'Unnatural' swerve deceives eye

March 17, 2006

In 1997, psychologist Cathy Craig watched Brazilian footballer Roberto Carlos's strike against France, hailed as one of the greatest free-kicks of all time, writes Olga Wojtas. But her attention was caught by the goalkeeper failing to move, assuming the ball was wide, and others ducking because they thought the ball was heading towards them.

"They all seemed to get the same information and come to the same, mistaken, conclusion," said Dr Craig, a lecturer at Queen's University Belfast. Dr Craig's research, involving stars from four leading European football teams, has revealed that the human eye is not equipped to track the curved course of a spinning ball struck at high speed.

As the World Cup approaches, her findings will provide encouragement for players such as David Beckham to beat opposition goalkeepers by putting spin on the ball.

Dr Craig, who began her project as a member of the Institute of Movement and Perception at the University of Aix-Marseille II in France, won funding from the Adidas Innovation Team for Football. Adidas-sponsored teams AC Milan, Olympique de Marseille, Bayer Leverkusen and Schalke 04 provided 11 players and nine goalkeepers to judge a series of simulated free-kicks through head-mounted virtual reality display units. Dr Craig discovered that the footballers were unable to gauge the trajectory of curved free kicks.

She concludes that we are not equipped to be attuned to fast-moving, unpredictable trajectories, since these do not occur in nature. While the force of gravity is constant, balls struck with spin are subject to a change of airflow, which creates a "Magnus force", making them curve.

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