*What does it mean to be double jointed? - Gen, aged 11, Melbourne, Australia.
If you're double jointed, it means you have a joint that can bend a lot more than in the average person. This has its upsides and downsides: apart from being a great party trick to show to friends, it might also mean you get injured more easily.
To understand what it means to be double jointed, it's helpful to know what a joint actually is and why we have joints. Let's start with the basics: a joint is any place in your body where bones touch each other. As you have more than 200 bones, you have lots of joints too.
Since bones cannot bend, joints give us a way to move our body. Every joint is different, but most joints can only bend a certain amount. For example, your shoulder joint can swivel all the way around so you can move your arm in a circle, while the joints between the 26 bones in your foot only move a small amount.
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How much each joint can move depends on the shape of the two bones touching each other, and the way they are connected. To see how the shape of the bones makes a difference, you can try an experiment with two tennis balls.
When you try to balance one ball on top of the other, the top ball will roll off easily. Now, try to balance one ball on the palm of your hand - the ball won't roll off so easily now. And if you make your hand into a cup, and hold the ball in there, it probably won't roll out at all.
This shows that the shape of the bones matters, because it affects how much a joint can move. So joints with a cup-shaped bone will move a lot less than joints with two round bones. But it also shows that some joints are quite wobbly, and could be quite difficult to control.
Limited by ligaments
That's where ligaments give our joints extra support. A ligament is a bit like a string or a rubber band, that attaches to both of the bones in a joint, to keep them in the right place and stop you from bending the joint too much. That way, you don't end up falling forward from your knees, or rolling sideways on your ankle.
Working together, the bones and ligaments limit how much your joints can move. If you are double jointed, it might mean that the bones are round at the end, like two balls, or that the ligaments which help hold the bones in place are loose, or even absent.
Being double jointed might mean you get injured more easily. For one thing, too much bending can dislocate a joint - that means the bones in the joint don't touch anymore, because the ligaments are too loose. Also, when a joint bends too much, the bones often get more wear and tear, which can cause pain.
You might still be wondering why people call this being "double jointed" - after all, it's not like the extra bendiness comes from having an extra joint. It actually just means that double jointed people are doubly bendy. That's why it looks so odd: when you have a double jointed joint, you can bend it in directions and positions most other people can't.
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Author: Marco Arkesteijn - Lecturer in Sport and Exercise Biomechanics, Aberystwyth University