Shoulder muscle

Deltoid, said DEL-toyd

Your shoulder muscle is a rounded cap that lifts your arm up high to wave.

Muscles can only pull. The biceps on top bends your elbow and the triceps underneath straightens it. Press Flex, or tap any part!

A triangle cap

The deltoid is a thick, rounded muscle on top of your shoulder. It's shaped like a triangle pointing down your arm.

Its name comes from delta, a letter in the Greek alphabet that looks like a triangle: Δ.

Try it: put your hand on top of your shoulder and lift your other arm out to the side. Feel the rounded muscle get firm?

Three parts, three directions

The deltoid wraps over the front, the side and the back of your shoulder.

The front part lifts your arm forward, like raising your hand in class. The side part lifts your arm out, like airplane wings. The back part pulls your arm back behind you.

When all three parts work together, you can reach in almost any direction.

The most movable joint

Your shoulder can move in more ways than any other joint in your body.

The round top of your upper arm bone sits in a shallow dish on your shoulder blade, a bit like a golf ball sitting on a tee. That lets your arm swing in big circles.

Because the dish is so shallow, muscles help hold the shoulder together. A group of deep muscles called the rotator cuff hugs the joint, and the deltoid works with them.

Try it: make big arm circles forward, then backward. Can your knee do that? Nope!

Shoulders at play

Throwing a ball, swimming, climbing, hanging on the monkey bars and reaching for a high shelf all use your deltoid.

Even waving hello to a friend is a little shoulder workout.

🤩 Wow facts

The deltoid is named after delta, a Greek letter shaped like a triangle: Δ.
Your shoulder can move in more directions than any other joint in your body.
The deltoid has three parts that lift your arm forward, out to the side and back.
Your shoulder joint works a bit like a golf ball sitting on a tee.
A river delta, where a river fans out into the sea, got its name from the same triangle-shaped letter.
Every time you raise your hand in class, your deltoid helps do the lifting.

💚 Look after it

Warm up your shoulders with gentle arm circles before throwing or swimming.
Don't throw for too long without a break, and rest when your arm feels tired.
Carry heavy things close to your body.

🩹 When something goes wrong

A sore shoulder. Throwing or swinging for a long time can make your shoulder ache. Rest it. If it hurts a lot, is hard to move or keeps hurting, tell a grown-up, who may take you to a doctor.

🙋 Questions kids ask

Why can my arm go in a circle but my knee can't?

Your shoulder is like a ball in a shallow cup, so it can swing every which way. Your knee works more like a door hinge: it mostly bends and straightens.

What does 'deltoid' mean?

It means 'shaped like delta,' a Greek letter that looks like a triangle.

🧠 Quick quiz

Question 1 of 2What shape is the Greek letter delta?

🔊 A triangle cap

🔊 Three parts, three directions

🔊 The most movable joint

🔊 Shoulders at play