Does the pop when you crack your knuckles come from the bones?

At a finger joint, the ends of two bones are enclosed by a pouch-like membrane filled with lubricating joint fluid. When you pull or bend a finger, the bone ends first cling together and resist, then suddenly separate at a certain point. The pressure inside drops sharply, gas dissolved in the fluid comes out, and an empty space forms.

In 2015, researchers at the University of Alberta in Canada pulled on 10 finger joints of one man while filming them with real-time MRI. In all 10, a dark empty space appeared inside the joint at the moment of the pop, and it remained there after the sound.

So does the sound come when the gap forms, or when it collapses? In 1971, British researchers concluded that the pop comes from a bubble collapsing, while the 2015 MRI study leaned toward the moment the gap forms. In 2018, a mathematical model showed that even a partial collapse of the bubble could produce a sound similar in loudness and pitch to the real thing, so the debate continues.

A joint usually won't pop again if you try right after cracking it. In a 1947 experiment, it took about 20 minutes before it would pop again.

People often say that cracking your knuckles a lot causes arthritis. One doctor recorded that he cracked only his left hand for 60 years and left his right hand alone, and neither hand developed arthritis. One person's experience is weak evidence, but a larger study found something similar.

In a 2011 US study of 215 people aged 50 to 89, the rate of hand arthritis was 18.1% among habitual knuckle crackers and 21.5% among those who didn't crack their knuckles, with no clear difference. The same held when the researchers looked at how many years and how often people had cracked their knuckles.

You can worry less about arthritis, but a joint doesn't need to pop to feel loosened up. If you just want to ease stiffness, slowly making and opening a fist or rolling your wrists is enough.

What to watch for is pain. If cracking hurts, a joint swells, or the popping started after an injury, it may be more than a simple bubble sound, so it's worth seeing a doctor.