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Referências sobre fadiga central

The effects of the 5-HT2C agonist m-chlorophenylpiperazine on elite athletes suffering from unexplained underperformance syndrome (overtraining).

Budgett R, Hiscock N, Arida RM, Castell LM.

British Olympic Association, United Kingdom.

The central fatigue hypothesis describes a possible link between the neurotransmitter, 5-hydroxytryptamine (5-HT), plasma tryptophan and branched chain amino acids concentration and exercise-induced fatigue. Recent investigations studied 5-HT receptors and neuroendocrine "challenge" tests, using prolactin release as an indirect measure of 5-HT activity. The present study combined the original hypothesis about the role of amino acids in increasing brain 5-HT with a neuroendocrine challenge test on elite athletes diagnosed with unexplained, underperformance syndrome (UUPS). There was an apparent increased sensitivity of 5-HT receptors in athletes with UUPS compared with fit, well-trained controls, as measured via increased prolactin release following a bolus dose of m-chlorophenylpiperazine, a 5-HT agonist. No changes were observed in plasma amino acid concentrations in either group. There is evidence that well-trained athletes have a reduced sensitivity of 5-HT receptors. The present study suggests that this adaptation may be lost in athletes with UUPS: this might explain some of their observed symptoms.

PMID: 18487257 [PubMed - as supplied by publisher]

: Acute noradrenaline reuptake inhibition decreases performance in normal and high ambient temperature.

Roelands B, Goekint M, Heyman E, Piacentini MF, Watson P, Hasegawa H, Buyse L, Pauwels F, De Schutter G, Meeusen R.

Vrije Universiteit Brussel.

Introduction: Combined inhibition of DA/NA reuptake improves exercise performance and increases core temperature in the heat. A recent study demonstrated that this effect may primarily be related to increased DA activity. NA reuptake inhibition (NARI), however, has received little attention in humans, certainly in the heat, where central fatigue appears to be a main factor influencing performance. Therefore the present study examines the effect of NARI (Reboxetine) on exercise capacity, thermoregulation and hormonal response in normal and high temperature. Methods: Nine healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (PLA;2*8mg) or Reboxetine (REBOX;2*8mg). Subjects exercised in temperate (18 degrees C) or warm (30 degrees C) conditions and cycled for 60min at 55%Wmax,, immediately followed by a time trial (TT; pla18/rebox18; pla30/rebox30) to measure exercise performance. Results: Acute NARI decreased power output and consequently exercise performance in temperate (p=0.018) and warm (p=0.007) conditions. Resting heart rate was significantly elevated by NARI (18 degrees C:p=0.02;30 degrees C:p=0.018). In rebox18 heart rate was significantly higher than in the pla18, while in the heat no effect of the drug treatment was reported during exercise. In rebox30 all hormone concentrations increased during exercise, except for GH that was significantly lower during exercise. In rebox18 PRL concentrations were significantly elevated, GH was significantly higher at rest, but significantly lower during exercise. Conclusions: Manipulation of the NA system decreases performance and modifies hormone concentrations, thereby indicating a central NA effect of the drug. These findings confirm results from previous studies that predominantly increased DA activity is important in improving performance. Key words: central fatigue, reboxetine, exercise, heat.

PMID: 18499777 [PubMed - as supplied by publisher]

3: Sports Med. 2008;38(5):357-67.

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Muscle fatigue during football match-play.

Reilly T, Drust B, Clarke N.

Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, UK. t.p.reilly@ljmu.ac.uk

One of the consequences of sustaining exercise for 90 minutes of football match-play is that the capability of muscle to generate force declines. This impairment is reflected in the decline of work-rate towards the late part of the game. Causes of this phenomenon, which is known as fatigue, and some of its consequences are considered in this article. The stores of muscle glycogen may be considerably reduced by the end of the game, especially if there has not been a tapering of the training load. Thermoregulatory strain may also be encountered, resulting in a fall in physical performance, or there may be a reduced central drive from the nervous system. The decline in muscle strength may increase the predisposition to injury in the lower limbs. Central fatigue may also occur with implications for muscle performance. Strategies to offset fatigue include astute use of substitutions, appropriate nutritional preparation and balancing pre-cooling and warm-up procedures. There is also a role for endurance training and for a pacing strategy that optimizes the expenditure of energy during match-play.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 18416591 [PubMed - in process]

4: J Physiol. 2008 Jun 1;586(Pt 11):2799-805. Epub 2008 Apr 10.

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Interpolated twitches in fatiguing single mouse muscle fibres: implications for the assessment of central fatigue.

Place N, Yamada T, Bruton JD, Westerblad H.

Department of Physiology and Pharmacology, Karolinska Institutet, S-171 77 Stockholm, Sweden. nicolas.place@ki.se

An electrically evoked twitch during a maximal voluntary contraction (twitch interpolation) is frequently used to assess central fatigue. In this study we used intact single muscle fibres to determine if intramuscular mechanisms could affect the force increase with the twitch interpolation technique. Intact single fibres from flexor digitorum brevis of NMRI mice were dissected and mounted in a chamber equipped with a force transducer. Free myoplasmic [Ca2+] ([Ca2+](i)) was measured with the fluorescent Ca2+ indicator indo-1. Seven fibres were fatigued with repeated 70 Hz tetani until 40% initial force with an interpolated pulse evoked every fifth tetanus. Results showed that the force generated by the interpolated twitch increased throughout fatigue, being 9 +/- 1% of tetanic force at the start and 19 +/- 1% at the end (P < 0.001). This was not due to a larger increase in [Ca2+](i) induced by the interpolated twitch during fatigue but rather to the fact that the force-[Ca2+](i) relationship is sigmoidal and fibres entered a steeper part of the relationship during fatigue. In another set of experiments, we observed that repeated tetani evoked at 150 Hz resulted in more rapid fatigue development than at 70 Hz and there was a decrease in force ('sag') during contractions, which was not observed at 70 Hz. In conclusion, the extent of central fatigue is difficult to assess and it may be overestimated when using the twitch interpolation technique.

Publication Types:

Research Support, Non-U.S. Gov't

PMID: 18403421 [PubMed - in process]

5: Med Sci Sports Exerc. 2008 Mar;40(3):457-61.

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Respiratory system determinants of peripheral fatigue and endurance performance.

Dempsey JA, Amann M, Romer LM, Miller JD.

John Rankin Laboratory of Pulmonary Medicine, University of Wisconsin-Madison, Madison, WI 53706, USA. jdempsey@wisc.edu

We briefly summarize recent evidence pertaining to how mechanisms primarily under the control of the respiratory system-namely, arterial oxyhemoglobin desaturation, respiratory muscle work and fatigue, and cyclical fluctuations in intrathoracic pressure-may contribute to exercise limitation. Respiratory influences on cardiac output and on sympathetic vasoconstrictor activity and blood flow distribution are shown to be important determinants of performance. We also address how a compromised O2 transport exacerbates the rate of development of peripheral muscle fatigue and, in turn, precipitates central fatigue and exercise limitation.

Publication Types:

PMID: 18379207 [PubMed - indexed for MEDLINE]

: Effect of voluntary activation on age-related muscle fatigue resistance.

Mademli L, Arampatzis A.

Institute of Biomechanics and Orthopaedics, German Sport University Cologne, Carl-Diem-Weg 6, D-50933 Cologne, Germany.

Ageing is associated with a higher fatigue resistance during submaximal or maximal fatiguing contractions. The present study aimed to investigate the contribution of the central and peripheral fatigue to the age-related differences in fatigue development of the plantar flexor muscles. Therefore, the voluntary activation, rest twitch moment and voluntary plantar flexor moment were examined before during as well as 2, 5 and 10min after a fatiguing task. This consisted of intermittent isometric submaximal plantar flexor contractions at equal intensity for both young and old adults (considering the age-related differences in muscle inhibition). Consequently, possible differences between young and old adults in voluntary activation during the maximal contraction utilised for determining the intensity of the fatiguing task, which can influence fatigue development, have been taken into account. The plantar flexors moment was calculated using inverse dynamics and the voluntary activation was measured using the twitch interpolation technique. Changes in voluntary activation and rest twitch moment during the fatiguing task were used to assess central and peripheral fatigue, respectively. In both young and old adults, peripheral ( approximately 20%) as well as central fatigue ( approximately 9%) contributed to the time to task failure. Old adults demonstrated greater time to task failure than young ones, but similar voluntary activation behaviour during the fatiguing task. We concluded that, the age-related enhancement in fatigue resistance is not attributable to voluntary activation but is linked to mechanisms located within the working muscle.

PMID: 18342865 [PubMed - in process]

: Neuroplasticity of Dopamine Circuits After Exercise: Implications for Central Fatigue.

Foley TE, Fleshner M.

Department Integrative Physiology, Center for Neuroscience, Clare Small Building, University of Colorado-Boulder, Boulder, CO, 80309-0354, USA.

Habitual exercise increases plasticity in a variety of neurotransmitter systems. The current review focuses on the effects of habitual physical activity on monoamine dopamine (DA) neurotransmission and the potential implication of these changes to exercise-induced fatigue. Although it is clear that peripheral adaptations in muscle and energy substrate utilization contribute to this effect, more recently it has been suggested that central nervous system pathways "upstream" of the motor cortex, which initiate activation of skeletal muscles, are also important. The contribution of the brain to exercise-induced fatigue has been termed "central fatigue." Given the well-defined role of DA in the initiation of movement, it is likely that adaptations in DA systems influence exercise capacity. A reduction in DA neurotransmission in the substantia nigra pars compacta (SNpc), for example, could impair activation of the basal ganglia and reduce stimulation of the motor cortex leading to central fatigue. Here we present evidence that habitual wheel running produces changes in DA systems. Using in situ hybridization techniques, we report that 6 weeks of wheel running was sufficient to increase tyrosine hydroxylase mRNA expression and reduce D2 autoreceptor mRNA in the SNpc. Additionally, 6 weeks of wheel running increased D2 postsynaptic receptor mRNA in the caudate putamen, a major projection site of the SNpc. These results are consistent with prior data suggesting that habitually physically active animals may have an enhanced ability to increase DA synthesis and reduce D2 autoreceptor-mediated inhibition of DA neurons in the SNpc compared to sedentary animals. Furthermore, habitually physically active animals, compared to sedentary controls, may be better able to increase D2 receptor-mediated inhibition of the indirect pathway of the basal ganglia. Results from these studies are discussed in light of our understanding of the role of DA in the neurobiological mechanisms of central fatigue.

PMID: 18274707 [PubMed - as supplied by publisher]

8: Appl Physiol Nutr Metab. 2007 Oct;32(5):857-64.

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Brain neurotransmitters in fatigue and overtraining.

Meeusen R, Watson P, Hasegawa H, Roelands B, Piacentini MF.

Department of Human Physiology and Sports Medicine, Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, B-1050 Brussels, Belgium. rmeeusen@vub.ac.be

Since the publication of the serotonin hypothesis, numerous theories involving the accumulation or depletion of different substances in the brain have been proposed to explain central fatigue. Although the theoretical rationale for the "serotonin-fatigue hypothesis" is clear, several seemingly well-conducted studies have failed to support a significant role for 5-hydroxytryptamine in the development of fatigue. As brain function appears to be dependent upon the interaction of a number of systems, it is unlikely that a single neurotransmitter system is responsible for central fatigue. Several other mechanisms are involved, with evidence supporting a role for the brain catecholamines. Fatigue is therefore probably an integrated phenomenon, with complex interaction among central and peripheral factors. When prolonged and excessive training happens, concurrent with other stressors and insufficient recovery, performance decrements can result in chronic maladaptations that can lead to the overtraining syndrome (OTS). The mechanism of the OTS could be difficult to examine in detail, perhaps because the stress caused by excessive training load, in combination with other stressors, might trigger different "defence mechanisms" such as the immunological, neuroendocrine, and other physiological systems that all interact and probably therefore cannot be pinpointed as the "sole" cause of the OTS. It might be that, as in other syndromes, the psychoneuroimmunology (study of brain-behavior-immune interrelationships) might shed a light on the possible mechanisms of the OTS, but until there is a definite diagnostic tool, it is of utmost importance to standardize measures that are now thought to provide a good inventory of the training status of the athlete. It is very important to emphasize the need to distinguish the OTS from overreaching and other potential causes of temporary underperformance such as anemia, acute infection, muscle damage, and insufficient carbohydrate intake.

Publication Types:

PMID: 18059610 [PubMed - indexed for MEDLINE]

9: J Appl Physiol. 2008 Feb;104(2):542-50. Epub 2007 Nov 21.

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A comparison of central aspects of fatigue in submaximal and maximal voluntary contractions.

Taylor JL, Gandevia SC.

Prince of Wales Medical Research Institute, Barker St., Randwick, Sydney, New South Wales, Australia. jl.taylor@unsw.edu.au

Magnetic and electrical stimulation at different levels of the neuraxis show that supraspinal and spinal factors limit force production in maximal isometric efforts ("central fatigue"). In sustained maximal contractions, motoneurons become less responsive to synaptic input and descending drive becomes suboptimal. Exercise-induced activity in group III and IV muscle afferents acts supraspinally to limit motor cortical output but does not alter motor cortical responses to transcranial magnetic stimulation. "Central" and "peripheral" fatigue develop more slowly during submaximal exercise. In sustained submaximal contractions, central fatigue occurs in brief maximal efforts even with a weak ongoing contraction (<15% maximum). The presence of central fatigue when much of the available motor pathway is not engaged suggests that afferent inputs contribute to reduce voluntary activation. Small-diameter muscle afferents are likely to be activated by local activity even in sustained weak contractions. During such contractions, it is difficult to measure central fatigue, which is best demonstrated in maximal efforts. To show central fatigue in submaximal contractions, changes in motor unit firing and force output need to be characterized simultaneously. Increasing central drive recruits new motor units, but the way this occurs is likely to depend on properties of the motoneurons and the inputs they receive in the task. It is unclear whether such factors impair force production for a set level of descending drive and thus represent central fatigue. The best indication that central fatigue is important during submaximal tasks is the disproportionate increase in subjects' perceived effort when maintaining a low target force.

Publication Types:

PMID: 18032577 [PubMed - indexed for MEDLINE]

10: Clin Neurophysiol. 2008 Jan;119(1):2-10. Epub 2007 Nov 26.

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Clinical neurophysiology of fatigue.

Zwarts MJ, Bleijenberg G, van Engelen BG.

University Medical Centre Nijmegen, Institute of Neurology, 920 Department of Clinical Neurophysiology, PO Box 9101, 6500 HB Nijmegen, The Netherlands. M.Zwarts@neuro.umcn.nl

Fatigue is a multidimensional concept covering both physiological and psychological aspects. Chronic fatigue is a typical symptom of diseases such as cancer, multiple sclerosis (MS), Parkinson's disease (PD) and cerebrovascular disorders but is also presented by people in whom no defined somatic disease has been established. If certain criteria are met, chronic fatigue syndrome can be diagnosed. The 4-item Abbreviated Fatigue Questionnaire allows the extent of the experienced fatigue to be assessed with a high degree of reliability and validity. Physiological fatigue has been well defined and originates in both the peripheral and central nervous system. The condition can be assessed by combining force and surface-EMG measurements (including frequency analyses and muscle-fibre conduction estimations), twitch interpolation, magnetic stimulation of the motor cortex and analysis of changes in the readiness potential. Fatigue is a well-known phenomenon in both central and peripheral neurological disorders. Examples of the former conditions are multiple sclerosis, Parkinson's disease and stroke. Although it seems to be a universal symptom of many brain disorders, the unique characteristics of the concomitant fatigue also point to a specific relationship with several of these syndromes. As regards neuromuscular disorders, fatigue has been reported in patients with post-polio syndrome, myasthenia gravis, Guillain-Barré syndrome, facioscapulohumeral dystrophy, myotonic dystrophy and hereditary motor and sensory neuropathy type-I. More than 60% of all neuromuscular patients suffer from severe fatigue, a prevalence resembling that of patients with MS. Except for several rare myopathies with specific metabolic derangements leading to exercise-induced muscle fatigue, most studies have not identified a prominent peripheral cause for the fatigue in this population. In contrast, the central activation of the diseased neuromuscular system is generally found to be suboptimal. The reliability of the psychological and clinical neurophysiological assessment techniques available today allows a multidisciplinary approach to fatigue in neurological patients, which may contribute to the elucidation of the pathophysiological mechanisms of chronic fatigue, with the ultimate goal to develop tailored treatments for fatigue in neurological patients. The present report discusses the different manifestations of fatigue and the available tools to assess peripheral and central fatigue.

Publication Types:

Review

PMID: 18039594 [PubMed - indexed for MEDLINE]

11: J Appl Physiol. 2008 Jan;104(1):306-14. Epub 2007 Oct 25.

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Cerebral blood flow and metabolism during exercise: implications for fatigue.

Secher NH, Seifert T, Van Lieshout JJ.

Department of Anesthesia, The Copenhagen Muscle Research Center, Rigshospitalet, Denmark. nhsecher@rh.regionh.dk

During exercise: the Kety-Schmidt-determined cerebral blood flow (CBF) does not change because the jugular vein is collapsed in the upright position. In contrast, when CBF is evaluated by (133)Xe clearance, by flow in the internal carotid artery, or by flow velocity in basal cerebral arteries, a approximately 25% increase is detected with a parallel increase in metabolism. During activation, an increase in cerebral O(2) supply is required because there is no capillary recruitment within the brain and increased metabolism becomes dependent on an enhanced gradient for oxygen diffusion. During maximal whole body exercise, however, cerebral oxygenation decreases because of eventual arterial desaturation and marked hyperventilation-related hypocapnia of consequence for CBF. Reduced cerebral oxygenation affects recruitment of motor units, and supplemental O(2) enhances cerebral oxygenation and work capacity without effects on muscle oxygenation. Also, the work of breathing and the increasing temperature of the brain during exercise are of importance for the development of so-called central fatigue. During prolonged exercise, the perceived exertion is related to accumulation of ammonia in the brain, and data support the theory that glycogen depletion in astrocytes limits the ability of the brain to accelerate its metabolism during activation. The release of interleukin-6 from the brain when exercise is prolonged may represent a signaling pathway in matching the metabolic response of the brain. Preliminary data suggest a coupling between the circulatory and metabolic perturbations in the brain during strenuous exercise and the ability of the brain to access slow-twitch muscle fiber populations.

Publication Types:

PMID: 17962575 [PubMed - indexed for MEDLINE]

12: Clin Physiol Funct Imaging. 2007 Sep;27(5):298-304.

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Central fatigue of the first dorsal interosseous muscle during low-force and high-force sustained submaximal contractions.

Eichelberger TD, Bilodeau M.

Graduate Program in Physical Therapy and Rehabilitation Science, Carver College of Medicine, University of Iowa, IA, USA. martin.bilodeau@uottawa.ca

The aim of this study was to compare the extent of central fatigue in the first dorsal interosseous (FDI) muscle of healthy adults in low, moderate and high-force submaximal contractions. Nine healthy adults completed four experimental sessions where index finger abduction force was recorded during voluntary contractions and in response to brief trains (five pulses at 100 Hz) of electrical stimulation. The ability to maximally activate FDI under volition, or voluntary activation, and its change with sustained activity (central fatigue) was assessed using the twitch interpolation technique. The fatigue tasks consisted of continuous isometric index finger abduction contractions held until exhaustion at four target force levels: 30%, 45%, 60% and 75% of the maximal voluntary contraction. The main finding was the presence of central fatigue for the 30% task, but not for the three other fatigue tasks. The extent of central fatigue was also associated with changes in a measure reflecting the status of peripheral structures/mechanisms. It appears that central fatigue contributed to task failure for the lowest force fatigue task (30%), but not for the other (higher) contraction intensities.

Publication Types:

Comparative Study

PMID: 17697026 [PubMed - indexed for MEDLINE]

13: Muscle Nerve. 2007 Oct;36(4):515-24.

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Mechanisms of fatigue differ after low- and high-force fatiguing contractions in men and women.

Yoon T, Schlinder Delap B, Griffith EE, Hunter SK.

Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, WI 53201, USA.

The magnitude of failure in voluntary drive after fatiguing contractions of different intensities in men and women is not known. The purpose of this study was to compare the time to task failure and voluntary activation of men and women for a sustained isometric contraction performed at a low and high intensity with the elbow flexor muscles. Nine men and nine women sustained an isometric contraction at 20% and 80% of maximal voluntary contraction (MVC) force until task failure during separate sessions. The men had a shorter time to failure than women for the 20% but not the 80% MVC task. Voluntary activation was reduced to similar levels for the men and women at the end of the fatiguing contractions but was reduced less after the 80% MVC task than the 20% MVC contraction. Twitch amplitude was reduced similarly at task failure for both sexes and to similar levels at termination of the 20% and 80% MVC tasks. The rate of change in mean arterial pressure was the main predictor of time to failure for the low-force sustained contraction. These results suggest that women experienced greater muscle perfusion, less peripheral fatigue, and a longer time to task failure than men during the low-force fatiguing contraction. However, the low-force task induced greater central fatigue than the high-force contraction for both men and women. Thus, low-force, long-duration fatiguing contractions can be used in rehabilitation to induce significant fatigue within the central nervous system and potentially greater neural adaptations in men and women.

Publication Types:

PMID: 17626289 [PubMed - indexed for MEDLINE]

14: Exerc Sport Sci Rev. 2007 Jul;35(3):110-8.

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Inadequate cerebral oxygen delivery and central fatigue during strenuous exercise.

Nybo L, Rasmussen P.

Institute of Exercise and Sport Sciences and Department of Anaesthesia, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark. lnnielsen@aki.ku.dk

Under resting conditions, the brain is protected against hypoxia because cerebral blood flow increases when the arterial oxygen tension becomes low. However, during strenuous exercise, hyperventilation lowers the arterial carbon dioxide tension and blunts the increase in cerebral blood flow, which can lead to an inadequate oxygen delivery to the brain and contribute to the development of fatigue.

Publication Types:

Review

PMID: 17620929 [PubMed - indexed for MEDLINE]

: Neuromuscular fatigue during sustained contractions performed in short-term hypoxia.

Szubski C, Burtscher M, Löscher WN.

1Department of Sports Science, University of Innsbruck, Innsbruck, Austria.

PURPOSE: Hypoxia is known to change neuronal activity in vitro and to impair performance in vivo. The present study was designed to study neuromuscular fatigue in acute hypoxia, and we hypothesized that hypoxia results in additional fatigue during sustained contractions, presumably because of increased central fatigue. METHODS: Twelve healthy subjects participated in a normoxic (NX) and hypoxic (HX) experiment performed on separate days. Hypoxia was induced by breathing an HX air mixture containing 12% oxygen. Before, during, and after a 90-s sustained voluntary maximal contraction (MVC) of the first dorsal interosseus muscle, we measured force, voluntary activation (VA), and parameters of motor cortical excitability (motor-evoked potentials (MEP) and silent periods (SP)). Measures of peripheral nerve and muscle function, compound motor action potential (M-wave), and muscle twitch forces were also taken. RESULTS: During the MVC, force declined similarly during both HX and NX. VA decreased throughout the contraction in HX, but, surprisingly, this decrease in VA in HX did not exceed that observed in NX. Also, motor cortical excitability changed to a similar degree in HX and NX; that is, MEP amplitude and SP duration increased. M-wave amplitude decreased significantly during the sustained MVC in NX and HX. The only difference observed between NX and HX was the quicker recovery of the muscle twitch in HX, which was even potentiated after 5 min of recovery. CONCLUSION: The present results show that peripheral and central neuromuscular adaptations during a sustained fatiguing contraction are similar in NX and HX. The quicker recovery and potentiation of twitch forces in HX suggest alterations in myosin phosphorylation, which may enhance contractile force.

PMID: 17545884 [PubMed - indexed for MEDLINE]

: Children are more susceptible to central fatigue than adults.

Streckis V, Skurvydas A, Ratkevicius A.

Laboratory of Human Motorics, Department of Physiology and Biochemistry, Lithuanian Academy of Physical Education, Kaunas, Lithuania.

Performance in high-intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19-27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12-14 years). The volunteers (n = 7 in each group) performed three 5-s maximal voluntary contractions (MVCs) and a continuous 2-min MVC. The VA and fatigue of the muscles was assessed by applying 250-ms 100-HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2-min MVC, VA in boys and girls was more depressed than in adults. The end-exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are more susceptible to central fatigue than adults. This should be taken into account when evaluating results of fitness tests that require high levels of motor unit activation.

PMID: 17487867 [PubMed - indexed for MEDLINE]

17: J Neurosci Res. 2007 Nov 15;85(15):3334-9.

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The brain at work: a cerebral metabolic manifestation of central fatigue?

Dalsgaard MK, Secher NH.

Department of Anaesthesia, The Copenhagen Muscle Research Centre, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark. madskd@gmail.com

Central fatigue refers to circumstances in which strength appears to be limited by the ability of the central nervous system to recruit motoneurons. Central fatigue manifests when the effort to contract skeletal muscles is intense and, thus, is aggravated when exercise is performed under stress, whereas it becomes attenuated following training. Central fatigue has not been explained, but the cerebral metabolic response to intense exercise, as to other modalities of cerebral activation, is a reduction in its "metabolic ratio" (MR), i.e., the brain's uptake of oxygen relative to that of carbohydrate. At rest the MR is close to 6 but during intense whole-body exercise it decreases to less than 3, with the uptake of lactate becoming as important as that of glucose. It remains debated what underlies this apparent inability of the brain to oxidize the carbohydrate taken up, but it may approach approximately 10 mmol glucose equivalents. In the case of exercise, a concomitant uptake of ammonium for formation of amino acids may account for only approximately 10% of this "extra" carbohydrate taken up. Also, accumulation of intermediates in metabolic pathways and compartmentalization of metabolism between astrocytes and neurons are avenues that have to be explored. Depletion of glycogen stores and subsequent supercompensation during periods of low neuronal activity may not only play a role but also link brain metabolism to its function.

Publication Types:

Review

PMID: 17394258 [PubMed - indexed for MEDLINE]

18: Pflugers Arch. 2007 Sep;454(6):957-69. Epub 2007 Mar 7.

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Central fatigue explains sex differences in muscle fatigue and contralateral cross-over effects of maximal contractions.

Martin PG, Rattey J.

Exercise and Sports Science Laboratories, School of Human Movement Studies, Charles Sturt University, Bathurst, Australia. peter.martin@unsw.edu.au

A sustained voluntary contraction increases central fatigue and produces a 'cross-over' of fatigue during a subsequent contraction of the contralateral limb. These studies compared the magnitude of these changes for men and women. Force and electromyographic responses from dominant (study 1; n = 8 men, 8 women) or non-dominant (study 2; n = 7 men, 8 women) leg extensors to nerve stimulation were recorded at rest and during brief maximal voluntary contractions (MVCs), before and after 100-s sustained MVCs performed with the dominant leg. For the dominant leg, force was reduced more for men (by approximately 24%) than women (by approximately 16%, P < 0.05) after the sustained contractions. Similarly, voluntary activation during these contractions was reduced more for men (by approximately 22%) than women (by approximately 9%, P < 0.05). Conversely, resting twitches changed similarly for both sexes (P > 0.05). For the non-dominant leg, men experienced a reduction in force (by approximately 13%, P < 0.001) and had greater deficits in activation than women ( approximately 9% vs approximately 3%, P < 0.05), after sustained contractions of the dominant leg. Therefore, sustained MVCs produce greater central fatigue and a more pronounced 'cross-over' of effects to the contralateral limb for men compared to women. These findings demonstrate distinct differences between sexes in the way the nervous system adapts to changes associated with fatigue.

Publication Types:

Comparative Study

PMID: 17342531 [PubMed - indexed for MEDLINE]