The breakdown of glucose in your cells is divided into two different phases, the first of which is called glycolysis. One of the products of glycolysis is a molecule called pyruvate, which would ordinarily undergo further oxidation in the citric acid cycle. When oxygen is in short supply, however, your cells use up pyruvate through lactic acid fermentation. This process is critical to continuing glycolysis, but it also has some disadvantages.
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During short bursts of activity like a sprint, your skeletal muscle fibres run out of the oxygen they need to continue aerobic respiration. Glycolysis reduces NAD+ to NADH, and if your muscle fibres don't oxidise the NADH back to NAD+, they'll run out of NAD+ for glycolysis and be unable to break down any more glucose for energy. To replenish their supply of NAD+, they reduce pyruvate to lactic acid, oxidising NADH to NAD+ in the process.
Glycolysis followed by lactic acid fermentation only extracts a fraction of the energy stored in each glucose molecule, producing a mere four ATP per glucose, compared with more than 30 per glucose for aerobic respiration. Cells relying on lactic acid fermentation need to consume more glucose to get the same amount of energy as cells using aerobic respiration. Fermentation also expends the energy stored by reduction of NADH on reduction of pyruvate, which is not useful to your cells.
The lactic acid generated by fermentation can be recycled by your liver, but this takes time. While you are running, lactic acid accumulates and reaches very high concentrations in extracellular fluid. This build-up creates the burning sensation you feel in very active muscles during a fast sprint or similar activity. It also hinders the breakdown of glucose, making it more difficult for your muscle fibres to sustain further exertion. Even well-conditioned athletes can only sprint for so long before they have to slow down or rest.
As your muscle cells burn glucose, they have to dig further into their store of glycogen, a polymer of glucose molecules your cells use to store glucose. Since the lactic acid fermentation process is inefficient, cells consume glucose rapidly, depleting their accumulated supply. Together with lactic acid build-up, these effects mean that your body has a very limited capacity for rapid and intense exertion, much more so than that of some other animals such as birds.
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