Lactic Acid System What becomes of this glucose? In the absence of oxygen, glucose is only partially broken down by a very complicated series of reactions. In man the end product is lactic acid; ATP is also formed. Lactic acid is actually a by-product, and when it accumulates to high levels, it causes muscular fatigue. This latter point is fundamental to the understanding of fatigue; more will be said about it later. The production of ATP, on the other hand, is the sole purpose of glycolysis. For every glucose molecule that undergoes glycolysis, a net of 2 ATP molecules is produced. This yield represents only about 5 per cent of the total yield possible when the same amount of glucose is completely broken down to CO 2 and H 2 O in the presence of oxygen (aerobic pathway). However, this anaerobic pathway, like the ATPPC system, is extremely important to us, primarily because it also provides us with a very rapid supply of ATP. For example, exercises that can be performed at maximum rate for only about 2 or 3 minutes, such as sprints and underwater swimming (breath holding), depend primarily upon the ATP-PC and lactic acid systems for ATP formation. The Aerobic Pathway (Oxygen System) In the presence of oxygen, a glucose molecule is completely broken down to CO 2 and H 2 O and a total of 38 molecules of ATP is produced. Two of these molecules of ATP come from glycolysis. This may come as a surprise to you since we just said that glycolysis is an anaerobic pathway. Actually, there is only one difference between the anaerobic glycolysis we discussed earlier and the glycolysis that occurs when there is a sufficient supply of oxygen -- lactic acid does not accumulate in the presence of oxygen. In other words, the presence of oxygen inhibits the accumulation of lactic acid but not the formation of ATP. Oxygen does this by diverting the majority of the lactic acid precursor, pyruvic acid, into the aerobic pathway after the ATP is formed. It is in the aerobic pathway that glucose, which has already been broken down to pyruvic acid, is further broken down to CO 2 and H 2 O, with the simultaneous production of additional ATP.		Home Components of Physical Fitness Clinical Examination Differences Between Fit and Unfit Men Physiological Elements of Fitness The Step Test The Effects of Training on the Physiological Systems Work Output Physical Condition of Muscle Application of Muscular Force Maintaining Optimum Strength Tests of Muscular Strength Strength and appearance Strength as a measure of physical fitness Strength as affected by organic problems Strength as prophylaxis Role of inactivity in production of disease Lack of Sociability in Work Lactic Acid System Muscular Form Capillary Growth Heart Size Heart Rate Training produces physicochemical changes Muscular Strength Nervous Stimulation and Muscular Strength Quality of Champions Factors Which Limit Skill Blood Supply Kinesthesis Balance Reaction Time Skill Muscular Tension Visual Aim Skill Center in the Brain Endurance for Exhaustive and Moderate Work Distribution of Energy