You exercise so intensely that your muscles burn and you gasp for breath. Then you slow down for a minute or two, catch your breath, and then go very fast again. This training technique has been used in all endurance sports since the 1920's. Now George Brooks of the University of California at Berkeley has shown why interval training makes you a better athlete ( American Journal of Physiology-Endocrinology and Metabolism, June 2006).
Inside each muscle cell are mitochondria, the little furnaces that burn fuel for energy. A major fuel for your muscles during exercise is the sugar, glucose. In a series of chemical reactions, glucose is broken down step by step, with each step releasing energy. When enough oxygen is available, the glucose releases all of its energy until only carbon dioxide and water remain; these are blown off through your lungs. However, if not enough oxygen is available, the chemical reactions stop at lactic acid which accumulates in the muscles and spills over into the bloodstream. Lactic acid makes muscles acidic and causes a burning feeling. This recent research shows that lactic acid is the most efficient source of energy for muscles. Anything that helps muscles to break down lactic acid faster will make you a better athlete because it will increase your endurance and allow you to move faster when you are tired.
Since lactic acid is burned for energy in the mitochondria, anything that enlarges the mitochondria builds a bigger furnace and helps to increase endurance. Lactic acid is carried from the cells into the mitochondria by special proteins called lactate transporter molecules, so anything that increases these molecules will build endurance. An enzyme called lactic acid dehydrogenase is needed to start the reaction, so anything that increases this enzyme will also help. Interval training does all three: it enlarges the furnace (mitochondria), increases lactic acid transporter molecules, and increases the amount of lactic acid dehydrogenase.