Excess post-exercise oxygen consumption (EPOC, informally called afterburn) is a measurably increased rate of oxygen intake following strenuous activity intended to erase the body's "oxygen deficit." In historical context the term "oxygen debt" was popularized to explain or perhaps attempt to quantify anaerobic energy expenditure, particularly as regards lactic acid/lactate metabolism; in fact, the term "oxygen debt" is still widely used to this day. However, direct and indirect calorimeter experiments have definitively disproven any association of lactate metabolism as causal to an elevated oxygen uptake.
In recovery, oxygen (EPOC) is used in the processes that restore the body to a resting state and adapt it to the exercise just performed. These include: hormone balancing, replenishment of fuel stores, cellular repair, innervation and anabolism. Post-exercise oxygen consumption replenishes the phosphagen system. New ATP is synthesized and some of this ATP donates phosphate groups to creatine until ATP and creatine levels are back to resting state levels again. Post-exercise oxygen is also used to oxidize lactic acid. Lactic acid is produced during exercise and then travels via the blood stream to the kidneys, cardiac muscle, and liver. An increased amount of oxygen is necessary to convert the lactic acid back to pyruvic acid at these locations. Another use of EPOC is to fuel the body’s increased metabolism from the increase in body temperature which occurs during exercise.
EPOC is accompanied by an elevated consumption of fuel. In response to exercise, fat stores are broken down and free fatty acids (FFA) are released into the blood. In recovery, the direct oxidation of free fatty acids as fuel and the energy consuming re-conversion of FFAs back into fat stores both take place.
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