How joints allow movement

Your moving skeleton

Same colour = same type of joint

Arm straight. To get here, the triceps contracted — it got shorter and pulled the forearm down — while the biceps relaxed. Darker red means that muscle is contracting.

Tap a joint to find out what type it is and how it moves. Then switch between Arm straight and Arm bent to watch the biceps and triceps swap jobs.

Zoom in on a joint

Peel a layer off, or tap a part

The muscle is not fixed straight onto the bone: a tendon joins it to the bone it moves.

Peel the layers off to see what sits where, then tap each part to see what it does.

Now try the knee

The knee is a hinge joint too. The pair of muscles that move it are the quadriceps and the hamstrings.

You kick a football. Your quadriceps contract and your leg straightens at the knee. What are your hamstrings doing?

What you need to know

• A joint is where two or more bones meet. Some joints are fixed, like those in the skull, but most are flexible and let the skeleton move.
• Hinge joints (elbow, knee) move backwards and forwards; ball and socket joints (hip, shoulder) move in a circle and give the greatest range of motion; gliding joints (spine) let bones slide past each other.
• Cartilage covers the ends of the bones and reduces friction; ligaments hold bones together; tendons attach muscles to bones.
• A contracting muscle gets shorter and pulls its tendon, which pulls the bone. Contraction needs energy. Muscles can only pull, never push.
• Antagonistic pairs pull a bone in opposite directions: when one muscle contracts, the other relaxes.

The big picture

A joint is where two or more bones meet. Some joints, like those in the skull, are fixed; most are flexible. Hinge joints (elbow, knee) move backwards and forwards, ball and socket joints (hip, shoulder) move in a circle with the greatest range, and gliding joints in the spine let bones slide past each other. Inside a joint, cartilage reduces friction, ligaments hold bone to bone and tendons attach muscle to bone. Muscles move bones by contracting and pulling — they can never push — so they work in antagonistic pairs: biceps and triceps at the elbow, hamstrings and quadriceps at the knee.

⚠ Watch out

Saying a muscle "pushes" a bone back. Muscles can only pull: a relaxing muscle just stops pulling, and its partner contracts to pull the bone back. And do not swap the cords — ligaments join bone to bone, tendons join muscle to bone.

Memory hook

Every movement is a tug-of-war where only one side pulls at a time. And to keep the cords straight: LigaMENT is bone to bone (it CEMENTs bones together); TENDON ties muscle ON to bone.

Check yourself

Without looking back: name the three types of flexible joint with an example of each. Then explain, using contract and relax, how your biceps and triceps bend and straighten your arm.