Editing Exercise physiology (section)

=== Rapid energy sources ===
Energy needed to perform short lasting, high intensity bursts of activity is derived from [[Bioenergetic systems#Anaerobic metabolism|anaerobic metabolism]] within the [[cytosol]] of muscle cells, as opposed to [[aerobic respiration]] which utilizes oxygen, is sustainable, and occurs in the [[mitochondria]]. The quick energy sources consist of the [[phosphocreatine]] (PCr) system, fast [[glycolysis]], and [[adenylate kinase]]. All of these systems re-synthesize [[adenosine triphosphate]] (ATP), which is the universal energy source in all cells. The most rapid source, but the most readily depleted of the above sources is the PCr system which utilizes the enzyme [[creatine kinase]]. This enzyme catalyzes a reaction that combines [[phosphocreatine]] and adenosine diphosphate (ADP) into ATP and [[creatine]]. This resource is short lasting because oxygen is required for the resynthesis of phosphocreatine via mitochondrial creatine kinase. Therefore, under anaerobic conditions, this substrate is finite and only lasts between approximately 10 to 30 seconds of high intensity work. Fast glycolysis, however, can function for approximately 2 minutes prior to fatigue, and predominantly uses intracellular glycogen as a substrate. Glycogen is broken down rapidly via [[glycogen phosphorylase]] into individual glucose units during intense exercise. Glucose is then oxidized to pyruvate and under anaerobic conditions is reduced to lactic acid. This reaction oxidizes NADH to NAD, thereby releasing a hydrogen ion, promoting acidosis. For this reason, fast glycolysis can not be sustained for long periods of time.{{cn|date=April 2025}}