Exercise physiology loss of integrity to the blood brain barrier. Hyperthermia also can lower cerebral blood flow, and raise brain temperature. Researchers once attributed fatigue to a build-up of lactic acid in muscles. However, this is no longer believed. Rather, lactate may stop muscle fatigue by keeping muscles fully responding to nerve signals. The available oxygen and energy supply, and disturbances of muscle ion homeostasis are the main factor determining exercise performance, at least during brief very intense exercise. Each muscle contraction involves an action potential that activates voltage sensors, and so releases Ca ions from the muscle fibre’s sarcoplasmic reticulum. The
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",