Soccer players jog or walk for much of a 90 minute-game, interspersed with regular bursts of full-out sprinting.
The sport makes for a rigorous workout, especially on the elite stage of the World Cup, which has quarterfinal matches starting Friday. Using various kinds of computerized tracking devices, researchers have found that midfielders in particular are covering a lot of ground — more than seven miles a game.
The evidence comes from portable devices that measure the amounts of oxygen and carbon dioxide that players inhale and exhale, said Barbara Ainsworth, a professor at Arizona State University's College of Health Solutions. These values allow scientists to calculate the number of calories a person is "burning" per kilogram of body mass each hour.
The end result is expressed as a metabolic equivalent — MET — the ratio between the metabolic rate of the sport in question and the rate when sitting. Swimming the butterfly, for example, has a MET of 13.8, meaning swimmers are burning calories 13.8 times as fast as when they are sitting.
The MET values for hundreds of human endeavors are ranked in the Compendium of Physical Activities, a long-running research project that Ainsworth oversees.
Some of the values are gleaned from scientific literature, while others are measured by Arizona State researchers.
In the case of soccer, an athlete's metabolic rate would depend on how much of the time is spent sprinting, jogging, or walking. For brief stretches, a soccer player's MET ratio would reach the mid-20s. For walking, however, the MET is just two.
The compendium lists competitive soccer with an average MET of 10, but Ainsworth said that figure could vary significantly depending on player and position.
It may even depend on who is winning. Other researchers have found that when a soccer team is ahead, its players tend to expend less energy — possibly because they have adopted a defensive, ball-control style of play.