Left or Right? Soccer Penalty Kicks Deconstructed

by Mary Martialay on June 29, 2010

Play the video, take the challenge. I guarantee it’s not easy.

Here’s how it works: Watch the “dot figure” of a kicker as he approaches a soccer ball to kick it. As soon as you can, guess whether the ball will go left or right.

The video is a demonstration of an experiment on soccer penalty kicks run by Gabriel Diaz, a doctoral student in Cognitive Science. Diaz wants to know if – in the split-second before foot meets ball - a soccer player’s body betrays the direction the ball will go, and if goalkeepers might be picking up on those tell-tales.

Diaz showed this video to 31 test subjects, and gave them up to a half-second after the screen goes black to hit a either the left or right shift key on a computer keyboard, indicating whether they thought the ball would go to the left or right.

Fifteen of them performed no better than chance (50/50).

But among the 16 who were able to do better than chance, further investigation revealed a high correlation between the correct result and four of the five indicators Diaz had identified.

How did you do?

This whole process is a lot more complex than it appears at first glance. Diaz conducted his research in the Perception and Action (PandA) laboratory of Rensselaer associate professor Brett Fajen. The lab itself is 20’X28’ room ringed with motion capture equipment like a 14 camera Vicon MXPro infra-red motion tracking system.

By placing sensors on 19 major joints of the body, Diaz was able to capture the motions of three college-level soccer players as they kicked – on his direction - to the left or right side of the goal. The video shows “stimuli” generated from the data Diaz collected.

In the first part of his experiment, Diaz tested the data he collected against 27 possible indicators of which direction the ball might go – twelve drawn from sports literature and 15 from a computer analysis that searched for patterns of coordinated movements.

The sports literature indicators are fairly simple and easy to describe – things like the angles of the kicking foot, kicking upper-leg, and kicking shank. He calls those “local” indicators because they restricted to area of the body.

The 15 patterns of coordinated movement – what he calls “distributed movement – throughout the body are harder to explain, and were found with a computer analysis of the motion capture data. Diaz explains them as combinations of movement coordinated over many repeated attempts to perform a task.

Here’s what he said:

“When, for example, you shift the angle of your planted foot, perhaps in an attempt to hide the direction of the kick, you’re changing your base of support. In order to maintain stability, maybe you have to do something else like move your arm. And it just happens naturally,” he said. “. If this happens over and over again, over time your motor system may learn to move the arm at the same time as the foot. In this way, the movement becomes one single distributed movement, rather than several sequential movements. A synergy is developed.”

His search turned up five reliable indicators of ball direction – two “local” indicators from sports literature, and three “distributed” indicators from the computer analysis.

Gabe’s research has developed quite a momentum over the past week, with stories by Discovery News, Scientific American and the Associated Press. It's also been translated into (at least) Italian, Spanish, and French. Also be sure to read my original story here.