Robert Drewe on knuckle crackers, and other important issues…


Robert Drewe looks at the strange art of becoming a crack knuckle-cracker and the even stranger disease High Altitude Flatus Expulsion (HAFE) – it’s a thing…

Has the sound of someone cracking their knuckles been a source of lifelong bafflement to you? Me neither. But the other day I read that the knuckle-popping noise that some men enjoy making (never a woman, in my experience) has always perplexed scientists.

Seriously? A topic of hot scientific debate? Anyway, never get between a well-funded research team and one of life’s lesser mysteries. It seems researchers have been working on the puzzle for years and are finally heading towards an answer. More or less.

The team from the Montreal Ecole Polytechnique‘s hydrodynamics laboratory first thought they had the solution in hand in 2015 after images were taken of one of their researchers cracking his knuckles in an MRI scanner. They discovered not only that not all knuckle joints can crack, but those that do crack can only crack once every 20 minutes.


A Eureka moment? According to Professor Abdul Barakat, co-author of the new study: “The cavity between two knuckles is filled with synovial fluid, and when you suddenly change the pressure in that fluid as a result of increasing the space between the knuckles, some of the gases in the fluid nucleate into a bubble.”

The verdict: the sound was caused by the rapid separation of the joint and the bubble formation. The team thought it was the collapse of such bubbles, formed of carbon dioxide and other gases, that caused the well-known crack.

But no. This year the researchers, working with a band of volunteers (all crack knuckle-crackers), delved deeper into the mystery and found that the formation of knuckle bubbles didn’t produce cracks of the observed magnitude and volume.

Alas, current imaging techniques couldn’t capture the high-speed dynamics of knuckle-cracking, so the team developed a mathematical model to explore whether collapsing knuckle bubbles were indeed behind the sound.

“What we find now is that you don’t need the full collapse of a knuckle bubble,” explained Professor Barakat, pointing out that even if the knuckle bubble just partially collapses, leaving a knuckle micro-bubble, it would still generate the knuckle crack.

Dr Greg Kawchuk from the University of Alberta, co-author of the original knuckle-popping study, doesn’t agree. “Their main finding, that theoretical knuckle-bubble collapse can create sound, is not surprising. But the case is not yet closed. The latest research is a mathematical model yet to be verified by experiment.”

While the knuckle-crack debate still rages across Canada, there’s another knuckle-cracking argument about whether the habit increases the risk of arthritis. Most studies point out that the father of knuckle-crack research, the Californian Dr Donald Unger, reported (at the age of 83, in 2009) that for 65 years he’d cracked his left-hand knuckles while leaving his right knuckles uncracked. Dr Unger proudly reported no sign of arthritis in either hand.


Incidentally, not everyone can produce a knuckle crack. “Some people cannot crack their knuckles because the space between their knuckles is too large,” explained Professor Barakat.

But for those of you who desire to join the crackers, or can crack already and enjoy the sensation (though I don’t wish to sit beside you at any gathering) he has a tip: “The more rapidly you pull on your knuckle, the faster you’re changing the pressure and the more likely you are to generate a knuckle crack.”

While on the subject of bodily gas mysteries, the International Society of Travel Medicine reported recently on a common condition called High Altitude Flatus Expulsion (HAFE), which might make air travellers feel less guilty about in-flight impulses.


The society’s president, Dr David Shlim, says HAFE is in-flight flatulence, a condition first observed in mountain climbers at high altitudes. This increased tendency to pass gas on a plane is not your fault. And don’t blame the airline food.

“Aircraft cabins are pressurized to between 6000 and 8000 feet, a significant altitude change for your body if you’ve come from sea level,” said Dr Shlim. “Just as the air in your water bottle expands at higher altitudes, the gas in your intestines expands on a plane, taking up about 30 per cent more room than usual. Obviously it needs to escape.”

Yes, even if it causes an international incident, as happened on the Dutch airline Transavia last month. Transavia was forced to abort a flight from Dubai to Amsterdam when a passenger constantly passing gas caused a commotion on board.

An argument ensued between complaining passengers and the airline crew, forcing the plane to make an unscheduled landing in Vienna. Local police and police dogs rounded up the gas protesters and ejected them from the plane. Four of them were banned from flying Transavia again.

The passenger with the flatulence problem, now with several rows to himself, was allowed to remain on the flight.

Robert Drewe’s latest book is The True Colour of the Sea





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