Monday, June 28, 2010

Predicting the Direction of a Penalty Kick

The 2010 FIFA World Cup is producing some exciting football. As we move through the knock out rounds and a winner must be decided for each game, the possibility of penalties for a game that ends in a draw becomes very likely.

ESPN's Sports Science program recently did a comparison between baseball hitting and the possibility of a goalkeeper saving a penalty kick. The presenter, John Brenkus, concludes that it is more difficult for a goalkeeper to make a save than for a batter to get a hit. The overall batting average for 2009 was .258 or 25.8%, while the annual save rate for penalty kicks is around 22%. Watch the Sports Science clip here.

Although it does seem almost impossible to make a penalty save, goalkeepers may be doing as well as they are because they are reading the kickers body language before deciding which way to dive.

A recent study by Gabriel J. Diaz of Rensselaer Polytechnic Institute, used motion capture technology in an attempt to determine clear indicators for the goalkeeper, that would help them to guess the direction the ball will be kicked from the penalty spot. This may suggest why goalkeepers guess the correct direction about 57% of the time (as suggested in the Sports Science clip), rather than only 50% of the time.

In his study, Diaz identified five reliable indicators of the direction the ball will be kicked. He discovered that four of five of these indicators were used by people (not goalkeepers) he tested that were able to identify the direction the ball would go before it was kicked. Read more here.

Two of these indicators are the angle of the hips as the kicking foot swings through and the angle of the support foot on the ground prior to kicking. These two indicators are known to football coaches and may already be used by goalkeepers.The other three indicators are "distributed movements", co-ordinated movements that balance the body, and have not been discussed by coaches or soccer players previously.

The idea that motion capture and or video analysis can be used to improve performance is not new to any of us here. Diaz rightly suggests that these predictors, including the distributed movements, could be pointed out to a goalkeeper and possibly give them an advantage. They would still need to be able to process these indicators and make a quick enough decision to gain an advantage and that may not be so easy.

Some great 2010 World Cup quarterfinal match ups are coming together. Argentina v. Germany and Brazil v. Netherlands should be some high tension and hopefully highly skilled encounters. We hope there are no penalty shootouts. But if there are, try and pick out the direction the kicker will go by watching his hips and standing leg and any other movements you think give it away. If you are a goalkeeper, let us know how you guess direction of the penalty kick.

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