The skeleton
The skeleton does four main jobs: it supports the body and gives it shape, protects vital organs (the skull guards the brain, the ribcage the heart and lungs), allows movement by giving muscles something to pull on, and makes blood cells in the bone marrow. Bone is a living tissue, strengthened by calcium and phosphate.
Joints
Bones meet at joints. A hinge joint (elbow, knee) allows movement in one plane, like a door; a ball-and-socket joint (shoulder, hip) allows movement in many directions. At a joint, cartilage cushions the bone ends, synovial fluid lubricates them, and ligaments hold the bones together.
| Structure | Function |
|---|---|
| Cartilage | cushions bone ends, reduces friction |
| Synovial fluid | lubricates the joint |
| Ligament | holds bone to bone |
| Tendon | attaches muscle to bone |
Muscles and antagonistic pairs
Muscles create movement by contracting — they can only pull, never push. So they work in antagonistic pairs: at the elbow, the biceps contracts to bend (flex) the arm while the triceps relaxes; to straighten (extend) it, the triceps contracts while the biceps relaxes. Muscles are joined to bones by strong tendons.
Muscles only pull
A muscle can contract (pull) but cannot actively lengthen (push). That is why they come in antagonistic pairs — one pulls the bone one way, its partner pulls it back.
A worked example
Explain how the biceps and triceps work together to bend and then straighten the arm.
- To bend the arm, the biceps contracts and shortens.A contracting muscle pulls on the bone.
- At the same time the triceps relaxes.Its partner must lengthen to allow the movement.
- To straighten the arm, the triceps contracts.Now the other muscle of the pair pulls.
- And the biceps relaxes.They act in opposition — an antagonistic pair.
The biceps and triceps are an antagonistic pair: biceps contracts (triceps relaxes) to bend the arm, triceps contracts (biceps relaxes) to straighten it.
Bone as living tissue
Bone is not dead scaffolding — it is living tissue with its own cells and blood supply, constantly being remodelled. It is made strong by calcium and phosphate salts deposited around tough protein fibres, giving it both hardness and a little flexibility. Because it is alive, a broken bone can heal, and the marrow inside long bones makes blood cells. A diet with enough calcium and vitamin D keeps bones strong.
Joints as levers
The skeleton and muscles form a system of levers that lets us move efficiently. At a joint, the bone acts as the lever, the joint as the pivot, and the muscle provides the effort to move a load. This is why a small muscle contraction at the elbow can move the whole forearm and hand. Together with the antagonistic pairs of muscles, this lever action gives the body its wide, controlled range of movement.
Bone, joint, muscle = a lever
The bone is the lever, the joint is the pivot (fulcrum), and the muscle supplies the effort. Antagonistic muscle pairs move the lever both ways.