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A study of penalties taken over all the World Cup matches between 1982 and 1994 showed that goalkeepers guessed the direction of the penalty kick 41% of the time and only saved 14 % of the time. The rules have changed since then giving the goalie a better chance.
Prior to 1997 a goal keeper had to stand on his line and keep still without moving forward. Today the keepers move but still not allowed to move forward before the ball is kicked. penalty takers have tried to thwart the keeper these days by doing a staggered run up trying to force the keeper to move early. This is allowed even though it does seem unfair as long as it is in one motion and no stopping in the action.
Science has proved that a penalty taker who doesn't look at the goal keeper will most likely score as just with power alone and placement a goal keeper has no chance. A football struck properly with power will travel at around 80 miles per hour and will hit the back of the net before a goal keeper can cover the corners of the goal. We are talking milliseconds though as a ball will cross the line in 500 milliseconds and a goal keeper takes 600 milliseconds to cover the dive to one of his corner posts.
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Goalie has no chance if the ball hits the top right or top left as it is out of his diving arc.
A football goal is 24 feet wide (7.2 m) and the penalty spot is 12 yards away and yet players still miss the goal altogether. It has been proven that kicking the ball into the top left or right corners of the goal it is impossible to stop and why penalty takers do miss. Trying to pick the spot with precision and accuracy sometimes doesn't work.
Pressure on the penalty taker is what makes the difference between a good penalty taker and someone who is just average. The good ones don't worry about the keeper knowing the odds are in their favor and should score a high percentage of the time. Hitting either top corners will give you a 100 percent success rate as it is out of the goalies reach in his diving arc.
What has changed these days is the information that is available for goal keepers to study the penalty takers of the various teams. More homework has gone into the art of defending their goals and they study any trigger movements aiding them to predict which way the ball is going. Most players have habits and they are unaware of the signals they are giving off subconsciously which make it slightly easier for the keepers today. Science still says if you place the ball accurately to one of the corners with power you will score 100 percent of the time as the pace of the ball will always win.