Glycolysis involved in glycolysis. The burning sensation in muscles during hard exercise can be attributed to the release of hydrogen ions during the shift to glucose fermentation from glucose oxidation to carbon dioxide and water, when aerobic metabolism can no longer keep pace with the energy demands of the muscles. These hydrogen ions form a part of lactic acid. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen reached high concentrations in the atmosphere between
American Oxygen shiver down your spine." Rob Le Donne of "The Guardian" wrote that the single features more "serious" Rihanna than the persona on her previous singles, "it's hard to tell exactly what Rihanna is trying to say – but maybe that’s the point." Further more, he praised the producers Alex da Kid and West and the "layered" production that they have done on the song. However, he thought that "American Oxygen" is not "radio-friendly" unlike Eminem and Rihanna's 2010 single "Love the Way You Lie", which was also produced by Alex da Kid, or Rihanna's songs from her previous album, "Unapologetic",