Grade by Grade
B. Life Systems · Human Organ Systems

Bones, Muscles and Joints: The Musculoskeletal System

B2.1 B2.2 A1.3 A1.5About 45 minutes
Learning goals
  • I can describe the jobs of the skeleton.
  • I can explain how muscles work in pairs to move bones.
  • I can name different kinds of joints and muscles.
  • I can design and build a model that moves like a finger.

Words to know

skeleton
All the bones of the body. An adult skeleton has 206 bones.
joint
A place where two bones meet.
cartilage
Smooth, bendy tissue that cushions the ends of bones at joints. It also shapes your nose and ears.
ligament
A strong band that holds bones together at a joint.
tendon
A strong cord that attaches a muscle to a bone.
contract
When a muscle contracts, it gets shorter and pulls.
bone marrow
Soft tissue inside some bones that makes blood cells.

Be a jellyfish

Try to stand up with no bones. You can't! Without a skeleton, you'd be a puddle on the floor, like a jellyfish out of water. And without muscles, your bones couldn't move at all. Your bones and muscles work as a team: the musculoskeletal system.

The skeleton's jobs

An adult has 206 bones. (Babies are born with more, but some join together as they grow.) Your skeleton:

  1. Supports your body and gives it shape.
  2. Protects soft organs: the skull protects the brain, the ribs protect the heart and lungs, and the vertebrae protect the spinal cord.
  3. Moves your body, with help from muscles.
  4. Makes blood cells in the bone marrow inside some bones.
  5. Stores minerals like calcium, which keep bones hard.

The longest bone is the femur (thigh bone). The smallest is the stirrup bone deep inside your ear. Bones are alive! They grow, and they can heal when they break.

A life-size model human skeleton hanging on a stand in a school science classroom.
A model skeleton shows the skull, rib cage, spine and long leg and arm bones. Find the bones that protect the brain, heart and lungs. AI-generated image

Joints

A joint is where two bones meet. The ends of the bones are covered in smooth cartilage, and ligaments hold the bones together.

  • Hinge joints bend one way, like a door: elbows, knees, fingers.
  • Ball-and-socket joints move in many directions: shoulders and hips.
  • Pivot joint: lets your head turn side to side (at the top of the neck).
  • Fixed joints don't move: the bones of the skull.
hingeelbow, knee,fingersbends one wayball-and-socketshoulder, hipmoves many wayspivottop of the neckturns side to side(no movement)fixedskull bonesstays still
Four kinds of joints. The arrows show how each one can move: a hinge bends one way, a ball-and-socket moves in many directions, a pivot turns, and a fixed joint does not move.

Muscles: pullers, not pushers

You have more than 600 muscles. There are three kinds:

  • Skeletal muscles are attached to bones by tendons. You control them: walking, writing, smiling.
  • Cardiac muscle is only in your heart. It works on its own.
  • Smooth muscle is in the walls of the stomach, intestines and blood vessels. It works on its own too.

A muscle can only pull (by getting shorter, or contracting). It can't push. So skeletal muscles work in pairs. In your upper arm:

  • The biceps contracts → your elbow bends.
  • The triceps contracts → your elbow straightens.

When one muscle of the pair contracts, the other relaxes.

biceps (contracts)triceps (relaxes)elbow jointbones
Bending the arm: the biceps contracts and pulls the forearm up while the triceps relaxes. To straighten the arm, the triceps contracts.
Worked example

Let's analyse a movement: kicking a soccer ball

Which parts of the musculoskeletal system work when you kick a ball?

  1. Joints: the hip (ball-and-socket) swings the leg; the knee (hinge) bends then straightens.
  2. Muscles in pairs: muscles on the back of the thigh contract to bend the knee. Then muscles on the front of the thigh contract to straighten it and kick.
  3. Tendons pass the pull of the muscles to the bones.
  4. Bones: the femur and lower leg bones act like levers.
  5. Another system helps: the nervous system tells each muscle when to contract.

Bones, joints and pairs of muscles work together, controlled by nerves.

Just for fun

Why didn't the skeleton go to the school dance?

It had no body to go with!

Hands-on activity

Engineering challenge: a model finger

You need: thin cardboard (like a cereal box), 3 drinking straws, string or yarn, tape, scissors, a grown-up.

  1. Problem: design a model finger that bends when you pull a string, like a tendon pulling a bone.
  2. Plan: a real finger has 3 bones and hinge joints. Draw your design first.
  3. Build: cut a cardboard strip about 2 cm wide and 12 cm long. Fold it in two places to make three "bones" with two "joints". Cut three short pieces of straw and tape one onto each section, in a line.
  4. Thread a string through all three straws and tape it at the tip. Pull the string: does the finger bend?
  5. Test and improve: can you add a second string on the other side to straighten it, like a muscle pair? Try to make it pick up a cotton ball.
  6. Communicate: which part stands for a tendon? a muscle? a joint?

Grown-up note: help with scissors. What to look for: the string = tendon, your pulling hand = muscle, the folds = hinge joints, the cardboard = bones.

Your turn

1

Name four jobs of the skeleton.

2

Which bones protect your heart and lungs?

3

What connects a muscle to a bone?

4

What holds two bones together at a joint?

5

Name the three kinds of muscle.

6

When you bend your elbow, which muscle contracts?

7

What kind of joint is your shoulder?

More practice: warm-up

1

How many bones does an adult have?

2

What is the longest bone?

3

What kind of joint is your knee?

4

Can a muscle push?

5

Where is cardiac muscle found?

More practice: on your own

1

Why do skeletal muscles work in pairs?

2

Compare a hinge joint and a ball-and-socket joint.

3

What does cartilage do at a joint?

4

Your stomach muscles churn food without you thinking. What kind of muscle is this?

5

Which system makes your blood cells, and where?

Challenge

1

Explain what the triceps does when you do a push-up and straighten your arms.

2

In your model finger, what was missing compared to a real finger? Name two things.

3

Why do you think a baby's skull bones are not fully joined yet?

Check your understanding

Quiz

1.

Which bones protect the brain?

2.

Muscles are attached to bones by...

3.

Which is a ball-and-socket joint?

4.

To bend your elbow, your biceps...

5.

The heart is made of...

6.

Blood cells are made in...

For parents and teachers

Big idea: bones support, protect, move, make blood cells and store minerals; joints let bones move; skeletal muscles work in pairs because they can only pull.

How to help at home:

  • Have your child hold their upper arm while bending and straightening the elbow to feel the biceps and triceps take turns.
  • Help with cutting in the finger model, then let your child lead the testing and improving.

Common difficulties:

  • Mixing up tendons (muscle to bone) and ligaments (bone to bone).
  • Thinking muscles push. They only pull.
  • Thinking bones are not alive. They grow and repair themselves.

Safety: supervise scissors.