Com­ment & Anal­y­sis

“THE CRACK COMES FROM THE SUD­DEN CRE­ATION OF THE CAV­ITY, AND IT’S LOUD BE­CAUSE IT’S A VI­O­LENT PROCESS”

BBC Earth (Asia) - - Contents - Dr He­len Cz­er­ski is a physi­cist and BBC science pre­sen­ter

He­len Cz­er­ski on crack­ing knuck­les

Along with pet­ting pup­pies and hav­ing just one more cho­co­late, I’ve added another thing to my list of temp­ta­tions that hu­mans can’t re­sist: knuckle-crack­ing. I men­tioned this odd habit in a talk that I used to give quite reg­u­larly, and al­most as soon as the words were out of my mouth, the pop­ping from the au­di­ence be­gan.

In the most ex­treme cases, usu­ally large au­di­to­ri­ums full of teenagers, it would grow like an avalanche: a few lone snaps at the back of the room rolling for­wards in an im­pres­sive crescendo as ev­ery­one else joined in. But fin­ger joints are tiny, and the process of crack­ing (pulling on a fin­ger joint un­til it pops) is rel­a­tively gen­tle. What’s mak­ing this dis­tinc­tive sound and why is it so loud?

The first step is a bit of anatomy. A joint is where two bones meet. The ends of your fin­gers’ bones are cov­ered with a layer of car­ti­lage, but be­tween the two car­ti­lage lay­ers is a cav­ity filled with gloopy stuff called syn­ovial fluid, which lu­bri­cates the joint as the bones move. Hav­ing that fluid-filled cav­ity there means that the joint can lengthen a lit­tle bit – there’s no rigid struc­ture hold­ing the whole thing to­gether. The would-be knuckle-cracker pulls on one of the bones, ap­ply­ing a force that could stretch the joint. The in­ter­est­ing thing is that nothing hap­pens im­me­di­ately. To pull the bones fur­ther apart, you need some­thing that’s go­ing to fill the new space you cre­ate. And then… pop! There it is: a bub­ble.

This does sound a bit like magic, but the gas was there all along, dis­solved in the syn­ovial fluid. Liq­uids don’t stretch much, so when you put a liq­uid un­der ten­sion, the only way for it to ex­pand sig­nif­i­cantly is for it to pull apart and cre­ate a hole. Then gas that was dis­solved in the liq­uid will es­cape be­cause the pres­sure has dropped (a bit like gas com­ing out of so­lu­tion in a fizzy drink when you take the lid off). The crack comes from the sud­den cre­ation of the cav­ity, and it’s loud be­cause it’s a vi­o­lent process.

The bub­ble will sit there un­til the gas re­dis­solves, which is why the mu­si­cal con­tri­bu­tion of my au­di­ences was short-lived. Once you’ve al­ready got a bub­ble, gas can move from the liq­uid to the bub­ble quite eas­ily, and if you pull on the joint, the bub­ble will just ex­pand slowly to fill the gap. It takes about 20 min­utes for the bub­ble to dis­solve, and you won’t get another crack un­til you’re able to form another bub­ble from scratch.

But the rea­son I men­tioned knuckle-crack­ing in my talk at all was to get to the tale of Don­ald Unger, who won an Ig No­bel Prize in 2009 for his long-term and very per­sonal in­ves­ti­ga­tion of the topic. Hav­ing been told as a child that this ‘bad habit’ of crack­ing his knuck­les would lead to arthri­tis, he cracked the knuck­les on his left hand (but not his right) twice a day for 50 years – just to see what hap­pened. At the end of that time, he didn’t have arthri­tis in ei­ther hand, and sub­se­quent med­i­cal science has backed up the idea that it won’t do you any harm.

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