Joints and Movement
Senior 2 · Biology
Joints and Movement
Lesson 1 of 3
22 min
Let's begin
Kicking a football, writing in your exercise book, dancing and carrying a jerrycan of water all depend on your joints. The adult skeleton has 206 bones, and wherever two of them meet there is a joint. Some joints do not move at all, some move a little, and some move freely in many directions. Their structure explains the difference.
Worked example
Compare how much movement happens (a) between the bones of the skull, (b) between two vertebrae of the backbone, and (c) at the shoulder.
These three examples match the three main groups of joints. A joint is a place where two or more bones meet.
| Type | How the bones are joined | Examples |
|---|---|---|
| Fixed (immovable), also called fibrous joints | tough fibrous tissue holds the bones tightly together; no cavity | the sutures between skull bones; teeth held in their sockets |
| Slightly movable, also called cartilaginous joints | a pad of cartilage between the bones | between the bodies of vertebrae (the intervertebral discs); where the two hip bones meet at the front |
| Freely movable, also called synovial joints | a fluid-filled cavity between the bones | shoulder, hip, elbow, knee, wrist, fingers |
| Part | Function |
|---|---|
| Articular cartilage | smooth, slippery tissue covering the ends of the bones; reduces friction and absorbs shock |
| Synovial membrane | lines the capsule and secretes synovial fluid |
| Synovial fluid | lubricates the joint so the bones move smoothly, and nourishes the cartilage |
| Fibrous capsule | tough outer covering that encloses the joint and holds the fluid in |
| Ligaments | strong, slightly elastic bands that join bone to bone, holding the joint together while allowing movement and preventing dislocation |
Do not confuse ligaments with tendons, which join muscle to bone (you will meet them in the next lesson).
| Type | Shape | Movement | Examples |
|---|---|---|---|
| Ball-and-socket | a rounded head fits into a cup-shaped socket | in all directions, including rotation | shoulder, hip |
| Hinge | a rounded end fits into a hollowed end, like a door hinge | in one plane only: bending and straightening | elbow, knee, joints between finger bones |
| Pivot | a rounded or peg-like part turns inside a ring of bone | rotation around one axis | between the first two neck vertebrae (turning the head to say "no"); between the radius and ulna (turning the palm up and down) |
| Gliding (plane) | flat surfaces | small sliding movements | between the small bones of the wrist and ankle |
| Saddle | two saddle-shaped surfaces | back and forth and side to side | base of the thumb |
| Condyloid | an oval end in an oval hollow | up and down and side to side, no rotation | wrist, knuckles |
Worked example
Why can you swing your arm in a full circle at the shoulder but only bend and straighten it at the elbow? Reasoning: at the shoulder, the ball-shaped head of the humerus sits in a cup-shaped socket, so it can turn in any direction. At the elbow, the curved end of the humerus fits into a groove in the ulna, which allows movement in one plane only, like a door on its hinge.
Tip
Ligament Links bone to bone; Tendon Ties muscle to bone.
Ball-and-socket = all directions; hinge = one plane; pivot = rotation; gliding = sliding. If asked to name the joint AND give an example, give both, for example "hinge joint, as at the knee".
Name the type of joint at (a) the hip, (b) the knee, (c) between the skull bones.
Solution: (a) ball-and-socket; (b) hinge; (c) fixed (fibrous) joint, called a suture.
What would happen to a synovial joint if it produced too little synovial fluid?
Solution: The joint would be poorly lubricated, so friction would increase. The cartilage would wear away and movement would become stiff and painful.
Why is it an advantage for ligaments to be slightly elastic?
Solution: They hold the bones together firmly but can stretch a little, allowing the joint to move without the bones separating.
Why are the joints between vertebrae only slightly movable?
Solution: The backbone must be flexible but also protect the spinal cord inside it. A pad of cartilage between vertebrae allows only small movements, which add up to bending of the whole spine without damaging the cord.
Remember
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