Hinge joint

A hinge joint is simple in that it allows movement in only one plane, rather like the hinge on a door. An example of this type of joint is the elbow joint.

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Ball-and-socket joint

This is perhaps the most complex joint in the body, and allows movement in several directions. In it, one bone has a rounded knob at its end, and this fits into a cup-shaped socket in another bone. The two bones have a layer of cartilage, and between the surfaces is synovial fluid to reduce friction to a minimum. Examples of this type of joint include the hip or shoulder joint.

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Antagonistic muscle pairs

Muscles can pull by contraction, but they are not able to push. This means that muscles often work in pairs to move bones at joints. A good example of such a pair of muscles is in the upper arm where the biceps and triceps produce movement in opposite directions. These pairs of muscles are called 'antagonistic muscle pairs'. Since they pull in opposite directions, they cannot both contract at the same time, or no movement would take place. The nervous system makes sure that as one muscle contracts, the other relaxes - and vice versa. In other words, the brain coordinates the actions of these muscles.

Find out more about bones and muscles

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