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In addition to the phosphagen reactions (CK and AK) and the oxidative mitochondrial respiration there is a third ATP generating system - glycolysis.
Glycolysis produces ATP from glucose (a carbohydrate). The significant waste products are pyruvate and protons.
Glycolysis of glucose to pyruvate
Both pyruvate and the protons can be consumed by the mitochondria (producing more ATP) or converted into lactate. If the protons are not used by either these processes then they accumulate and contribute to acidosis and fatigue. This will happen during intense activity as the consumption of waste products cannot keep up with production.
Lactate Dehydrogenase reaction (LDH)
It should be noted that the LDH reaction consumes protons and therefore the production of lactate is a buffer to acidosis. This is in contrast to the view expressed in many exercise biochemistry/physiology information sources where they erroneously assume lactic acid is a byproduct of glycolysis and contributes to acidosis. The opposite is the truth. Lactate is transported out of the cell into the blood and is associated with rising acid levels. This is because it is a product of an acid buffering mechanism rather than a cause of the acid. Lactate production is positive in terms of reducing acidosis and measuring it will underestimate the amount of acid being produced by ATP hydrolysis and glycolysis (because lactate only buffers some of the protons).
The lactic acid myth runs through a great amount of sports science and therefore undermines a lot of work being done in the subject.
Glycolysis is a fast process and therefore has a high power (ATP per second). However, it has a limited capacity due to limited storage of glucose.
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