Lactic acid fermentation occurs in muscle cells when levels are low. When builds up in muscles it causes pain and muscle fatigue. Usually, the lactic acid diffuses into the bloodstream over time and is filtered out by the and converted into glycogen and then to glucose. However, with proper training, lactic acid can transfer to the of muscle cells, where it is broken apart for its energy. HighLighter PopupUnderline Selected TextGreen Color HighlightOrange Color HighlightYellow Color HighlightBlue Color HighlightPink Color HighlightRemove Color Highlight