So what's going on with these flexible folks? What people chalk up to double-jointedness is actually called hypermobility or joint laxity. People with this characteristic can move certain joints farther than most people, and they can do it without preparation or pain. While some people with hypermobility are able to stretch farther due to loose ligaments, others get their flexibility from the shape of their bones. Ball-and-socket joints (like hips and shoulders, for example) allow for the largest range of motion. If the socket is a little on the shallow side, the ball part of the joint can twist farther. Check out the video below for more on the myth of double-jointedness.
It Is Impossible For A Human To Be Double-Jointed
Even if you've never claimed to be double-jointed, you've definitely heard someone else brag about it. But we've got news for those people proudly cranking their thumbs backwards to touch their forearms: there's no such thing as being double-jointed. The idea that some people have two joints in certain places on their bodies that increase flexibility is not only virtually impossible, but it actually sounds kind of silly, if you think about it...
Explaining The Myth Of Double-Jointedness
Though you may be super flexible, you definitely do not have twice as many joints.
from Tech Insider
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Why Your Joints May Hurt When It Rains
Is this really a thing, or just in your head?
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Does Cracking Your Knuckles Cause Arthritis?
It's another myth you've heard, but it's time to bust it.
from Today I Found Out
Key Facts In This Video
No study has ever shown a link between cracking your knuckles and any form of arthritis. (0:25)
There are no muscles in the fingers. Movement is controlled by forearm muscles pulling on finger tendons. (1:12)
When you crack your knuckles, you stretch out the synovial capsule, creating a negative pressure inside the joint. This pulls the gases out of the synovial fluid in the form of a bubble, which pops to create the cracking sound. This is why you have to wait to crack your knuckles again: you have to wait for more gases to build up. (1:38)