Association football at the elite level has developed vastly over recent years and many studies into match performance and training have been performed. It is clear that this research has enabled science to be incorporated to a greater extent into the training conducted in football. Earlier studies looked into the physiological demands of the game, by performing physiological measurements before and after the game or at half-time. In addition to this earlier research, some up to date studies have scrutinized changes in both performance and physiological responses with a special focus on the most demanding activities and periods in the game.

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Another area to have received considerable attention individual differences in the physical demands players are exposed to throughout the games and in training. These can be affected by training status, playing position and to the specific tactical roles assigned to the players. Thus, most top level clubs have incorporated the tactical and physical demands of the players into their fitness training.
This paper will look into the demands of different activites of football, aerobic and anaerobic energy production in match play, the fatigue experienced in football matches and the training of top level players.
Aerobic Energy Production in a Football Match
Association football is an intermittent sport in which the aerobic energy system is utilized majorly, with mean heart rate at around 85% of maximal and peak heart rate at around 98% of maximal, Taking these values, it is possible to discover oxygen uptake using the relationship between heart rate and oxygen uptake. Though, it is unlikely that the heart rates measured during a match will be accurate enough to lead to a correct estimation of oxygen uptake, since variables such as dehydration, hyperthermia, and mental stress elevate the heart rate without affecting oxygen uptake. However, taking these factors into account, the heart rate measurements received during a game suggest the average oxygen uptake is around 70% of VO2 max.
This is supported by core temperature data measured during the match. Since a linear relationship has been reported between rectal temperature and relative work intensity (Saltin & Hermansen, 1966), core temperature can be used as an indirect measure of energy production. Throughout a bout of continuous cycling, completed at 70% VO2 max, the rectal temperature was 38.7°C. In association football, the core temperature increases relatively more compared with the average intensity due to the intermittent nature of the game. Hence, it is pragmatic that a 60% of VO2 max work rate, the core temperature was 0.3°C higher during intermittent than continuous exercise (Ekblom et al., 1971). All the same, core temperatures of 39-40°C for the duration of a game propose that the average aerobic energy production rate for the period of a game is around 70% VO2 max (Mohr et al., 2004).
Conversely, a factor of more interest than the average oxygen uptake may possibly be the rate of rise in oxygen uptake during the many short intense actions throughout the duration of the game. A player’s heart rate during a game is rarely below 65% of maximum, which means that oxygen delivery is continuously high. However, the oxygen kinetics during the constant flow from low to high intensity during match play appear to be restricted by the oxidative capacity of the contracting muscles (Krustrup, Hellsten, & Bangsbo, 2004).
Anaerobic energy production in a Football Match
Top football players complete approximately 150-250 short duration, intense actions (sprints, shooting, tackling etc.) throughout a game (Mohr et al., 2003). This suggests the rate of anaerobic energy production will vary from low to high during the game. Albeit, not studied directly, the intense exercise leads to a high rate of creatine phosphate breakdown, which in some measure is resynthesized in the low-intensity exercise periods (Bangsbo, 1994). On However, creatine phosphate levels may decrease during periods of the game if the intense activities are completed with short recovery periods. Creatine phosphate in muscle biopsies obtained after intense exercise periods during a game have provided values above 70% of those at rest, although could be due to the delay in attaining the biopsy (Krustrup et al., 2006).
A range of blood lactate concentrations of 2-10 mmol·l−1 have been observed during matches, from a variety of research (Krustrup et al., 2006). These findings suggest that the rate of muscle lactate production is high during match-play. However, it is important to consider that muscle lactate has been measured in only one study. In a non-competitive match between non-professional teams, data indicated that muscle lactate increased by 400% in comparison with resting values, after intense periods in both halves, (Krustrup et al., 2006). A study in 2003 by Krustrup, found values over three times those observed previously. However, more interesting was the fact that muscle lactate was not correlated with blood lactate. This is supported by research when participants performed repeated intense exercise using the Yo-Yo intermittent recovery test (Krustrup et al., 2003). This is in contrast to continuous exercise where the blood lactate concentrations are lower but reflect well the muscle lactate concentrations during exercise. This difference between intermittent and continuous exercise are most likely caused by the different turnover speeds of muscle and blood lactate during the two types of exercise, with muscle lactate being removed more readily than blood lactate (Graham, & Saltin, 1993).
The relationship between muscle lactate and blood lactate also appears to be influenced by the activities immediately before sampling (Krustrup & Bangsbo, 2001). Thus, the rather high blood lactate concentration often seen in football may not correspond to a high lactate production in the activity just performed, but instead, an accumulated reaction to a sequence of high-intensity activities (Krustrup et al., 2006). This is important to take into account when looking at the relationship between blood lactate concentration and muscle lactate concentration. Yet, it is suggested that the rate of glycolysis is high for short periods of time during a game based on the finding of high blood lactate and moderate muscle lactate concentrations during match-play,
Fatigue in a Football Match
Several studies have suggested that players’ ability to perform the high-intensity activities associated with football,is reduced towards the end of games in both elite and non-professional football (Krustrup et al., 2006; Mohr et al., 2003). Therefore, it has been established that the amount of sprinting, tackling, shooting, and the distance covered are lower in the second half compared to the first half of a game (Mohr et al., 2003). What’s more, it has been suggested that the amount of sprinting decreases in the final 15 min of a top-class soccer game (Mohr et al., 2003). However, there is a wide range of mechanisms that have been suggested to explain the decrease in exercise performance at the end of the football match. One particular mechanism is the depletion of glycogen stores, since the onset of fatigue during intermittent exercise has been linked to a lack of muscle glycogen. Furthermore, it has been demonstrated that increasing muscle glycogen before intermittent exercise by carbo-loading enhances performance during exercise (Balsom et al., 1999). A study by Krustrup et al. (2006), found that the muscle glycogen concentration at the end of the match was reduced to 150-350 mmol·kg. Thus, there was still glycogen available. However, histochemical analysis revealed that about half of the individual muscle fibres of both types were almost depleted or depleted of glycogen. This reduction can be linked to the reduction of sprint performance at the end of the match, and it was suggested a depletion of glycogen in some mucsle fibres does not allow for a maximal effort in single and repeated sprints. Nevertheless, it is unclear what the mechanisms are behind the possible causal relationship between muscle glycogen concentration and fatigue during prolonged intermittent exercise (Maughan, 2007).
Dehydration has also been linked to the onset of fatigue in the later stages of a football game (Magal et al., 2003). Elite players have been reported to lose up to 3 litres of fluid during games (Maughan, 2007) and it has been observed that 5 and 10 m sprint times are slowed by dehydration which amounts to 2.7% of body weight (Magal et al., 2003). On the other hand, in a study by Krustrup et al. (2006) a significant decline in sprint performance was found, although the fluid loss of the subjects was only about 1% of body mass. Thus, it would appear that fluid loss is not always an important component in the impaired performance seen towards the end of a game.
Current research via analysis of professional male football players during games has pointed out that players become fatigued at stages in a game (Mohr et al., 2003). Accordingly, in the five minutes subsequent to the most intense time of the match, the ability to complete  high-intensity exercise was decreased to levels below the average. Fatigue throughout a match is a complex and one with a wide range of explnations. One of these may be cerebral in nature, especially during hot conditions (Meeusen, Watson, & Dvorak, 2006). Nevertheless, it has been suggested that the cause of fatigue, in elite level athletes only, is a muscular mechanism. In the study by Krustrup et al. (2006), the decrease in performance for the period of the game was correlated to muscle lactate. Conversely, the connection was very weak and the alteration in muscle lactate were not particularly clear. What’s more, numerous studies have publicized that the build up of lactate does not cause fatigue (Krustrup et al., 2003). A further mechanism suggested to be responsible muscle fatigue at some point in intense exercise is a low muscle pH (Sahlin, 1992). Nonetheless, muscle pH is not reduced dramatically, only to about 6.8, throughout a game and no correlation with performance level has been observed (Krustrup et al., 2006). Nevertheless, none of these explanations offer a clear picture into what is the primary cause of the fatigue during the game, and further research is needed to reveal the mechanisms causing fatigue throughout the match.
Conclusions
It is clear to see that association football utilizes both the aerobic and anaerobic energy production systems heavily, and could not be described as predominantly either aerobic or anaerobic. With the players travelling on average 10-13 km through a 90 minute game, the aerobic system is very important and training needs to focus on aerobic exercise. However, as the players complete, on average, 150-250 intense activity exercises throughout the 90 minute game, and blood and muscle lactate levels both dramatically increasing throughout the game, anaerobic exercise would also need to be focused on in order to improve this part of the game. It is the players that  can managed the balance between aerobic and anaerobic exercise that reach the top level of the game, and differences are seen between international players and other professionals, like they are non-international players and non-professional players.
Based on the analysis of the demands of association football it is evident that the training of elite football players should focus on enhancing their ability to perform intense exercise and to recover rapidly from these periods of high-intensity activity. This can be achieved by performing an aerobic and anaerobic training regime on a regular basis (Bangsbo, 2005), which is easy for elite level football players who are played to train every day. However, for those who are wanting to become a professional football player, it is more difficult to train regularly, while potentially completing other work to earn money. 
In a typical week for a professional football team with one match to play, the players might have six training sessions in 5 days, with the day after the match used to recover. For the average person, this sort of time is hard to find, and restricts an individual, who has not come through the academy system, wanting to become professional.