Part of the show Science Q&A Show
When I'm cracking my knuckles back and forth, is it true that it causes arthritis? What's happening when you do this?
People have wondered about this for some time because it probably is deleterious to your joints. So what's going on here? Well when you put your joints under tension, what you're doing is reducing the pressure inside the joint. You get the little clicks because all the ligaments around the side of the joint that stabilise it get sucked in against the fluid in the joint. Because the pressure in the fluid is reduced, gas that's dissolved in that fluid can then come into one big bubble and pops into existence. The bubble takes up about 15% of the joint space that's there for the fluid to occupy normally. As this is something that's quite a big space in something that's full of fluid, it pushes all of the little ligaments that were pushing in inside of the joint back out again, and they go snap. That's the cracking sound that you hear. The energy that's unleashed in making and breaking that bubble is about 7% of the energy you'd need to damage cartilage, so it shouldn't in theory be able to cause arthritis. Some facts and figures though: there was a man called Daniel Unger who cracked the joints of his left hand for fifty years but not the joints of his right hand. He did not have any signs of his left hand being more arthritic than his right hand. There's been one other study, bit dicey, but there were 300 people who all cracked their joints for 35 years and there was no evidence of increased risk of arthritis among those people, but there grip strength was much weaker - only about 25% of what it should be.