The cause of muscle fatigue depend on whether the work being done is mainly anaerobic or mainly aerobic . during anaerobic work glucose is catabolized in the absence of oxygen into two molecules of lactic acid and two molecule of atp only about 2% of the energy of glucose in transferred to atp .during aerobic work glucose or fatty acid is combined with oxygen and catabolized to carbon dioxide and water. Approximately 36 molecule of atp are produces with is 40% of the available energy within a glucose or fatty acids molecule

Anaerobic exercise cannot continue for long because lactic acid accumulated in the muscles and gradually lowers the pH which cause pain and interferes with normal chemical activities and the inefficient use of glucose cause the muscle glycogen to be used up rapidly. In an all out sprint for example the glycogen reserve are depleted in approximately 45 seconds.

Aerobic exercise can continue for long period of time because it uses efficiently two fuels glucose and fatty acids. Although glucose reserves are always limited and average sized person has enough stored fat to walk1000 miles. Exercise more rapid than a slow walk uses both glucose and fatty acids .fatigue develops when glycogen reserves are depleted and the more rapid the pace of the exercise the sooner depletion occurs.