Answer
After the phosphagen system is depleted , the muscles switch to anerobic fermention.;
The fuel used by the muscles during this period is stored glycogen of the muscles and glucose in the blood This system produces 2 ATPs per molecule of glucose and generates lactic acid . This lactic acid is both a waste product and with a store of energy. However this just a transient measure until the cardiopulmonary hits its stride and is able to catch cup with the increased demand for oxygen. This process can supply the body's requirement for ATP for 30-40 minutes.
Work Step by Step
After 40 minutes of activity ( very vigorous exercise) the cardiopulmonary system is in high gear and is able to meet the body's (muscles') demand for oxygen, When the muscles switch to aerobic respiration the production of ATP rises dramatically. Whereas only 2 ATPs were generated per glucose molecule by anaerobic fermentation, the same molecule of glucose would generate 34 ATPs as glucose goes to pyruvate and pyruvate passes through the process of aerobic respiration.
