Editing Weakness (section)

==Pathophysiology==
{{See also|Muscle contraction}}
Muscle cells work by detecting a [[Action potential|flow]] of electrical impulses from the [[brain]], which signals them to [[Muscle contraction|contract]] through the release of [[calcium]] by the [[sarcoplasmic reticulum]]. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This makes less calcium available for the muscle cell. In addition, the Columbia researchers propose that an enzyme activated by this released calcium eats away at muscle fibers.<ref>{{cite news| url=https://www.nytimes.com/2008/02/12/health/research/12musc.html | work=The New York Times | title=Finding May Solve Riddle of Fatigue in Muscles | first=Gina | last=Kolata | date=February 12, 2008}}</ref>

[[Substrate (biochemistry)|Substrates]] within the muscle generally serve to power muscular contractions. They include molecules such as [[adenosine triphosphate]] (ATP), [[glycogen]] and [[Phosphocreatine|creatine phosphate]]. ATP binds to the [[myosin]] head and causes the 'ratchetting' that results in contraction according to the [[Sliding filament mechanism|sliding filament model]]. Creatine phosphate stores energy so ATP can be rapidly regenerated within the muscle cells from [[adenosine diphosphate]] (ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5–7 seconds. Glycogen is the intramuscular storage form of [[glucose]], used to generate energy quickly once intramuscular creatine stores are exhausted, producing [[lactic acid]] as a metabolic byproduct. Contrary to common belief, lactic acid accumulation doesn't actually cause the burning sensation felt when people exhaust their oxygen and oxidative metabolism, but in actuality, lactic acid in presence of oxygen recycles to produce [[pyruvate]] in the liver, which is known as the [[Cori cycle]].{{citation needed|date=August 2021}}

Substrates produce metabolic fatigue by being depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions. In essence, the muscle stops contracting because it lacks the energy to do so.{{citation needed|date=August 2021}}