There are primarily three energy systems in the human responsible for the production of fuel for your muscles. Learning about these energy systems gives you a more complete and overall picture of what is happening to your body when you exercise. This knowledge will also enable you to differentiate between different exercises to achieve a particular goal or maximize physiological adaptation. A person's ability to perform an activity is based on the ability to gain required energy. Energy requirements may be short term or long term depending on the activity. Long-term performance, usually requires a great deal of energy from aerobic (meaning utilizing oxygen) sources. Those who train to enhance their endurance or cardiovascular system will train aerobically. In contrast, short-term performance requires its greatest energy from anaerobic sources (meaning without utilizing oxygen) and is thus referred to as anaerobic activity or training. There is a variety of different training modes for anaerobic exercise including everything from weight training to sprinting, all of which do not use oxygen as a limiting factor in performance. ATP (adenosine triphosphate) serves as the energy currency for all cells, however, its quantity is limited. Thus, ATP must be continuously resynthesized to continuously supply energy for biological work. Although anaerobic and aerobic exercises ultimately provide the same resulting energy (ATP) to working muscles, their delivery and mechanisms vary greatly. Let us examine these energy systems more thoroughly. There are two types of anaerobic energy sources, the phosphogen system and glycolysis. The phosphagen system provides ATP immediately to the muscle, so the energy needs of fast and powerful movement are met in the immediate area of the muscle's contractile machinery. This type of energy is vital for sports of very intense and short duration, such as sprinting, power lifting, wrestling, football, etc. The ATP generated lasts less than 10 seconds. Sprinters utilize the phosphagen system for 6-10 seconds for explosive energy and high power output producing more work in a shorter period of time. Glycolysis is the breakdown of glucose to pyruvic acid and the conversion of this intermediate, in the absence of oxygen, to lactic acid. The primary food source of glucose for the body is carbohydrates. This reaction needed to break down the glucose creates a greater time delay in energy production compared to the phosphagen system. Sports such as boxing and basketball use a relatively high ratio the glycolytic system. After 20 to 30 seconds, the anaerobic contributions to delivery ATP decrease and aerobic pathways steadily increase. Gymnasts typically rely heavily on anaerobic glycolysis for increased performance usually lasting less than 1 minute. Aerobic or endurance training has a direct physiological adaptation associated with increased oxygen uptake and cardiac output. Metabolic changes include an elevated respiratory capacity, lower blood lactate leves, increased mitochondrial density and improved enzyme activity. Aerobic athletes are able to show progressive improvement in their abilities to extract and utilize oxygen. Aerobic training may also change and alter body composition by decreasing relative percentage body fat and has little or no significant effect on fat free mass. Marathoners use the aerobic energy system to provide the maximum energy release over extend time periods. For combinations of energy systems used in various sports, please review the following chart: