Tissues in Action
Bones, joints and the skeleton
See how bones, joints and muscles work together.
Bones, muscles, joints, cartilage, tendons and ligaments together form the musculoskeletal system. It lets you stand, move and keep your posture, and it protects your organs, all under the control of the nervous system.
"How does a bone move?" A muscle can only pull, never push. When it contracts (shortens), its tendon passes the pull to the bone, and the bone moves at a joint. A joint is simply where two or more bones meet; it can't move anything by itself.
How heavy is your skeleton? In an adult, bones are about 12 to 15 per cent of body weight. Muscle is about 40 to 50 per cent in men and 30 to 40 per cent in women.
Joints are named by the movement they allow:
| Joint | Where | Movement | Try it |
|---|---|---|---|
| Ball and socket | Shoulder | Forwards, backwards, sideways, in circles | Bowl a cricket ball |
| Hinge | Elbow, knee | Bends and straightens one way, like a door | Bend your elbow |
| Pivot | Where the skull sits on the backbone | Turns side to side, like a doorknob | Shake your head "no" |
| Fixed | Between the flat bones of the skull | No movement | Keeps the brain safe |
The kneecap protects the knee joint.
The backbone (vertebral column) is a chain of small bones called vertebrae, with discs of cartilage between them for cushioning. Because it is made of many small pieces, you can bend. A single straight bone would leave you stiff.
The rib cage has 12 pairs of ribs, joined to the backbone at the back and the breastbone (sternum) at the front by bendy cartilage. It protects the heart and lungs, and it moves out and in as you breathe.
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