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While the average distance covered by a top-class outfield player during a 90-minute match is over 10,000m, at an average speed of over 7km per hour, these figures do not accurately represent the full demands placed on a player. In addition to running, a player must jump, change direction, tackle, accelerate and decelerate, etc, and each of these individual tasks requires an energy input over and above that required simply to cover a similar distance at a constant speed. Scientific investigation has shown that the true demands on a player can be approximated at roughly 70%VO2 Max. This is based on evidence of heart rate, sweat loss, increase in body temperature, and depletion of carbohydrate stores within the muscles (intramuscular glycogen).

The specific demands of the different positions within a team are not as clearly defined as in some other team sports, such as rugby union. The obvious exception to this is, of course, the goalkeeper. A keeper relies little on the aerobic system for energy production since all the important phases of play for him last a relatively short time. The key performance quality of the keeper is probably agility, and this can be broken down further to include speed, power, strength and flexibility. If he happens to be tall, it's clearly an added bonus!

Popular training programmes for keepers include repetitions of short sprints performed at maximal speed, with many changes of direction involved. Obviously, an element of skill can be built into this training by having to save a bombardment of shots at goal. This way, another important constituent of training is then automatically introduced, namely, the ability to regain one's feet in order to save a follow-up shot at goal. However, to gain the edge in physical development, the keeper should also train away from the pitch so that upper and lower body strength and power can be improved in the weights room. In addition, plyometric training lends itself perfectly to improving the qualities necessary for agility around the goal mouth. Plyometric training does need to be conducted correctly which includes the provision of generous rest periods between sets of exercises, but if done so can produce some significant improvements in the ability to move one's own body weight at speed.

As far as the rest of a soccer team goes, the differing demands are less obvious. However, a systematic analysis of soccer matches on video has shown that midfield players tend to cover the most distance, and other studies have - not surprisingly - shown these players to have the highest VO2 Max scores, and to show the least fatigue when performing many repeated sprints in succession. Compared to forwards and defenders, midfield players tend to have a more continuous involvement in the game. However, while forwards and defenders usually have more time to recover between sprints, they also need to perform those sprints at a faster speed to be successful in their crucial phases of play.

Implications for training should become apparent. Clearly, the midfield players need more of an all-round fitness profile, with an emphasis on both aerobic and anaerobic capacity. Aerobic capacity relates to sustained performance (20-40 minutes), or performance during lengthy repetitions, each of 2-3 minutes in duration. Anaerobic capacity can be related to performance of a repeated nature, but with work/rest intervals of equal length, and not over 30 seconds.

The players regularly involved in attacking/ defending situations will need more training emphasis on speed. Speed training can itself be broken down into at least two phases - an acceleration component and a maximal speed component. For improvements in acceleration, repeated sprints of not less than six seconds in duration, performed from a standing or walking start, will be useful in training. This will help develop the neuromuscular function of the athletes. For development of maximal speed, a gentle increase in speed to about 85 per cent followed by a sustained burst at maximum speed for about six seconds will produce more specific improvements. This will help develop both the metabolic and neuromuscular qualities of the muscles involved. Put simply, to improve acceleration, accelerate as fast as possible in training. To improve maximal speed, the length of time spent running at current maximal speed during training should be increased. A relatively gentle acceleration phase before a sustained burst can best achieve this.

If the coach can accomplish these sorts of training goals by using drills which involve ball skills, then the players will become used to performing the skills under conditions of fatigue. As many will appreciate, it is under conditions of fatigue and mental pressure such as a competitive match that skills often become lost - unless they are both well-drilled for their own sake and practised under simulated conditions of fatigue.

Moving away from training methods for a moment but continuing the analysis of the physical demands of the game, there is an interesting form of player behaviour that playing experience seems to encourage. It is a phenomenon that many players will recognise as common without perhaps understanding why. The behaviour in question is the avoidance of prolonged high-intensity activity that would require a corresponding long period of recovery - which can rarely be afforded in a competitive situation. For instance, if a defender is involved in high-intensity activity as he assists in an attacking phase of play, he often will not attempt to return to his defending position in time for the immediate counter-attack. While this might be perceived as laziness, it may benefit both the individual player and the team in the longer term, providing the rest of the team has sufficient cover to deal with the counterattack.

Sound physiological reasoning provides the basis for this. It has been shown that short periods of intense exercise (e.g., less than 15 seconds), when interspersed with rest periods of similar duration, produce a fairly low build-up of lactic acid in the muscles (a strong indicator of fatigue) even when this activity pattern is continued for some time. However, periods of intense exercise of about 30 seconds or more, even when accompanied by equal rest periods of 30 seconds (such that the work:rest ratio is till 1:1 as in the previous example), produce a far higher concentration of lactic acid in the muscles and also greater fatigue.

This situation is exactly what the experienced player is trying to avoid when he decides to return more slowly to his main position on the pitch. However, this obviously requires a large degree of teamwork, with team-mates prepared to cover for the defender concerned. If a team can achieve this sort of co-operation, it helps reduce player fatigue and increases performance capacity throughout the match as a whole. Clearly the role of the coach is paramount in organising this sort of team approach in spreading the workload, especially with inexperienced players.

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