News in Science

Penalty kicks may be predictable: study

In soccer, some games are decided on a penalty kick. Now, new research might give goalies the upper hand.

Faced with a ball that rockets toward him from 11 metres away at more than 100 kilometres per hour, a goalkeeper needs to decide which way he's going to dive before the kicker's foot even makes contact with the ball. More than 80% of the time, studies show, even the best goalies guess wrong.

Using motion-sensing technologies and computer analyses, a scientist has identified a few early signs that reliably predict which direction a penalty kick will go. The study also found that some people are much better at picking up on those signs than others.

"There's some interesting data suggesting that there's a big difference in performance among expert goalkeepers out there playing on national teams," says Gabriel Diaz, a cognitive scientist at Rensselaer Polytechnic Institute, New York. Some are adept at blocking penalty kicks, while others are no better than novices.

"In the future, I would like to create a perceptual training regime," he says. "Maybe we can train people's attention to focus on the most reliable information."

Mind game

Penalty kicks can determine which team wins a soccer game, especially when shoot-outs are used to settle a draw. Although there is plenty of physical skill involved in placing the ball just out of the goalie's reach, a penalty shot is ultimately a mind game between player and goalkeeper. In a split second, each opponent tries to guess what the other will do.

Past studies have looked at how a goalie's emotions influence whether he dives or stands still, how his stance affects where the kicker places the ball, and how the kicker's anxiety affects his success rate.

Taking a different approach, Diaz wanted to find out if any of the kicker's body movements before the kick might betray where the ball will go.

In a large room, Diaz created a scaled down penalty kick scenario that was one-third shorter than the official distance between ball and goal. The experimental setup also had a goal that was one-third smaller than normal.

With more than 40 motion sensors on 19 major joints, three college-level soccer players kicked more than 100 penalty kicks to both sides of the goal. Even the ball wore a motion sensor. Cameras tracked every move.

Out of 27 possible body movements, Diaz found five that turned out to be reliable predictors of where the ball would go, including the direction of the kicker's planted foot and the angle of his hips.

If a kicker tried to do something unexpected, such as point his planted foot one way but kick the ball in the opposite direction, the rest of his body could also give him away, says Diaz. The player might, for example, have to move his arms in a certain way to keep his balance.

Some better than others

In a second experiment, Diaz showed animated video footage of penalty kicks from the first experiment to about 30 novice soccer players. They saw the kick just to the point where the kicker's foot touched the ball. Then, the screen went blank. They had half a second to decide which way the ball would go.

Results, which Diaz plans to submit to a scientific journal when he's done with follow-up studies, showed that half of the people were terrible at guessing where the ball went. The other half, however, excelled at it.

Those who did well may have zeroed in a few specific body movements, the study suggested. They also tended to wait slightly longer to make a decision.

But that's time that goalies just don't have in real-game situations, says Dr Richard Ginsburg, co-director of the Massachusetts General Hospital Sport Psychology Program in Boston.

While the new findings might eventually give both kickers and goalies new ways to outsmart each other, he says, dissecting a sport into its scientific parts can never produce a perfect player.

With high-stake games, lots of emotion flowing, and tremendous amounts of pressure on the players, psychology starts to play a big role. That will always make results unpredictable.

"This kind of science can only take you so far," says Ginsburg, who has also played and coached soccer. "There are an enormous number of factors that go into this. There will always be these other variables that will be unknown."