Fast-twitch muscle fibers rapidly produce lactic acid.
Lactic acid is mainly produced in muscle cells and red blood cells. It forms when the body breaks down carbohydrates to use for energy when oxygen levels are low.
Lactic acid is formed within the muscle cells during glycolysis to clear the cells of accumulating pyruvate, a by-product of glycolysis. While still in the muscle cells, the hydrogen ion is what is responsible for lowering the pH of the muscle tissue, making it more acidic.
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