The three energy systems work as follow. The ATP/PC system last for around 10 secs and requires no oxygen. This system is when the phosphate from phosphocreatine (PC) is broken away and joins with Adenosine Diphosphate (ADP) to form Adenosine Triphosphate (ATP). The ATP/PC produces no waste product which is a benefit. The Anaerobic Glycolysis/Lactic Acid System takes over once the alactic threshold is reached. The lactic acid system relies on glycolysis which is the break down of carbohydrates into glucose. Glycolysis occurs without oxygen and produces 2 ATP molecules. It unfortunately produces lactic acid as a waste product which has to be cleared to avoid muscle fatigue. After roughly 4 minutes the Aerobic system takes full effect of energy production. This system use proteins, carbohydrates and fats to resynthesize ATP molecules. The difference between this cycle and the Lactic acid system is because oxygen is required to complete the break down of glucose, with no lactic acid production, instead carbon dioxide and water are produced which are much easier to remove. This system can be used continuously for extended periods of time and yields at total 38 molecules of ATP pre glucose
Bibliography: Parkin D, Smith R, and Schokman P (1987) Premiership Football: How to play, train, and coach Australian Football (2nd ed.)