I have been asked this question a surprising amount of times over the years! After being asked it again a couple of days ago, I thought I’d look into it and write a post about it here.

Knuckle cracking occurs when you lace your fingers together, turn your palms so they are facing away from you, and then bend the fingers back. You are all probably familiar with the loud ‘popping’ sound that results, and the habitual knuckle crackers out there are no doubt familiar with the “stop that or you’ll get arthritis” retort!

Why does the cracking occur?

Some of the articles and papers I used for this research seemed fairly certain of the cause, whilst others were more tentative, claiming we don’t know for certain. I’ll outline the most cited cause anyway. Synovial joints (such as the interphalangeal and metacarpophalangeal joints in the fingers) are surrounded by connective tissues such as the joint capsule, tendons and ligaments. Between the articulating bones within a joint, a thick liquid called synovial fluid resides, encapsulated by the joint capsule. Within the synovial fluid a number of gases are dissolved including oxygen, nitrogen and carbon dioxide.

When we crack our knuckles we are causing the joint capsule to be stretched, thereby increasing the volume and decreasing the pressure within the joint. As pressure within the joint decreases, gases that are dissolved within the synovial fluid become less soluble and form bubbles (a process called cavitation). When you stretch the fingers far enough, the pressure within the joint decreases so far that these bubbles actually burst, which many attribute as the cause for the popping sound.

It takes time to re-dissolve the gases into the synovial fluid (called the refractory period) – many different times were cited, ranging from a few seconds to hours. During this period, it is stated you won’t be able to ‘pop’ your knuckles again.

Are any other reasons suggested?

Another explanation for the popping sounds is the sudden snapping of a tendon returning to its original position with respect to a joint, after it has been forced to shift during a movement. The ‘snapping’ of a tendon or scar tissue over a protuberance (something sticking out) can also purportedly create a ‘popping’ sound. Other reasons posited include the rapid stretching of ligaments as you move a joint, or intra-articular breaking of adhesions (think of these as scar tissue within a joint).

Does it actually cause arthritis?

There is a dearth of research on this topic; but what there is appears to conclude there is no apparent correlation between knuckle cracking and osteoarthritis, although signs of damage were found for habitual knuckle crackers including soft tissue damage to the joint capsule and a decrease in grip strength. I obviously don’t advocate the cracking of knuckles!

Should I worry about any of my joints ‘popping’ or ‘cracking’?

The consensus seems to be the only time to start worrying is when the ‘popping’ is accompanied by joint pain or swelling, or if you are losing joint mobility or function; in these instances, medical attention should be sought.

Why do people do it?

Many reasons are posited, including a liking for the sound or a penchant for the resultant cringing that invariably follows from others (my personal favourite!) Some assert that the action of knuckle cracking causes muscles around the cracked joint to relax due to activation of Golgi tendon organs, increasing finger mobility (due to released gases increasing joint volume by 15-20%), making them feel looser and invigorated.

Can we crack any other joints?

Joints reported as being able to crack, either deliberately or as a natural consequence of moving, are your hips, knees, ankles, toes, wrists, elbows, jaws, neck and back.