinnervation and suggest that self-reinnervation can produce complete interconversion of muscle fiber types. Cross-reinnervation of FDL muscle by SOL motoneurons (SOL----FDL; n = 12) produced muscles that were smaller (about half the normal wet weight) and more red than normal. SOL----FDL muscle contracted more slowly than normal or FDL----FDL muscles and had much higher proportions of histochemical type I muscle fibers. In those SOL----FDL muscles, in which little or no unwanted self-reinnervation could be demonstrated, greater than 95% of the muscle fibers were type I. Forty-one individual motor units in SOL----FDL muscles were isolated by intracellular penetration in functionally identified SOL alpha-motoneurons. Their muscle units were all type S by physiological criteria (absence of "sag" in unfused tetani and marked resistance to fatigue). SOL----FDL muscle units had contraction times and fatigue properties that were essentially identical to those of type S units in the normal FDL. All of the seven units, successfully studied by glycogen depletion, exhibited histochemical type I fibers. SOL motoneurons that innervated FDL muscle units had slightly shorter afterhyperpolarization durations than normal SOL cells, but axonal conduction velocities were normal.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Cross-reinnervated motor units in cat muscle. I. Flexor digitorum longus muscle units reinnervated by soleus motoneurons. 299 48

We investigated the relationship between oxidative capacity, substrate depletion, and fatigability of muscle grafts in situ. Isometric contractile properties and fatigability were measured in stabilized grafts and control muscles from age-matched rats. Intramuscular glycogen and triglyceride concentrations were measured before and after the fatigue protocol. Even though stabilized soleus grafts have a reduced oxidative capacity compared with control soleus muscles, no difference was observed between groups in terms of resistance to fatigue. Glycogen concentrations did not decrease significantly during the fatigue protocol in either group. Compared with control muscles, the triglyceride concentration in the grafts was significantly higher before the fatigue protocol and it decreased significantly more following the fatigue protocol.
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PMID:Intramuscular substrate depletion and fatigability of soleus grafts in rats. 316 98

Ice hockey is characterized by high intensity intermittent skating, rapid changes in velocity and duration, and frequent body contact. The typical player performs for 15 to 20 minutes of a 60-minute game. Each shift lasts from 30 to 80 seconds with 4 to 5 minutes of recovery between shifts. The intensity and duration of a particular shift determines the extent of the contribution from aerobic and anaerobic energy systems. The high intensity bursts require the hockey player to develop muscle strength, power, and anaerobic endurance. The length of the game and the need to recover quickly from each shift demands a good aerobic system. Physical characteristics of elite players show that defensemen are taller and heavier than forwards probably due to positional demands. Hockey players are mesomorphic in structure. They are relatively lean since excess mass is detrimental to their skating performance. There is a large interindividual variability in VO2 during skating. Both the aerobic and anaerobic energy systems are important during a hockey game. Peak heart rates during a shift on the ice exceed 90% of HRmax with average on-ice values of about 85% of HRmax. Blood lactate is elevated above resting values confirming the anaerobic nature of the game. Glycogen depletion studies show a preferential utilisation of glycogen from the slow twitch fibres but also significant depletion from the fast twitch fibres. Elite hockey players display a muscle fibre composition similar to untrained individuals. Physiological profiles of elite hockey teams reveal the importance of aerobic endurance, anaerobic power and endurance, muscular strength and skating speed. Training studies have attempted to improve specific components of hockey fitness. Using traditional laboratory tests, a season of hockey play shows gains in anaerobic endurance but no change in aerobic endurance. On-ice tests of hockey fitness have been recommended as an essential part of the hockey player's physiological profile. Existing training procedures may develop chronic muscular fatigue in hockey players. Lactic acidosis is associated with the onset and persistence of muscle fatigue. Muscle force output remains impaired throughout the hockey player's typical cycle of practices and games. A supplementary programme of low-intensity cycling during the competitive phase of training was unsuccessful in altering VO2max. Strength decrements during the hockey season are attributed to a lack of a specifically designed strength maintenance programmes. On-ice and off-ice training programmes should focus on the elevation of aerobic endurance, anaerobic power and endurance, muscular strength and skating speed.
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PMID:Physiology of ice hockey. 328 Dec 10

To study the relation between the response of respiratory muscle to inspiratory loads and glycogen content, we subjected unanesthetized sheep to moderate and severe inspiratory flow resistive (IFR) loads. Only severe IFR loads eventually led to a decrease in transdiaphragmatic pressure (Pdi) and a concomitant rise in PaCO2. Respiratory and nonrespiratory skeletal muscle samples were obtained at necropsy. Glycogen content was determined biochemically in muscle homogenates. Frozen sections were stained with periodic acid-Schiff (PAS) for glycogen and fibers were typed using myosin ATPase stain. Fibers were categorized as full, intermediate, or devoid of glycogen by a subjective scoring system of PAS staining intensity. We found that glycogen content decreased in the costal and crural diaphragm and in the intercostal muscles as the duration of moderate IFR loaded breathing was increased. With severe loads glycogen content decreased significantly, reaching about 40 and 22% of control levels in the costal and crural diaphragm, respectively (P less than 0.01). In addition, with severe IFR loads, a statistically significant proportion of both type I and type II muscle fibers was depleted of glycogen when compared with that of controls (P less than 0.05), but more type II fibers were depleted than type I fibers (50 vs 23%). These data indicate that in sheep subjected to IFR loads: (1) glycogen content in the respiratory muscles decreases as the severity and duration of loaded breathing increases and (2) respiratory muscle fatigue occurs at a time when considerable glycogen is still present in type I fibers in the diaphragm.
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PMID:Respiratory muscle response to load and glycogen content in type I and II fibers. 339 Dec 57

1. The rat lateral gastrocnemius muscle (LG) is a complex of four muscle compartments, each defined in terms of its unique innervation by a single primary nerve branch of the muscle nerve. A study has been made of the topographical distribution of motor units in the medial compartment of the LG (LGM) both before and after the loss of polyneuronal innervation that accompanies development. 2. Glycogen depletion methods showed that the distribution of single motor units depended on the rostro-caudal origins of their axons in the spinal cord: rostral axons possessed motor units almost exclusively confined to the medial half of the LGM; intermediate axons possessed motor units primarily in the intermediate and lateral part of the LGM; caudal axons possessed motor units that were not restricted to any particular part of the LGM. 3. Myosin ATPase staining showed that about 80% of the LGM consists of type II A fibres, whilst the remainder are type II B. Physiological determination of the contractile properties of motor units indicated two classes of units: those that were relatively fatigue resistant and did not show a sag property (like fast-twitch, fatigue-resistant fibres or FR) and those that were relatively fatigable and did show a sag property (like fast-twitch, fatigable fibres or FF). 4. Glycogen depletion was also used to determine the distribution of motor units in the LGM at 7 days post-natal, when most fibres still receive a polyneuronal innervation. The LGM primary nerve branch innervated a confined sub-volume of muscle fibres which is similar to the mature pattern. However, rostral axons possessed motor units that extended into the lateral half of the LGM, a position from which they are excluded in the adult. 5. These observations suggest that the axons of rostral and intermediate units form a topographical map within adult FR motor units (type II A fibres) in the LGM. The results suggest that competition between axon terminals for synaptic sites plays a role in the elimination of inappropriately positioned terminals and subsequent emergence of the topographical map.
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PMID:The formation of topographical maps in developing rat gastrocnemius muscle during synapse elimination. 341 2

To study changes in muscle energy state during prolonged exercise, especially in relation to fatigue, muscle biopsies were obtained from seven healthy males working until exhaustion on a cycle ergometer at 68% (63-74%) of their maximal oxygen uptake. Biopsies were taken at rest, after 15 and 45 min of exercise and at exhaustion, and analysed for ATP, ADP, AMP, inosine monophosphate (IMP) and hypoxanthine content by high performance liquid chromatography (HPLC), and for creatine phosphate (CP), lactate and glycogen by enzymatic fluorometric techniques. Glycogen content at exhaustion was approximately 30% of the pre-exercise level. The CP content decreased steeply during the first 15 min of exercise (P less than 0.01) and continued to decrease during the rest of the exercise period (P less than 0.05). Pronounced increases in contents of IMP (64% P less than 0.001) and hypoxanthine (69%, P less than 0.05) were found when exhaustion was approaching. Furthermore, energy charge [EC; (ATP + 0.5 ADP)/(ATP + ADP + AMP)] was decreased at exhaustion (P less than 0.05). The increases in IMP and hypoxanthine which occurred when exhaustion was approaching during prolonged submaximal exercise together with the decrease in EC during this phase of exercise suggest a failure of the exercising skeletal muscle to regenerate ATP at exhaustion.
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PMID:ATP breakdown products in human skeletal muscle during prolonged exercise to exhaustion. 342 83

Experiments are described which suggest that the loss of force generating capacity seen during fatigue from intermittent, submaximal voluntary contractions of the quadriceps muscle cannot be explained by any of the usual factors thought to be responsible for fatigue. During the first 30 min of intermittent contractions at 30% MVC the force generated periodically by a brief test train of 50 Hz stimulation and by brief maximal voluntary contractions both declined by 50%. Yet no significant changes were seen in the muscle lactate, ATP or phosphocreatine. Glycogen depletion was confined only to the type I and type IIA fibres, with less than 10% totally depleted. The depletion patterns indicated that the type IIAB and type IIB motor units were not recruited during the first 30 min. The central nervous system appeared to remain capable of generating full muscle activation since the force from maximal voluntary efforts declined in parallel with that from 50 Hz stimulation. We suggest that, in this type of fatigue, the loss of force may be largely due to impaired excitation/contraction coupling. This possibility is supported by the disproportionate depression of the twitches recorded between contractions compared with that from 50 Hz stimulation (low frequency fatigue). The single unit EMG recordings suggest that, in sustained and repeated submaximal contractions, muscle contractile failure is compensated by recruitment of additional motor units rather than by rate coding of those already active. During intermittent contractions large increases in the surface EMG were associated with only modest increases in firing rates. In sustained contractions when the EMG was held constant the discharge rates declined in parallel with the force. In constant force contractions involving about 35% muscle contractile failure no changes in discharge rates were seen despite substantial increases in EMG.
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PMID:Fatigue of submaximal static contractions. 347 Oct 51

The purpose of this study was to determine whether the postponement of fatigue in subjects fed carbohydrate during prolonged strenuous exercise is associated with a slowing of muscle glycogen depletion. Seven endurance-trained cyclists exercised at 71 +/- 1% of maximal O2 consumption (VO2max), to fatigue, while ingesting a flavored water solution (i.e., placebo) during one trial and while ingesting a glucose polymer solution (i.e., 2.0 g/kg at 20 min and 0.4 g/kg every 20 min thereafter) during another trial. Fatigue during the placebo trial occurred after 3.02 +/- 0.19 h of exercise and was preceded by a decline (P less than 0.01) in plasma glucose to 2.5 +/- 0.5 mM and by a decline in the respiratory exchange ratio (i.e., R; from 0.85 to 0.80; P less than 0.05). Glycogen within the vastus lateralis muscle declined at an average rate of 51.5 +/- 5.4 mmol glucosyl units (GU) X kg-1 X h-1 during the first 2 h of exercise and at a slower rate (P less than 0.01) of 23.0 +/- 14.3 mmol GU X kg-1 X h-1 during the third and final hour. When fed carbohydrate, which maintained plasma glucose concentration (4.2-5.2 mM), the subjects exercised for an additional hour before fatiguing (4.02 +/- 0.33 h; P less than 0.01) and maintained their initial R (i.e., 0.86) and rate of carbohydrate oxidation throughout exercise. The pattern of muscle glycogen utilization, however, was not different during the first 3 h of exercise with the placebo or the carbohydrate feedings. The additional hour of exercise performed when fed carbohydrate was accomplished with little reliance on muscle glycogen (i.e., 5 mmol GU X kg-1 X h-1; NS) and without compromising carbohydrate oxidation. We conclude that when they are fed carbohydrate, highly trained endurance athletes are capable of oxidizing carbohydrate at relatively high rates from sources other than muscle glycogen during the latter stages of prolonged strenuous exercise and that this postpones fatigue.
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PMID:Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. 352 2

The contractile properties of single motor units of rat plantaris were measured in situ 7 days following muscle partial denervation, achieved by section of radicular nerve L4. Partially denervated muscles weighed less, generated weaker twitch and tetanic forces, and contained denervated fibers, as evidenced by indirect/direct stimulation force ratios less than 1. Fast motor units (over 90% of unit pool) showed elevated twitch and tetanic responses (222% and 171% of controls, respectively) and elevated twitch-to-tetanic force ratios. Although partial denervation did not alter the mean fatiguability of fast motor units, fewer proportions of units remained in the extreme categories of fatigue resistance, with a clustering of units in the intermediate ranges. Slow units, while showing elevated twitch and tetanic responses, did not change in fatiguability. Glycogen depletion of the fibers of two fast motor units in partially denervated muscles revealed the presence of fibers varying in size, and in staining intensities for succinate dehydrogenase and ATPase, within the same motor unit, as a result of motoneurone sprouting.
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PMID:Force and fatiguability of sprouting motor units in partially denervated rat plantaris. 360 4


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