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GRUNTING, THAT LOUD, sometimes obnoxious noise produced when we exercise — has been an ongoing source of debate among psychologists.

From a scientific standpoint, the grunting noise is made as we “exhale against a closed, or partially closed, vocal fold,” said Dennis O’Connell, a professor of physical therapy at the Holland School of Sciences & Mathematics in Texas.

The vocal folds, or vocal cords, refer to the two bands of muscle tissue that open into the windpipe. The vocal folds are open and relaxed when we breathe in, sometimes producing a rushing noise. But when the vocal folds close as we exhale “you are going to hear some turbulence,” said O’Connell.

The reason we grunt is slightly more complicated. Some experts say it improves performance; others say it’s just blowing smoke.

Belisa Vranich, a psychologist at willspace, a boutique training studio in New York City, believes grunting is a natural and necessary physical response to exerting effort.

“Think about tennis,” she said. “When you hit the ball, it’s hard to be explosive and not grunt.”

And if you’re not grunting, you should be, because it gives you more power. “It helps you push more weight, hit harder, and throw farther,” said Vranich.

When we do something like squats or power lifting, we take air in and hold our breath for a moment. By doing that, the middle of our body gets squished together, creating a pressure ball in our belly that makes the body rigid in order to stabilize and protect our spine from injury, said Vranich.

All that pressure built up inside our gut means that when we finally do exhale “it’s almost impossible to let that energy out without making a grunt,” said Vranich.

Scientists don’t know exactly why a sharp yell or grunt gives people that extra oomph when they do things like lift weights or swat at a tennis ball, but it’s probably related to a communication signal from the part of the brain that controls breathing to the part that controls muscle function, said O’Connell.

When we forcefully push air out, the brain sends information down to the muscles, which either excites muscle groups or decreases inhibition — or both. The result is enhanced performance.

The new gym at Ulster Rugby's redeveloped facility. Presseye / Darren Kidd/INPHO Presseye / Darren Kidd/INPHO / Darren Kidd/INPHO

This beneficial effect of grunting was demonstrated in a study of college tennis players. O’Connell’s students found that grunting increased serve and forehand velocity by an average 4.5 mph. It didn’t matter if the players were regular grunters, if they were male or female, or how they felt about grunting.

Although grunting seems like a natural reaction, Vranich said unnecessary or excessive grunting is annoying. That kind of testosterone-pumped animalistic sound designed to psych the person up and get adrenaline going — or even intimidate a competitor — is rooted more in psychology than in physiology.

The controversy over whether grunting should be allowed or discouraged is growing, particularly at the gym and on the tennis court, where it can be distracting.

In 2006, a bodybuilder was kicked out of a Planet Fitness for grunting, which violated the club’s strict “no-grunting policy.” Shortly after that incident, a man was attacked at an Equinox gym in New York City for being too noisy in spin class.

O’Connell recognises that grunting maximises force production. He also believes that all the hooting and hollering can be controlled, and that there are quieter breathing techniques for achieving comparable muscle activity and peak force.

A recent study from O’Connell’s department found that in a simulated forehand stroke, athletes got the same power boost from grunting as they did by forcefully expelling air.

“Forced exhalation without the annoying sound is just as good at increasing force production as exhaling with the annoying sound,” said O’Connell.

O’Connell offered an anecdote from his daughter’s softball games, where he would overhear dads telling their kids to grunt. If you can be coached to grunt, he said, then you could certainly be coached to exhale without grunting.

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