Energy systems of the body is divided into 2 categories the Anaerobic (functions without oxygen) and Aerobic Energy systems (functions with oxygen). These systems are subdivided into ATP-PC and Anaerobic Glycolysis which are anaerobic in nature and Aerobic glycolysis and lypolysis. The different energy systems are used to meet different energy demands and use different fuel sources. While one energy system may be dominant in a specific activity, all 3 energy systems are used simultaneously during rigorous activity. The 3 energy systems are therefore almost always at work and overlap each other. Different sports use the different energy systems to a more or lesser extent. The aerobic system is much more dominant for a long distance runner compared to a 400m sprinter that would predominantly tap into the anaerobic energy systems. A trainer must know the dominant system or correct relation in which the energy systems are used in different sports to enable an athlete to perform optimally.