He cracked his knuckles; you won’t BELIEVE what happens next (hint: not arthritis)



Knuckle-cracking is a divisive ritual: some people do, some people cringe and don’t.

I do- I began this satisfying staccato ritual in highschool, alongside other teen-angst palliatives like biting the inside of my cheek and making iron-on Smashing Pumpkin t-shirts.

My brother Owen is one of the many who hates knuckle-cracking. I fondly remember irritating the bejesus out of him in the car by crushing my balled fists against the headrest next to his ears, delighting in both the cathartic joint-popping and his vexed complaints that it sounded like ‘disgusting skeletons’.

However, every time I cracked my knuckles, a small pop would sound near the back of MY head: “this may seem worth it to aggravate your brother now, but what about arthritis?”

Boning up

The ‘pop’ sound of knuckles cracking is caused by small bubbles bursting in the synovial fluid that lubricates your joints. Synovial fluid fills the cavity between your bones, underneath layers of tendons and ligaments. The protective aqueous solution cushions the innermost joint, and allows it to move without friction:

via: http://thewannabescientist.com/
via: http://thewannabescientist.com/

Synovial fluid is interesting stuff- it’s about the same consistency of egg-white, and it’s secreted by the tissues and cartilage around your joints. It’s one of the few rare fluids that becomes more viscous under pressure, allowing it to cushion the joint from both friction and shock. After the stress of movement relieves, the fluid instantly becomes thinner and less viscous, allowing it to move freely around the joint.

To crack, or not

The man who first set out to prove that knuckle-cracking is harmless is named Dr. Donald Unger. Unger meticulously cracked the fingers on his left hand, but not his right, for over 60 years, likely as a rebuke of his mother’s no-crack policy. After 6 decades of daily popping, he reported that knuckle-cracking had not induced arthritis in his left hand.

Generally, the scientific community frowns on experiments of sample size 1, and it’s worth noting that this bizarre life-long undertaking is not rooted firmly in scientific principle. Perhaps Unger’s genetic disposition protects him from developing arthritis generally; perhaps he was cracking his knuckles too frequently, or too infrequently, to precipitate the condition; perhaps if he’d kept up another year, arthritis would have smote him down with a harrowing bony fist.

Though Unger’s experiment doesn’t meet the criteria of rigorous scientific experimentation, it does begin to counteract the years of falsely accusing knuckle-cracking of causing arthritis. In fact, numerous other studies have produced similar results repudiating the connection between cracking and arthritis.

HOWEVER, chronic crackers shouldn’t rejoice just yet: cracking induces mild swelling in the hands and finger joints, and likely causes reduced grip strength.


There is no evidence, scientific or otherwise, that associates knuckle-cracking with arthritis. Crack on, you crazy diamonds- just don’t complain when life gives you lemons and your hands are too swollen and feeble to make lemonade.

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