Where tendons lie close to bone, tiny sacs called bursae sit between the tendon and the bone to reduce friction. Muscles are attached to bones with thick, tough bands of connective tissue called tendons.
In freely movable joints, the entire joint is enclosed inside a membrane filled with lubricating synovial fluid, which helps to provide extra cushioning against impact. Smooth cartilage prevents friction as the bones move against one another. Joints are held together and supported by tough bands of connective tissue called ligaments.
Allow rotation without displacing the bones: for example, the head as it turns from side to side swivels the cervical vertebrae on top of one another. Allow a circular movement – this is how ball and socket joints work: for example, the shoulder. Increase or decrease the angle between the two bones – this only occurs in the long bones of the body (arms and legs): for example, when the arm is bent or extended. Slide one broad and flat surface across another – examples include the bones in the wrist or ankle. Smooth surfaces slip over one another, allowing limited movement, such as the wrist joints. Pivot joint – one bone swivels around the ring formed by another bone, such as the joint between the first and second vertebrae in the neck. Condyloid joint – this permits movement without rotation, such as in the jaw or finger joints. Hinge joint – the two bones open and close in one direction only (along one plane) like a door, such as the knee and elbow joints. Saddle joint – this permits movement back and forth and from side to side, but does not allow rotation, such as the joint at the base of the thumb. Ball and socket joint – the rounded head of one bone sits within the cup of another, such as the hip joint or shoulder joint. The six types of freely movable joint include: Freely movable – most joints within the human body are this type. Slightly movable – two or more bones are held together so tightly that only limited movement is permitted – for example, the vertebrae of the spine. The joints of the skull are called sutures. Immovable – the two or more bones are in close contact, but no movement can occur – for example, the bones of the skull. 
The three broad classes of joints include: Joints are described by how much movement they allow. This is because greater range of movement reduces the strength of the joint. Generally speaking, the more movement that is possible through a joint, the higher the risk of injury. A joint is also known as an articulation. The shape of a joint depends on its function. Every bone in the body – except for the hyoid bone in the throat – meets up with at least one other bone at a joint. A joint is the part of the body where two or more bones meet to allow movement.
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