At any party, there's always that guy who can bend his fingers freakishly far back, and then brags about being "double-jointed" but there's actually no such thing.
The term double-jointed implies that a person with unusual flexibility has twice the average number of joints, which allows for their increased range of motion. But that's anatomically impossible except for rare instances, the majority of people have the same number of joints, the points in the body where two bones meet .
People with "double-jointedness" actually have hypermobility syndrome, a condition that allows them to move a bone within a joint to its fullest capability, but without experiencing the pain and discomfort that the average person experiences when extending a joint beyond its normal range.
What makes a person seem double-jointed can actually have more to do with the soft tissue that's near the joints than the actual joints themselves. Most joints are wrapped in ligaments, which connect bone to bone, and tendons, which connect muscle to bone . Everyone's joints have the same range of motion, but the flexibility of a person's ligaments and tendons determines a person's ability to, say, flex at the hips in order to touch their toes.
The body has several types of joints, which range from the immovable joints of your skull to highly mobile joints called synovial joints. The ball-and-socket joint is the most mobile synovial joint and involves a domed bone that rolls inside a curved socket. The ball-and-socket joints in your shoulders and arms are what give your arms and legs such a wide range of motion.
A joint's shape can also influence a person's range of motion. Some people with hypermobility syndrome are born with ball-and-socket joints that have unusually shallow sockets, which allow the domed bone more mobility. The shallower a socket, the more "give" it provides to the domed bone, which increases that joint's mobility.
In some cases, the "ball" part of the joint can be moved partially or completely out of its socket, which is how some people are able to purposely and, for people with hypermobility, painlessly dislocate their shoulders. Now there's a cool party trick.
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