Lactic acid
During the 1990s, the lactic acid hypothesis was created to explain why people experienced burning or muscle cramps that occurred during and after intense exercise. A lack of oxygen inside of the muscle cells resulted in lactic acid fermentation. This is due to the cell needing oxygen as a terminal electron acceptor to produce ATP. Without oxygen present, the cells needed to create energy through a different method. Lactic acid, or lactate and H+ were created as a byproduct. This buildup of lactic acid causes a burning sensation inside of the muscle cells, causing leg cramps and discomfort.