Q1.
You may have noticed that some parts of our body can move easily in many directions, while others move only in a single direction. Why does this happen?
Answer
Because the type of joint is different at different places, and a joint is simply the junction between two or more bones shaped in a particular way.
| Joint | How the bones fit | Movement allowed | Example |
|---|---|---|---|
| Ball and socket | Rounded top of one bone sits in a shallow hollow of the other | Forward, backward, sideways and circular | Shoulder, hip |
| Hinge | Bone ends shaped so they swing in one plane only, like a door hinge | Bending and straightening in one direction | Elbow, knee (protected by the kneecap) |
| Pivot | One bone turns within a ring formed by another | Rotation — side to side, like a doorknob | Skull on the backbone (neck) |
| Fixed | Flat bones joined edge to edge | None | Bones of the skull |
Why the body does not simply use ball-and-socket joints everywhere: freedom of movement is bought at the cost of stability. A shoulder can swing anywhere, and it is also the joint most easily dislocated. A knee has to carry your whole body weight, so it is built to move in one plane and to refuse every other direction — the ligaments see to that. And where nothing must move at all, as around the brain, the joints are fixed so the skull stays a rigid protective case even while you run and jump.