Football is a sport that requires repetition of various movements, such as jogging, running, sprinting, and jumping. The energy expenditure associated with competition has been estimated to be about 5700 kJ for a male weighing 75 kg with a maximum VO2max of 60 ml/kg/min (Stølen, Chamari, Castagna, & Wisløff, 2005). The average work intensity during a match is approximately 85% of maximal heart rate, and the mean rate of energy expended is 70% of an athlete's VO2max (Bangsbo, 1994).
The primary energy system used in football is the aerobic system. Playing football shows a similar metabolic response as seen with endurance exercise (Bangsbo, 1994). "Off the ball" incidents are primarily aerobic, while "on the ball" or when directly involved in play, the primary system used is the anaerobic system. Movements performed during a game typically consist of: standing (19.5%), walking (41.8%), jogging (16.7%), running (16.8%), sprinting (1.4%), and other (3.7%) movements (Mohr, Krustrup, & Bangsbo, 2003). High aerobic capacity has been shown to aid with recovery during high-intensity bursts that are commonly performed in football (Reilly, 1997). Improving aerobic capacity can lead to an increase in distance covered, work intensity, number of sprints, and involvement with the ball during a game (Helgerud, Engen, Wisløff, & Hoff, 2001).
Max effort sprints occur roughly every 90 seconds on average, and a high intensity effort occurs every 30 seconds on average (Reilly & Stratton, 1995). Football requires players to repeatedly produce maximum or near-maximum bursts for a short duration with low recovery periods (Spencer, Bishop, Dawson, & Goodman, 2005). The repetitiveness of random bursts of high-intensity anaerobic and aerobic activity increases blood lactate concentration, which is mainly responsible for fatigue during matches (Reilly, 1997). Actions such as turning, tackling, and heading also require energy and can be taxing on the body.
The physiological demands vary depending on the playing position, as displayed in the Table. Central midfielders cover the most distance per game (12 KM), while central defenders cover the least distance among outfield players (10.6 KM). Wide positions and forwards cover more high-intensity distances (>23 km/h) compared to more central positions. The greatest aerobic requirements are for outfield players, specifically midfielders. Centre backs have the least aerobic demand. The most work is done in all positions in the 0-11 km/h range (Di Salvo, 2007).
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