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How do joints work?

The shoulder joints are the bendiest in the bodyWhere the ends of two or more bones meet, they form joints. Most of these joints are movable. They let your body bend and take up different positions. The bendiest joints include the shoulders, knees, hips and elbows. Hinge joints, like the elbow and knee, work like a door hinge. They allow up and down movement, but not from side to side. The ball and socket joints in the shoulder and hip allow a wider range of movements, including circular movement.



Four different kinds of joint
A knee joint 

Inside a joint

In a movable joint, the ends of the bones are covered with a smooth, slippery substance called cartilage (or gristle). It stops the bones rubbing against each other and wearing away. An oily fluid fills the space between the bones. It keeps the joint moist, allowing it to move without friction. Movable joints are held in place by straps called ligaments.
 

Hips

The most mobile joint in the body is the shoulder joint. It allows your arm to move freely in most directions. But it is also the joint most likely to be dislocated—where the ends of the bones in the joint are shifted out of their normal positions.

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