As the name suggests, anaerobic respiration is done without oxygen. When we exercise, we use more oxygen than normal, and we can't breathe in enough to keep up with the necessary reactions taking place in our bodies. The body then starts using anaerobic respiration, more commonly known as Lactic Acid Fermentation, to make energy. A biproduct of this reaction is Lactic acid, which is why your muscles burn when you exercise.
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