What Makes That Popping Sound?

If you’re somebody who likes to crack your knuckles before tackling a tough task, listen up. A study led by researchers at the University of Alberta has cracked the secret behind the popping sound knuckles make when they’re cracked.

The secret to the sound is a momentary, vacuum-like gap formed in the lubricating synovial fluid between the finger and knuckle bones. When the joints are pulled apart too far, a momentary gas-filled gap forms in between, triggering the sound.

Lead author and U of A professor Greg Kawchuk says the “pull my finger” study could lead to future breakthroughs in joint health, including better analysis of spinal injuries, knee problems and other joint difficulties.

“It may be that we can use this new discovery to see when joint problems begin long before symptoms start,” Kawchuk said in a statement.

Kawchuk’s international team of scientists used MRI scans to watch a knuckle crack in super-slow motion. A video of the MRI shows a momentary pocket of darkness between the knuckle and finger bones as they are pulled apart.

“You can actually see very clearly what is happening inside the joints,” said Kawchuk.

The MRI also revealed a brief flash of white just before the cracking sound occurs. Kawchuk says no one has ever observed that flash before, and he hopes to analyze it further with more powerful MRI machines in the future.