How does an outfielder know where to run for a fly ball?

January 21, 2010
Researchers at Brown tested the three theories explaining how outfielders judge fly balls. It turns out to be a matter of optical acceleration cancellation. Credit: David Silverman/Brown University

( -- Faced with a fly ball soaring deep into center field during the 1954 World Series, New York Giants center fielder Willie Mays turned his back on the ball, ran straight to the center field fence and caught the ball over his shoulder.

"The Catch" is among the best-known images in all of sports.

How did Mays do it? Did he predict where and when the ball was going to land just from seeing it hit? How does any outfielder actually catch a fly ball?

Brown University researchers have concluded that prediction has little to do with catching a fly ball. By using a virtual environment in which volunteers ran after computer-generated fly balls, researchers were able to confirm one of three major theories about how baseball players catch fly balls: It's a matter of optical acceleration cancellation.

Instead of predicting the ball's likely landing point, the outfielder's eyes continuously track the ball as its visual velocity increases or decreases, and the outfielder runs backward or forward to compensate. "The Catch" is famous precisely because fielders rarely turn their back on the ball. Mays may have added luck to the equation. Details are online at the Journal of Vision.

"All the fielders need to do is track this optical variable and it will lead them to a successful catch," said William H. Warren, professor of cognitive and linguistic sciences at Brown and the paper's senior author. "They don't have to do a lot of heavy computation in their heads to predict the landing point."

Warren collaborated on the paper with lead author Philip W. Fink, a former postdoctoral associate at Brown who is now lecturer in exercise and sport science at Massey University in New Zealand. Co-author Patrick S. Foo, another former postdoctoral associate, is now assistant professor of psychology at the University of North Carolina-Asheville.

Warren said that understanding how outfielders catch fly balls might help coaches advise ballplayers and teach children how to play baseball, but the significance of the finding goes well beyond baseball.

"This is a classic example of visual-motor control, because the limits of human ability are really being tested," Warren said. "If we can understand these key cases, it gives us some insight into how visual control works in more everyday situations."

The finding is contrary to currently popular models of visual-motor control that assume people have a model of the world and its physical laws in their heads, Warren said.

In conducting the research, Fink, Foo and Warren wanted to test three major theories of how an outfielder catches a fly ball:

  • The mental model of trajectory. Outfielders rely on subconscious internal models of the physical world to calculate the ball's trajectory and predict its landing point.
  • Linear optical trajectory. Outfielders run in such a way that their visual image of the ball appears to form a straight line.
  • Optical acceleration cancellation. First published in 1968, the theory proposed that the outfielder tracks the elevation angle of the ball after it is hit. If the angle accelerates, the ball will land behind the ballplayer; if it decelerates, the ball will land in front. The fielder moves forward or backward to compensate.
Eight varsity baseball players and four varsity softball players took part in the study, fielding "virtual" fly balls in Brown's Virtual Environment Navigation Lab. Each wore a head-mounted display that allowed them to see the virtual ball as it was launched, and they ran freely in the 40-by-40-foot research space to make their virtual catches. Their movements were consistent with the optical acceleration cancellation theory.

Conducting the research virtually allowed Warren and the team to test the three theories by making the ball fly on a physically impossible trajectory. They then recorded the subjects' head and glove positions to figure out how they were trying to catch the ball.

While all three models can predict successful catches and similar running paths, the digitally altered or physically impossible trajectories included in this research show that baseball players continuously track the ball and run forward or backward to cancel optical acceleration. "Catching is clearly a perceptual motor skill that you have to learn," Warren said, "but it does raise interesting questions about why some people are so much better at it than others."

Warren said some individuals may have a greater sensitivity to visual acceleration or may use it to calibrate their movements more precisely.

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not rated yet Jan 21, 2010
What is an "impossible trajectory"??

Why do unrealistic scenario's give us a realistic approach?

The reason outfielders dont run to a point on the ground, is because they arent looking at it. They look at the ball. The field is simply an assumed infinite plane in which to run on, so its independent of the action involved.

However, initial contact with the bat is important because reading how the ball comes off the bat helps with intial motion and trajectory of the ball.

The hardest ball to determine such things, is the one hit right at you.

not rated yet Jan 21, 2010
As an outfielder for many years all I can say is how I did it. Initial contact with the bat isn't what I look at. We watch the speed and trajectory of the ball from the instant it leaves the bat throughout its flight until it's in the glove. We make constant adjustments to our running path adjusting it to where we think the ball is going to land based on the observations of the speed and trajectory right from the start. Sometimes we can go straight at that point right away, other times our best guess from the start is wrong and we have to adjust our running path to our adjusted guess of the landing point. My guess is there are few one-eyed outfielders. Without stereoscopic vision intersecting the ball would be pure guesswork. The best outfielders are the people whose internal ballistic computer comes up with the correct impact point the quickest and the most often. If the running path is curved it's inefficient and the fielder did not get the impact point right away.
not rated yet Jan 21, 2010
I think really good outfielders use a variety of visual cues and experience to predict the flight of the ball, sometimes even before the batter steps up to the plate. Example: Some batters flip their wrists more than others, imparting extra spin and lateral motion to the ball. Knowing this beforehand, a savvy outfielder will pre-position himself a step or two to that side. Coaches make charts of where balls land for each player, sometimes including where they hit balls off particular pitchers. So, if Joe Mauer faces a lefty reliever with a sweeping slider, you can bet the lefty will try to get Mauer to flail at a pitch that sweeps outside and shallow. If Mauer does hit it, it will either be a weak grounder to third or a line drive to left field. A good outfielder knows this before the pitch. He also knows that the ball will curve towards the line, because Mauer's swing is a slap. After he sees the initial trajectory, he will head to where such slaps usually land, plus corrections.
not rated yet Jan 22, 2010
How is it that a basketball player can make the minute adjustments to shoot the ball from 1ft to 25ft in a split second without even thinking about it? As a basketball player I don't think about it. I just do it. The brain is an incredibly fast processor that uses all the variables of stimulus to control the muscles. The longer the player practices the better the accuracy. This is self evident in all aspects of life from walking to shooting a bow and arrow.
not rated yet Jan 22, 2010
I honestly don't think this is right.I played left field for about 15 years of intramural and city league ball. What I lacked in speed I made up for in judgment. By knowing the pitch and seeing the swing, I would immediately go for a spot on the field. Sure, I would correct on the way, but the goal was to be standing and waiting under the ball when it came down. In practices we would try to go to the "spot" without taking a 2nd look at the ball. I think this helped a lot. Just trying to intersect the ball's path cuts the possible range that an outfielder can have. When your goal is to beat the ball to the spot, you cover more ground and get "burned" less often because you can instantly pick your direction and focus on speed, then fine tune, or stand there and wait for the ball, at the end.
not rated yet Jan 24, 2010
Never played professionally, but I was unable to catch fly balls as a child until I got glasses that corrected a horizontal astigmatism. Without the glasses, I was never able to get to the right place as the ball came down.

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