Football - or soccer - is a multiple sprint sport requiring players to spend some of their time sprinting, and other times running fast or slowly, walking or even standing still.
Varying degrees of fitness are required, depending on the player's position in the team, the amount of time spent on the field and the level at which the game is played. A full 90-minute match, for example, demands high levels of aerobic fitness.
The intermittent nature of the game means that energy demands are sometimes extremely high and sometimes fairly low; however, studies have shown that at professional levels the average work rate (based on percentage of maximum oxygen uptake) of midfield players and strikers is similar to that of a good marathon runner.
This means football players must have a good aerobic endurance base if they are to last the full game without becoming excessively tired. Because the game involves acceleration and deceleration, changes in direction, angled runs and running backwards - all unorthodox methods of running - there's also an increased energy cost of the exercise.