NEW YORK • Emitting a high-decibel grunt or squeal while striking a backhand or uncorking a swirling roundhouse kick could substantially increase the power of that movement and sneakily bedevil your opponent.

A revelatory new study finds that yelling during sports could have greater benefits for performance than many of us might expect, even if it might cause spectators to look aghast and cover their ears.

Grunting during physical activities that demand sudden short, sharp bursts of power, such as weight training, probably has been used since our prehistoric ancestors hefted stones.

Tennis players are particularly famous for grunting. Their shouts with each serve and stroke can be so loud that some players, including Martina Navratilova, have declared that the noise is a form of cheating, meant to confound the other player and not to increase the power of a stroke.

But there has been limited past scientific scrutiny of the impact of grunting. A 2014 study of college tennis players determined that grunting could indeed increase the power of players' groundstrokes and serves.

And a study from 2010 found that such sounds can be distracting for others. In that study, participants watched videos of a tennis player striking the ball while a loud, gruntlike yell played or did not. The viewers pressed keys to indicate their snap judgments about which side of the court, right or left, the ball was heading towards. When the stroke coincided with the noise, the viewers were noticeably slower and more inaccurate in their picks.

But those studies could not determine whether the usefulness of grunting was confined only to tennis or how the ballistic squawks were affecting onlookers and opponents. Were the screams masking the sound of the racket striking the ball, making it difficult for people to judge the right trajectories? Or were the sounds more directly distracting people, drawing their attention away from the onrushing ball and befuddling their reactions?

To learn more, some of the same scientists decided, for the new experiment, which was published last month in PLOS One, to look closely at mixed martial arts and grunting.

They chose that sport for several reasons, the first being that, like tennis, it demands sudden, explosive movements, meaning punches and kicks, to which grunting conceivably could add power.

Perhaps even more important, martial arts moves do not involve inherent noise, unlike the ping of a tennis racket meeting a ball. So if an onlooker proved to be worse at judging a fast-approaching kick when someone grunted, it would be because the yell had directly confused the watcher, not because it had masked some other noise.

The researchers began by recruiting 20 local recreational martial arts practitioners, both male and female, and having them kick a specially prepared heavy bag. The bag contained a device that measured force.

In separate sessions, the athletes kicked the bag multiple times while producing a mighty, oomphing yell or remaining silent.

The researchers also videotaped some of the kicks in close-up.

Then they gathered 22 undergraduate students and had them watch the videos, which showed the athletes' feet advancing towards the viewers at intimidating speed.

The students had to rapidly decide whether the kick would land high or low and press a corresponding computer key.

During about half of the kicks, a sound like a grunt (standardised to avoid changes in volume or pitch) accompanied the motion. Otherwise the videos were quiet.

The researchers then checked all of the data. They found that grunting while kicking had definitely improved the athletes' power.

They had generated about 10 per cent more force with each kick while yelling. The noise also had affected viewers, although not favourably. They had proven to be much slower in responding and more prone to errors in judging direction when the kick had come with a grunt.

These results indicate that "the advantage that a grunter gains" in terms of impact on an opponent "are due to distraction" and not to other, useful sounds being drowned out, says Scott Sinnett, an associate professor of psychology at the University of Hawaii at Manoa, who led this study and one of the earlier experiments with tennis players.

The findings also show that grunting is beneficial for upping power, he says, which undercuts the argument that it is a kind of cheating. If it only bothered opponents, he says, it might be considered unfair.

But making a noise does amplify force and is not banned by sports governing bodies, so can be considered a useful and sporting competitive tool.

Of course, this experiment was conducted in a laboratory, not a real-world competition, and with martial arts athletes. The results do not show whether grunting loudly in other situations and sports would produce the same results.

It is unlikely, for instance, that grunting would be beneficial in long-distance running or walking, which require little explosive force, Dr Sinnett says.

This study also cannot tell us whether consciously deciding to scream during sports would be beneficial, if noise is not natural to you. You might wind up distracting yourself with your grunts and playing worse, Dr Sinnett says.

NYTIMES