Skeleton, muscles and movement

Your skeleton is a frame of bones inside your body: 206 of them in an adult. It holds you up, protects the soft organs inside you and, with your muscles, lets you move. Muscles pull on bones across joints, and because muscles can only pull, they work in pairs.

Key facts

  • The skeleton has four jobs: support, protection, movement and making blood cells (in the bone marrow).
  • The skull protects the brain, the ribs protect the heart and lungs, and the backbone (made of vertebrae) protects the spinal cord.
  • The femur (thigh bone) is the longest bone. Other bones: humerus (upper arm), radius and ulna (forearm), tibia and fibula (lower leg), pelvis (hips).
  • Joints: hinge joints (elbow, knee) bend one way; ball-and-socket joints (shoulder, hip) move in all directions; fixed joints (skull) do not move.
  • Tendons join muscles to bones. Ligaments join bones to bones at a joint. Cartilage cushions the ends of bones.
  • Muscles can only pull, so they work in antagonistic pairs: to bend the arm the biceps contracts and the triceps relaxes; to straighten it the triceps contracts and the biceps relaxes.
  • Animals with a backbone (vertebrates) have a skeleton inside (endoskeleton). Insects, spiders and crabs have a hard outer skeleton (exoskeleton). Worms and jellyfish have no hard skeleton.

Worked examples

Example 1

Name the bone that protects each organ: the brain, the heart and lungs, the spinal cord.

  1. The brain is soft and very important: it sits inside the skull.
  2. The heart and lungs are in the chest, inside the cage of ribs.
  3. The spinal cord runs down through a tunnel in the vertebrae of the backbone.

Answer: skull; ribs (ribcage); backbone (vertebrae)

Example 2

Explain how the biceps and triceps bend and straighten your arm.

  1. Muscles can only pull (contract). They cannot push.
  2. The biceps is on the front of the upper arm. When it contracts it pulls the forearm up: the arm bends. The triceps relaxes.
  3. The triceps is on the back. When it contracts it pulls the forearm down: the arm straightens. Now the biceps relaxes.
  4. Two muscles that work against each other like this are an antagonistic pair.

Answer: to bend: biceps contracts, triceps relaxes; to straighten: triceps contracts, biceps relaxes

Example 3

What type of joint is the shoulder, and how is it different from the elbow?

  1. In the shoulder, the round end of the humerus (the ball) fits into a cup in the shoulder blade (the socket).
  2. So the arm can swing forwards, backwards, sideways and round in circles.
  3. The elbow is a hinge joint: like a door hinge, it only bends and straightens in one direction.

Answer: a ball-and-socket joint: it moves in all directions; the elbow is a hinge joint that bends one way

Example 4

A sprinter tears the tissue that joins her calf muscle to her heel bone. What has she torn: a tendon or a ligament?

  1. Tendons join muscles to bones. Ligaments join bones to bones.
  2. This tissue joins a muscle (the calf) to a bone (the heel).
  3. So it is a tendon: the Achilles tendon.

Answer: a tendon (the Achilles tendon)