Tennis players who grunt loudly when they hit the ball appear to have a competitive edge over their opponents, according to a study published on Friday.

The noise accompanying a hard shot makes an opponent slower to respond and more likely to misjudge exactly where the ball is going -- so it is tougher to hit it back, said Canadian and American researchers.

"Conservatively, our findings suggest that a tennis ball traveling 50 miles per hour (80 kph) could appear 24 inches 2 feet (60 cm) closer to the opponent than it actually is," said Scott Sinnett, an assistant at the University of Hawaii.

The researchers tested their theory on students in a laboratory at the University of British Columbia, using sounds that were comparable in volume to grunts of tennis stars Maria Sharapova and Rafael Nadal.

The results were published in the online issue of Public Library of Science ONE.

Sinnett and his colleagues say there are several possible explanations for why grunting has an effect.

Some professional tennis players try to judge the spin and velocity of a ball from the sound it makes hitting a racket, so a loud grunt would mask those clues, while also serving as a general distraction, the researchers suggested.

Grunting is a controversial subject in tennis circles, with nine-time Wimbledon champion Martina Navratilova having called it "cheating pure and simple."

"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?" Sinnett said.

A Wimbledon match this year between Serena Williams and Portuguese teenager Michelle Larcher de Brito was described as a "decibel Derby" for all the noise the players were making.

Sinnett said the researchers now planned to look at whether the world's top tennis players had developed strategies to mitigate the effects of their opponents' grunts