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.

Sympathy for the Referee

The last week has seen numerous bad refereeing decisions in both the World Cup Soccer and the NBA finals. As the NBA finals are won and done (Lakers in 7) and in basketball, a 7 game series means that the bad decisions tend to even out over its course, let's turn our attention the the FIFA World Cup.

In soccer bad refereeing decisions are etched into every fans memory.
Quickly, what is the most famous FIFA World Cup goal? The first goal that comes to mind is Maradona's "Hand of God" goal in the 1986 quarter-final win over England. A goal that should not and would not have been awarded, had the referee seen Maradona's fist punching the ball into the net.

The 2010 FIFA World Cup is trending along a similar line. Siphiwe Tshabalala's spectacular opening strike of the World Cup for South Africa or David Villa's excellent skill to get Spain going, should be among the most memorable goals, but most of us are still talking about and replaying the goal by USA's Maurice Edu, which was disallowed by Mali referee Koman Coulibaly for an unknown reason.

Before damning the referees to a far less stressful existence in their day jobs, we may consider the difficulty they have controlling the game, making correct calls every time and what these controversies mean to soccer in general.

Difficult decisions are part of the referees job. We have previously written about the difficulty of getting into the correct position to see all that goes on in a soccer game. Even with TV we often don't see what happened until we get the instant replay from 3 different angles. Sometimes, even then, we are still not sure.

The referee needs to make his decisions quickly. Sure he can get some help from his assistant referees, but decisions also need to be prompt so that the game can continue to flow and be entertaining for the spectators. Player skill, speed, aggressiveness and even simulation, sometimes diving with theatrical eloquence to win free kicks and penalties, make it even harder.

In the USA vs Slovenia game on June 18, 2010, the referee was attempting to keep track of at least 3 different tussles between the players in the penalty box. In most cases, to those of us who got to see it more than once and from more than one angle, it looked obvious that the Slovenia players were fouling the Americans and that a penalty should have been awarded to the USA. The referee however had one view and one chance to see all that was happening and made his call.

Of course we hear the argument that the referee should simply have explained the call. Perhaps he should have, but would that really have appeased the United States players, coach or fans? After all, the goal was still disallowed and video replay is not currently used in soccer to overturn or review a refereeing decision.

This leads me to the other benefit of a poor decision - Interest in the game. The USA is not normally a country to be gripped by soccer fever, but the excitement for the deciding group game against Algeria has been raised a few levels because of the refereeing error. Americans who usually label soccer boring will be tuning in to see if their team can win and qualify for the round of 16, despite the terrible judgement that has befallen their team.

A similar enhancement of the game can be seen around the world. Every game England plays against Argentina, whether Maradona is Argentina's coach or not, is at an elevated level because the the "Hand of God" goal. The next time the Republic of Ireland play France, Irish revenge will be on everyones mind because Thierry Henry used his hand to assist in a goal for France over Ireland to qualify for the 2010 FIFA World Cup. The list goes on, everyone creating renewed interest, excitement and rivalries.

Let's have some sympathy for the referee. After all they are only human and can make human errors. And we so enjoy the rivalries they may have created by disallowing or allowing a goal here and there.

OK with sympathy we can now try to understand what Mali referee, Koman Coulibaly saw in this goal to disallow it. Could this video have the answer?

The Unpredictable World Cup Soccer Ball "Jabulani"

Can you feel it? The FIFA World Cup Soccer Finals produce an electricity unmatched by any other world sporting event with only the Summer Olympic Games being in contention.

To add to the excitement every 4 years since 1970, FIFA has introduced a new soccer ball for the World Cup tournament. As the World Cup nears the opening game on June 11, 2010, we once again hear the anguished cries of the forsaken goalkeepers in reference to this years Adidas "Jabulani" ball.

Every 4 years goalkeepers complain that the new balls flight path is more unpredictable. “Highly unpredictable when hit from long distances.” says Wendell, the Brazilian goalkeeper coach about the "Jabulani" ball. In 2006 England's goalkeeper Paul Robinson criticised the then World Cup Ball named "Teamgeist" for being too light and behaving unpredictably.
Sound familiar?

The "Jabulani" ball to be used at the World Cup hosted in South Africa, is made with 8 3D molded panels. This is fewer than the 2006 ball, made with 12 panels and of course far less than the original soccer ball made with numerous black and white panels that were stitched together. The "Jabulani" also has aero grooves, which are there to stabilize flight in the air and will provide grip in the wet for both strikers and goalkeepers. The ball is also said to be rounder than any previous soccer ball because of the 3D molded panels.

Time to use our video analysis to see what the fuss is all about. Take a look at a recent goal scored from a free kick by China's Deng Zhuoxiang against France on June 4th 2010.



Those of us who have watched football and particularly free kicks in the past will have seen this type of thing before. Watch as the ball moves toward the goalkeepers left at the start of the flight and then swerves right after passing the wall.

The French goalkeeper Lloris moves left to cover the shot and then has to change direction to his right as the ball swerves past him. Great free kick, whether Deng intended it or not.

Now take a look at a goal by Danny Rose of Tottenham Hotspurs against Arsenal in February 2010.


This goal is scored with the old "Teamgeist" ball and the movement through the air can be considered as dramatic as that for Deng's goal.

The goalkeepers might not like it very much but each time a new ball is created it does improve the control that the strikers have over the ball.

The top spin that Nadal uses to destroy opponents in tennis, the curve ball thrown in baseball and the reverse swing in cricket are all examples of so called "unpredictable" behaviour of a round ball. The fact is that with a more better ball, a player that can create enough speed and spin in a specific direction, can use physics to make the soccer ball move around "unpredictably" for the goalkeeper.

To measure the movement on a perfect round ball lets assume that a free kick is taken 30 meters out and hit at a good 30 meters per second or around 70 mph with a spin of about 10 revolutions per second. With this information and knowledge of the balls coefficient of drag we could calculate the distance the ball would "move" swerve off course at about 4 meters or 12 feet, which would be more than enough to beat a goalkeeper.

This coefficient of drag is what is being altered by the new ball. The fewer panels, rounder shape and aero grooves all have an effect. The fewer panels and rounder ball allow the striker to get more of the ball in his shot and also control the rotation on the ball. This should result in higher ball speeds off the players boot and cut the initial drag. The increased rotation creates the effect of moving air on one side of the ball faster than on the other, creating a force which makes the ball swerve more. The aero grooves produce turbulent flow around the ball (similar to the grooves on a golf ball) allowing it to also cut down on drag and fly through the air faster and further.

The extra movement on the ball occurs as the ball slows down and the drag forces take effect. The higher the ball speed and spin initially the more these drag forces can pull on the ball as it slows down. The design of the new ball has allowed us to use physics to better determine what will happen to it. The multi-paneled soccer balls of the past were far more unpredictable because of the seams, stitching and shape which affected its flight and movement.

To simplify all that, the "Jabulani" ball gives the strikers more control than they have ever had before and the ball flies faster through the air, making movement of the ball more dramatic off a well hit shot. The striker still needs to have control and power and be capable of imparting them to the ball in a perfect sequence to have the desired "unpredictable" effect.

Goalkeepers will learn to adjust to the new flight and speed of the "Jabulani" ball and then we can wait in anticipation for World Cup Finals Brazil 2014 to hear the new complaints about the "unpredictable" new ball.

In the meantime we will enjoy watching some spectacular goals and watch out for video that we can analyze that might show how the strike made the ball move like that.

Go Bafana Bafana and USA soccer.
Laduma! (Goal !)

Henry Gives France a Hand into The World Cup

Another week and another call for the use of video replay in sports.
This time from the football (soccer) World Cup qualifier between France and the Republic of Ireland.

Frances' William Gallas scores from an assist by team mate Thierry Henry. Replays and video evidence clearly show that Henry used his hand, intentionally, to control the ball before crossing it.

Watch the video below. Although I believe that Irish football commentary is often biased for their team, the commentary in this video is not biased. It is obvious that this goal should have been disallowed by the referee or the linesmen.



Henry admitted after the game that he had used his hand. I am not sure that any player with the desire to win this do or die World Cup qualification game would not have risked the same. I am sure he even expected the goal to be disallowed. Henry conned the referee and linesmen. Similar cases, particularly of players diving to win penalties, are prevalent in football at all levels and throughout the world. Players should never in my opinion deliberately try to con the referee. However I have no intention of passing judgement on Henry's character or that of any other professional athlete. This website is about video analysis, so once again we will harp back to a previous discussion on the use of technology and particularly video replay in sports.

In this particular instance we had some clear evidence of the hand ball from the TV footage. But again we need to point out that the evidence is not always that clear. See our previous post on video technology for refereeing. There have been numerous calls in soccer when the decision could have gone either way because of lack of clear video footage.

Watch this video of Wayne Rooney falling to win a penalty against Arsenal earlier this year. This decision is less obvious, was Rooney already falling or did the Arsenal goalkeeper touch his foot and bring him down? Was the ball close enough to be retrieved before going over the goal line? Even on watching the video it is a difficult decision to make.



The video technology in this case has let us down. A side on view of this same incident would have enabled us to better assess the position of the ball and the point at which Rooney begins to fall. We would also have been able to tell when Almunia, the Arsenal goalkeeper makes contact. So here is a decision that even with video replay remains controversial.

I believe that some amount of controversy actually brings excitement and emotion to the game. As a fan of a losing team, what better way to make yourself feel better than to suggest that the team would have won if the referee/player/coach/linesman hadn't cheated. In fact had Henry's goal for France been disallowed, Ireland would not have automatically won as many of the Irish believe (the score was 1-1 on aggregate before the goal went in), and may even have lost in the end anyway. Now just think of the great grudge match when France and Ireland meet again, even in sports other than soccer.

The problem is the blaming. With a referee in charge there are bound to be honest mistakes on occasion. Sometimes there may even be some blatant bias. The players too can make honest mistakes. A player falling over in the penalty box is not always trying to win a penalty, he may be trying to avoid injury.

Video replay technology can give increased accuracy to the decision. A referee is not always in the perfect position to make the most accurate call. Video from different angles can give the official added perspectives to help make his decision more accurate.

Video technology can remove all of this bias and simplify things. However it currently is not 100% accurate and controversial decisions will remain when the perfect camera angle is not available.

As technology progresses and even now with a good camera placement, such as those used in cricket and with the Hawk-Eye system for tennis, even more accurate decisions can be made.

It's time to bring in the video replay for all sports that rely on a referee or umpire to make a decision.