Both ATP and muscle glycogen can be metabolized without the need for oxygen. Thus this energy system involves anaerobic metabolism (occurring in the absence of oxygen). As exercise continues, the body has to rely on the metabolism of carbohydrates (more specifically, glucose) anti fats to generate ATP. This is second energy system requires oxygen and is therefore referred to as aerobic metabolism (occurring in the presence of oxygen).
In most activities both aerobic and anaerobic systems function simultaneously. The degree to which the two major energy systems are involved is determined by the intensity and duration of the activity. If the intensity of the activity is such that sufficient oxygen can be supplied to meet the demands of working tissues, the activity is considered to be aerobic. Conversely, if the activity is of high enough intensity or the duration is such that there is insufficient oxygen available to meet energy demands, the activity becomes anaerobic. Consequently, an oxygen debt is incurred that must be paid back during the recovery period. For example, short bursts of muscle contraction, as in running or swimming sprints, use predominantly the anaerobic system. However, endurance events depend a great deal on the aerobic system. Most sports use a combination of both anaerobic and aerobic metabolism.
