Crack + Joints

Your mama told you to steer right clear, but it felt so good - so relaxing - that you just couldn't stop. Next thing you know, you're doing it at the dinner table, on dates, in meetings ...

For some of us, this is the familiar start of a life of sidelong looks + stern orders to "stop it". But once you've popped that neck + cracked those knuckles, there's no turning back.

What actually happens when you crack a joint, and why does it feel so good? More importantly, is this harmful or beneficial?

"Pop!", by Roy Lichtenstein for 25 April 1966 cover of Newsweek

Knowing your clicks from your cracks

To be clear, we are not talking here about clicking or snapping sounds produced by movements that can be repeated over and over again, which are likely tendons flicking over bones or lumps (usual in hips + knees + ankles). This form of clicking is not accompanied by a relaxing feeling of release, and has not been reported to be "addictive".

We are instead talking about the louder pop of a neck or spine, or the crack of your knuckle + finger joints that gloriously releases tensions, but like all good things that make us wait, can only be re-perform after around 20 minutes.

You'll never gas what just happened

Why does it pop? Imagine the gap where 2 bones meet. This gap is filled with synovial fluid that lubricates / cushions your bones. With the passage of time, gases are formed in the synovial fluid, and the joint feels stiff or tight. By bending or pulling the connecting bones, or through an abrupt motion, you can decrease the pressure in the synovial fluid and displace the gases - much like cracking open a can of coke. Whilst scientists agree the release is linked to the sound, there is no consensus on what actually causes the noise.

Joint cavitation diagram, Bodymotion 2016.

Whatever the reason, the result is clear - a satisfying bone fart that relieves the pressure in the joint. The gases account for some 15% of your synovial fluid, you can imagine how relaxing this must feel. The gases remain in the joint, and dissolves back into synovial fluid after 20 minutes or so, and you're ready for another pop.

For a fascinating discussion on joint-cracking techniques, and the many places in your body that you never thought could be cracked (ears, anyone?), visit jointcrackers.com.

Hard to distract me

How much strength does it take to pop a joint? It all depends. Some joints are easier to crack, and the more often you do it, the looser the joint gets, hence making it easier to crack.

"if you’ve ever washed up glass plates, you’ll know they can be hard to separate when they are wet. The film of water between them creates a tension that needs to be overcome. It’s similar with joints. When you pull on them, they resist at first, and then suddenly give way." - Dr Greg Kawchuk, leading authority on joint cracking.

Physiotherapists apply up to 14kg distraction force to restore neck and spine movements, and 8-12kg for wrists + fingers, which produces the same kind of pop or tension relief. Harvard Health cites a study where the author likened the relative force of knuckle cracking to "the forces responsible for the destruction of hydraulic blades and ship propellers". Interestingly, whist a similar force could in theory cause fractures or tissue damage, none of these were identified in people who cracked their joints.

The nay-sayers

"Crack Down", Keith Haring 1986

COLAB+ interviewed a handful of people who cringe at the sound of a joint popping for reasons why they disliked joint cracking (see dropdown). Some are down to personal opinion, some plain flimflam. We'll let you be the judge.

What the nay-sayers say:

+ Causes arthritis: This myth has (literally) been single-handedly debunked by 2009 Ig Nobel Prize in Medicine winner, Donald Unger, who cracked his left hand at least twice a day for 50 years. The result - no difference between his battered left and his control right hand for signs of arthritis.

+ Swollen knuckles: The science is contractictory. A 1990 study suggests a correlation, but more recent literature disagrees. The origins of this could be the swelling in a joint where the ligaments were damaged, and perhaps accompanied by an incidental pop when the injury was sustained.

+ Dislocated joint: "Without an intervening external force, the ability to dislocate a joint is highly unlikely" says Jay from COLAB+'s Fitness team. If 8-14kg of force is the going rate for joint popping, you'd have to be pretty determined to do yourself grave harm to be able to dislodge your joint, or pull a finger out.

+ Paralysis: If you crack your neck, you could end up paralysed. For the many who crack their necks daily, this is clearly not true, but we have not found a study that can conclusively say this is not a risk. If you'd like work done on your neck or spine, best use a qualified therapist.

+ Grip strength: Australian scientist Dr Karl Kruszelnicki suggests that habitual joint cracking can reduce grip strength by up to 75%, rendering one unable to open a jar of vegemite in one's old age. The backing for this claim is unclear, and has raised doubts amongst medical practitioners.

My physiotherapist, Daniel, says all of the above is up for debate, but in his experience, those who came in to see him for sprains had one thing in common - they did not have a habit of cracking their joints. You can of course crack joints in the privacy of your own home - in the same way people are free to chew their food with open mouths when no one's around.

Finally, Castellenos & Axelrod's 1990 study raises an interesting but undeveloped finding. Habitual knuckle cracking was associated with manual labourers with higher incomes, smoking, alcohol drinking and nail-biting. Perhaps joint cracking releases more than just the tightness in a joint, but also tension more generally? Perhaps those prone to sedentary or high stress jobs suffer particularly from stiff joints?

What are your thoughts? Let us know at colabmailbox@gmail.com or get in touch with our Fitness + Rehab team to find out more.

References:

Sample, I., "Cracked it! Scientists solve puzzle of why knuckles pop when pulled", The Guardian, 15 April 2015.

Kawchuk, G.N., Fryer, J., Jaremko, J.L., Zeng, H., Rowe, L., et al. (2015) Real-time visualization of joint cavitation. PLOS ONE 10(4): e0119470.https://doi.org/10.1371/journal.pone.0119470

Cleland, J.A., Flynn, T.W., Childs, J.D., Eberhart. S., (2007) The audible pop from thoracic spine thrust manipulation and its relation to short-term outcomes in patients with neck pain. Journal of Manual Manipulation Therapy, 2007;15(3):143-54. https://doi.org/10.1179/106698107790819828

Protopapas, M.G., Tyler, C.C., Joint cracking and popping: understanding noises that accompany articular release. Journal of American Osteopath Association: Clinical Practice. 102(5), 283-7.

Castellenos, J. & Axelrod, D. (1990) Effect of habitual knuckle cracking on hand function. Annals of Rheumatic Diseases, 1990(49), 308-309.