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Exercise physiology
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== Fatigue == === Intense activity === Researchers once attributed fatigue to a build-up of lactic acid in muscles.<ref>{{Cite book |last=Hermansen |first=L |title=Ciba Foundation Symposium 82 - Human Muscle Fatigue: Physiological Mechanisms |work=Ciba Foundation Symposium |year=1981 |isbn=9780470715420 |series=Novartis Foundation Symposia |volume=82 |pages=75β88 |chapter=Effect of Metabolic Changes on Force Generation in Skeletal Muscle During Maximal Exercise |doi=10.1002/9780470715420.ch5 |pmid=6913479}}</ref> However, this is no longer believed.<ref>{{Cite journal |last=Brooks |first=GA |year=2001 |title=Lactate doesn't necessarily cause fatigue: Why are we surprised? |journal=The Journal of Physiology |volume=536 |issue=Pt 1 |pages=1 |doi=10.1111/j.1469-7793.2001.t01-1-00001.x |pmc=2278833 |pmid=11579151}}</ref><ref>{{Cite journal |last=Gladden |first=LB |year=2004 |title=Lactate metabolism: A new paradigm for the third millennium |journal=The Journal of Physiology |volume=558 |issue=Pt 1 |pages=5β30 |doi=10.1113/jphysiol.2003.058701 |pmc=1664920 |pmid=15131240}}</ref> Rather, lactate may stop muscle fatigue by keeping muscles fully responding to nerve signals.<ref>{{Cite journal |vauthors=Pedersen TH, Nielsen OB, Lamb GD, Stephenson DG |year=2004 |title=Intracellular acidosis enhances the excitability of working muscle |journal=Science |volume=305 |issue=5687 |pages=1144β7 |bibcode=2004Sci...305.1144P |doi=10.1126/science.1101141 |pmid=15326352 |s2cid=24228666}}</ref> The available oxygen and energy supply, and disturbances of muscle ion homeostasis are the main factors determining exercise performance, at least during brief very intense exercise.{{cn|date=April 2025}} Each [[muscle contraction#Skeletal muscle|muscle contraction]] involves an [[action potential]] that activates voltage sensors, and so releases [[Voltage-dependent calcium channel|Ca<sup>2+</sup> ions]] from the [[muscle fibre]]'s [[Sarcoplasmic reticulum#Sarcoplasmic reticulum|sarcoplasmic reticulum]]. The action potentials that cause this also require ion changes: [[Sodium channel#Voltage-gated|Na influxes]] during the [[depolarization]] phase and K effluxes for the [[repolarization]] phase. [[Chloride channel|Cl<sup>β</sup> ions]] also diffuse into the sarcoplasm to aid the repolarization phase. During intense muscle contraction, the ion pumps that maintain homeostasis of these ions are inactivated and this (with other ion related disruption) causes ionic disturbances. This causes cellular membrane depolarization, inexcitability, and so muscle weakness.<ref>{{Cite journal |last=McKenna |first=MJ |last2=Bangsbo |first2=J |last3=Renaud |first3=JM |year=2008 |title=Muscle K<sup>+</sup>, Na<sup>+</sup>, and Cl disturbances and Na<sup>+</sup>-K<sup>+</sup> pump inactivation: Implications for fatigue |journal=Journal of Applied Physiology |volume=104 |issue=1 |pages=288β95 |doi=10.1152/japplphysiol.01037.2007 |pmid=17962569 |s2cid=25190764}}</ref> Ca<sup>2+</sup> leakage from type 1 [[ryanodine receptor]]) channels has also been identified with fatigue.<ref>{{Cite journal |last=Bellinger |first=AM |last2=Reiken |first2=S |last3=Dura |first3=M |last4=Murphy |first4=PW |last5=Deng |first5=SX |last6=Landry |first6=DW |last7=Nieman |first7=D |last8=Lehnart |first8=SE |last9=Samaru |first9=M |last10=Lacampagne |first10=A. |last11=Marks |first11=A. R. |display-authors=8 |year=2008 |title=Remodeling of ryanodine receptor complex causes "leaky" channels: A molecular mechanism for decreased exercise capacity |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=105 |issue=6 |pages=2198β202 |bibcode=2008PNAS..105.2198B |doi=10.1073/pnas.0711074105 |pmc=2538898 |pmid=18268335 |doi-access=free}}</ref> [[File:Dorando Pietri 1908.jpg|right|thumb|300px|[[Dorando Pietri]] about to collapse at the Marathon finish at the 1908 London Olympic Games]] === Endurance failure === After intense prolonged exercise, there can be a collapse in body [[Human homeostasis|homeostasis]]. Some famous examples include: * [[Dorando Pietri]] in the [[1908 Summer Olympics|1908 Summer Olympic]] [[Athletics at the 1908 Summer Olympics β Men's marathon|men's marathon]] ran the wrong way and collapsed several times.{{cn|date=April 2025}} * [[James Peters (athlete)|Jim Peters]] in the marathon of the [[1954 Commonwealth Games]] staggered and collapsed several times, and though he had a five-kilometre (three-mile) lead, failed to finish. Though it was formerly believed that this was due to severe dehydration, more recent research suggests it was the combined effects upon the brain of hyperthermia, hypertonic hypernatraemia associated with dehydration, and possibly hypoglycaemia.<ref>{{Cite journal |last=Noakes |first=T |last2=Mekler |first2=J |last3=Pedoe |first3=DT |year=2008 |title=Jim Peters' collapse in the 1954 Vancouver Empire Games marathon |journal=South African Medical Journal |volume=98 |issue=8 |pages=596β600 |pmid=18928034}}</ref> * [[Gabriela Andersen-Schiess]] in the woman's marathon at the Los Angeles [[1984 Summer Olympics]] in the race's final 400 meters, stopping occasionally and shown signs of [[heat exhaustion]]. Though she fell across the finish line, she was released from medical care only two hours later.{{cn|date=April 2025}} === Central governor === [[Tim Noakes]], based on an earlier idea by the 1922 [[Nobel Prize in Physiology or Medicine]] winner [[Archibald Hill]]<ref>{{Cite journal |last=Hill A. V. |last2=Long C. N. H. |last3=Lupton H. |year=1924 |title=Muscular exercise, lactic acid and the supply and utilisation of oxygen. Parts IβIII |journal=Proc. R. Soc. Lond. |volume=97 |issue=679 |pages=438β475 |doi=10.1098/rspb.1924.0037 |doi-access=free}}</ref> has proposed the existence of a [[central governor]]. In this, the brain continuously adjusts the power output by muscles during exercise in regard to a safe level of exertion. These neural calculations factor in prior length of strenuous exercise, the planned duration of further exertion, and the present metabolic state of the body. This adjusts the number of activated skeletal muscle motor units, and is subjectively experienced as [[Muscle weakness|fatigue]] and exhaustion. The idea of a central governor rejects the earlier idea that fatigue is only caused by mechanical failure of the exercising muscles ("[[Muscle weakness#Peripheral|peripheral fatigue]]"). Instead, the brain models<ref>{{Cite journal |last=St Clair Gibson |first=A |last2=Baden |first2=DA |last3=Lambert |first3=MI |last4=Lambert |first4=EV |last5=Harley |first5=YX |last6=Hampson |first6=D |last7=Russell |first7=VA |last8=Noakes |first8=TD |year=2003 |title=The conscious perception of the sensation of fatigue |journal=Sports Medicine |volume=33 |issue=3 |pages=167β76 |doi=10.2165/00007256-200333030-00001 |pmid=12656638 |s2cid=34014572}}</ref> the metabolic limits of the body to ensure that whole body homeostasis is protected, in particular that the heart is guarded from hypoxia, and an emergency reserve is always maintained.<ref>{{Cite journal |last=Noakes |first=TD |last2=St Clair Gibson |first2=A |last3=Lambert |first3=EV |year=2005 |title=From catastrophe to complexity: A novel model of integrative central neural regulation of effort and fatigue during exercise in humans: Summary and conclusions |journal=British Journal of Sports Medicine |volume=39 |issue=2 |pages=120β4 |doi=10.1136/bjsm.2003.010330 |pmc=1725112 |pmid=15665213}}</ref><ref>{{Cite journal |last=Noakes |first=TD |last2=Peltonen |first2=JE |last3=Rusko |first3=HK |year=2001 |title=Evidence that a central governor regulates exercise performance during acute hypoxia and hyperoxia |journal=The Journal of Experimental Biology |volume=204 |issue=Pt 18 |pages=3225β34 |doi=10.1242/jeb.204.18.3225 |pmid=11581338}}</ref><ref>{{Cite journal |last=Noakes |first=TD |year=2000 |title=Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance |journal=Scandinavian Journal of Medicine & Science in Sports |volume=10 |issue=3 |pages=123β45 |doi=10.1034/j.1600-0838.2000.010003123.x |pmid=10843507 |s2cid=23103331}}</ref><ref>{{Cite journal |last=St Clair Gibson |first=A |last2=Lambert |first2=ML |last3=Noakes |first3=TD |year=2001 |title=Neural control of force output during maximal and submaximal exercise |journal=Sports Medicine |volume=31 |issue=9 |pages=637β50 |doi=10.2165/00007256-200131090-00001 |pmid=11508520 |s2cid=1111940}}</ref> The idea of the central governor has been questioned since 'physiological catastrophes' can and do occur suggesting that if it did exist, athletes (such as [[Dorando Pietri]], [[James Peters (athlete)|Jim Peters]] and [[Gabriela Andersen-Schiess]]) can override it.<ref>{{Cite journal |last=Esteve-Lanao |first=J |last2=Lucia |first2=A |last3=Dekoning |first3=JJ |last4=Foster |first4=C |year=2008 |editor-last=Earnest |editor-first=Conrad P. |title=How do humans control physiological strain during strenuous endurance exercise? |journal=PLOS ONE |volume=3 |issue=8 |pages=e2943 |bibcode=2008PLoSO...3.2943E |doi=10.1371/journal.pone.0002943 |pmc=2491903 |pmid=18698405 |doi-access=free}}</ref> === Other factors === Exercise fatigue has also been suggested to be affected by: * brain hyperthermia<ref>{{Cite journal |last=Nybo |first=L |year=2008 |title=Hyperthermia and fatigue |journal=Journal of Applied Physiology |volume=104 |issue=3 |pages=871β8 |doi=10.1152/japplphysiol.00910.2007 |pmid=17962572}}</ref> * [[glycogen]] depletion in brain cells<ref name="Dalsgaard, M. K. 2006" /><ref>{{Cite journal |last=Dalsgaard |first=MK |last2=Secher |first2=NH |year=2007 |title=The brain at work: A cerebral metabolic manifestation of central fatigue? |journal=Journal of Neuroscience Research |volume=85 |issue=15 |pages=3334β9 |doi=10.1002/jnr.21274 |pmid=17394258 |s2cid=23623274 |doi-access=free}}</ref> * depletion of muscle and liver glycogen ''(see "[[hitting the wall]]")''<ref>{{Cite journal |last=Smyth |first=Barry |date=2021-05-19 |title=How recreational marathon runners hit the wall: A large-scale data analysis of late-race pacing collapse in the marathon |journal=PLOS ONE |language=en |volume=16 |issue=5 |pages=e0251513 |bibcode=2021PLoSO..1651513S |doi=10.1371/journal.pone.0251513 |issn=1932-6203 |pmc=8133477 |pmid=34010308 |doi-access=free}}</ref> * [[reactive oxygen species]] impairing skeletal muscle function<ref>{{Cite journal |last=Ferreira |first=LF |last2=Reid |first2=MB |year=2008 |title=Muscle-derived ROS and thiol regulation in muscle fatigue |journal=Journal of Applied Physiology |volume=104 |issue=3 |pages=853β60 |doi=10.1152/japplphysiol.00953.2007 |pmid=18006866}}</ref> * reduced level of [[glutamate]] secondary to uptake of ammonia in the brain<ref name="Nybo, L. 2005" /> * Fatigue in [[Muscles of respiration|diaphragm and abdominal respiratory muscles]] limiting breathing<ref>{{Cite journal |last=Romer |first=LM |last2=Polkey |first2=MI |year=2008 |title=Exercise-induced respiratory muscle fatigue: Implications for performance |url=http://bura.brunel.ac.uk/handle/2438/10000 |journal=Journal of Applied Physiology |volume=104 |issue=3 |pages=879β88 |doi=10.1152/japplphysiol.01157.2007 |pmid=18096752}}</ref> * Impaired oxygen supply to muscles<ref>{{Cite journal |last=Amann |first=M |last2=Calbet |first2=JA |year=2008 |title=Convective oxygen transport and fatigue |url=http://cris.ulpgc.es/jspui/bitstream/10553/6567/5/Convective_oxygen_transport.pdf |journal=Journal of Applied Physiology |volume=104 |issue=3 |pages=861β70 |doi=10.1152/japplphysiol.01008.2007 |pmid=17962570 |s2cid=22648694 |hdl-access=free |hdl=10553/6567}}</ref> * Ammonia effects upon the brain<ref name="Nybo, L. 2005" /> * [[Serotonin]] pathways in the brain<ref>{{Cite book |last=Newsholme |first=EA |title=Fatigue |last2=Blomstrand |first2=E |year=1995 |isbn=978-1-4899-1018-9 |series=Advances in Experimental Medicine and Biology |volume=384 |pages=315β20 |chapter=Tryptophan, 5-Hydroxytryptamine and a Possible Explanation for Central Fatigue |doi=10.1007/978-1-4899-1016-5_25 |pmid=8585461}}</ref>
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