The muscle require a chemical (ATP). This is produced in three ways. Two are anaerobic (witout oxygen) as soccerismylife mentioned. Cellular respiration is an aerobic proceess for producton of ATP. People who train use the cellular respiration method more effectively. Less lactate (lactic acid) is produced in people who train. Anaerobic fermentation produces ATP by breaking glucose down to lactic acid. The buildup of lactic acid in the muscles causes the muscles to not relax momentarily. This leads to cramping and even fatique. Simply stated, ateletes can consume oxygen more effectively and break down fatty acids instead of glucose. Courtesy of Humen Biology 8th Edition.