Fatigue

==== Physical ====
Physical fatigue, or [[muscle fatigue]], is the temporary physical inability of muscles to perform optimally. The onset of muscle fatigue during physical activity is gradual, and depends upon an individual's level of physical fitness – other factors include [[sleep deprivation]] and overall health.<ref name="WebMD">{{cite news|title=Weakness and fatigue|newspaper=Webmd|url=http://www.webmd.com/a-to-z-guides/weakness-and-fatigue-topic-overview|publisher=Healthwise Inc.|access-date=2 January 2013|archive-date=30 December 2012|archive-url=https://web.archive.org/web/20121230140117/http://www.webmd.com/a-to-z-guides/weakness-and-fatigue-topic-overview|url-status=live}}</ref> Physical fatigue can be caused by a lack of energy in the muscle, by a decrease of the efficiency of the [[neuromuscular junction]] or by a reduction of the drive originating from the [[central nervous system]], and can be reversed by rest.<ref>{{cite journal | vauthors = Gandevia SC | title = Spinal and supraspinal factors in human muscle fatigue | journal = Physiological Reviews | volume = 81 | issue = 4 | pages = 1725–1789 | date = October 2001 | pmid = 11581501 | doi = 10.1152/physrev.2001.81.4.1725 }}</ref> The central component of fatigue is triggered by an increase of the level of [[serotonin]] in the central nervous system.<ref name="pmid10919962">{{cite journal | vauthors = Davis JM, Alderson NL, Welsh RS | title = Serotonin and central nervous system fatigue: nutritional considerations | journal = The American Journal of Clinical Nutrition | volume = 72 | issue = 2 Suppl | pages = 573S–578S | date = August 2000 | pmid = 10919962 | doi = 10.1093/ajcn/72.2.573S | doi-access = free }}</ref> During motor activity, serotonin released in synapses that contact [[motor neuron]]s promotes [[muscle contraction]].<ref>{{cite journal | vauthors = Perrier JF, Delgado-Lezama R | title = Synaptic release of serotonin induced by stimulation of the raphe nucleus promotes plateau potentials in spinal motoneurons of the adult turtle | journal = The Journal of Neuroscience | volume = 25 | issue = 35 | pages = 7993–7999 | date = August 2005 | pmid = 16135756 | pmc = 6725458 | doi = 10.1523/JNEUROSCI.1957-05.2005 }}</ref> During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extrasynaptic receptors located on the [[axonal initial segment]] of motor neurons with the result that [[Action potential|nerve impulse]] initiation and thereby muscle contraction are inhibited.<ref>{{cite journal | vauthors = Cotel F, Exley R, Cragg SJ, Perrier JF | title = Serotonin spillover onto the axon initial segment of motoneurons induces central fatigue by inhibiting action potential initiation | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 12 | pages = 4774–4779 | date = March 2013 | pmid = 23487756 | pmc = 3607056 | doi = 10.1073/pnas.1216150110 | doi-access = free | bibcode = 2013PNAS..110.4774C }}</ref>

Muscle strength testing can be used to determine the presence of a [[neuromuscular disease]], but cannot determine its cause. Additional testing, such as [[electromyography]], can provide diagnostic information, but information gained from muscle strength testing alone is not enough to diagnose most neuromuscular disorders.<ref>{{cite journal | vauthors = Enoka RM, Duchateau J | title = Muscle fatigue: what, why and how it influences muscle function | journal = The Journal of Physiology | volume = 586 | issue = 1 | pages = 11–23 | date = January 2008 | pmid = 17702815 | pmc = 2375565 | doi = 10.1113/jphysiol.2007.139477 | author-link = Roger M. Enoka }}</ref>