Image caption Maria Sharapova is notorious for her grunting

The loud grunts of some tennis players can give them a real advantage over opponents, a scientific study says.

Canadian and American researchers said tests had shown that "extraneous sound interfered with participants' performance, making their responses both slower and less accurate".

Some top tennis stars, including Martina Navratilova, regard grunting as unfair, or even as cheating.

Maria Sharapova and Rafael Nadal are among the game's big grunters.

Speed and spin

The study, which appeared in the Public Library of Science ONE journal, tested 33 students at the University of British Columbia in western Canada.

Hundreds of video clips were shown of a player hitting a ball to either the left or right. The students had to determine the direction quickly, but on some shots were subjected to noises simulating grunting.

Lead report author Scott Sinnett told the BBC: "The findings were unequivocal. Basically, when the video clips did have a grunt, the participants were not only slower to react but they had lower accuracy levels. So they were basically slower and could actually be wrong-footed, if you could extend that to a real-world tennis court."

The report said the grunt could also hamper a receiver who was trying to judge the spin and speed of a ball from the sound made off the racket.

Mr Sinnett said: "The study raises a number of interesting questions for tennis. For example, if Rafael Nadal is grunting and Roger Federer is not, is that fair?"