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Generally, lactic acid fermentation occurs only when aerobic cells are lacking oxygen. However, some aerobic mammalian cells will preferentially use lactic acid fermentation over aerobic respiration. This phenomenon is called the Warburg effect and is found primarily in cancer cells. Muscles cells under great exertion will also use lactic acid fermentation to supplement aerobic respiration. Lactic acid fermentation is somewhat faster, although less efficient, than aerobic respiration, so in activities like sprinting it can help quickly provide needed energy to muscles.