The Science of Knuckle Cracking

The Science of Knuckle CrackingThe Science of Knuckle Cracking

Almost everyone cracks their knuckles sometimes. But did you ever wonder what makes that sound?  Dr. Jerome Fryer, a knuckle cracker himself, wondered. So he teamed up with a group of scientists to figure this out.

For the past forty years, many have believed that the popping sound comes from bubbles popping in the fluid between the joints. Now, however, scientists have a new idea. Instead of bubbles popping, some researchers reported that a bubble actually forms during the cracking sound. This new study looks at knuckle cracking using the modern medical tool, Magnetic Resonance Imaging (MRI) to watch inside the finger joints while they are cracking. The researchers learned that a gas bubble forms at the time of the cracking sound, and the bubble doesn’t pop.

Jerome Fryer can crack all his knuckles, and he volunteered to have them cracked while his hand was inside an MRI machine. The researchers took pictures and movies of each of Dr. Fryer’s ten fingers as they were cracked. One by one, a tube was placed over one of Dr. Fryer’s fingers and tightened onto the finger. The tube was pulled so that the joint cracked. The videos showed that a dark air space formed when the joints were pulled apart. You can watch it here.

The video shows that the popping sound happens when the bubble forms, and the bubble is still there after the popping sound is over. The researchers believe the bubble is formed by gases coming out of the liquid between the joints. At first, the joint fluid makes the two bone ends stick together, just like how a little water helps suction cups stick to something. When the finger is pulled with enough force, the bone ends separate quickly. This reduces the pressure in the liquid, making a bubble form.

Knuckle cracking is very common, but until now, no one really knew what was going on. Now, modern technology has answered another question about our bodies. What do you hope to learn as we keep building better machines?

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