BrightKidz Library
Subjects
The two muscles of an elbow: one pulling, one let go A single arm bone lying across the picture with a knobbed end at each side, set out the way a museum lays out a specimen. A second bone is hinged to its right-hand end and slopes up and away, and a curved arrow beside that second bone shows it swinging upwards. Two muscles run from the far left of the big bone across to the second bone, one above and one below, both drawn clear of the bone so that each can be seen, and both striped along their length. The muscle above is the one doing the work: it has bunched up short, thick and taut, and the arrow shows the movement it is making. The muscle below has let go, and hangs long, thin and slack. Neither muscle can push. Each can only pull and then let go, which is why it takes two of them, taking turns, to bend the joint and straighten it again.

The Human Body: Discovering How Your Muscles Work

About 4 minutes

You have more than 600 muscles in your body, and every one of them can only pull, never push. Moving anything at all takes at least two muscles working as a pair — one pulling one way, and a second pulling back the other way when it lets go.

What are Muscles?

Muscles are made of stretchy tissue that can contract (get shorter) and relax (get longer). Squeezing shorter is the only thing a muscle can do — that one motion is what moves every part of you.

How Muscles Help You Move

Muscles work in pairs. One muscle pulls a bone in one direction, and the other pulls it back the other way. In your arm, the bicep (on top) bends your elbow, and the triceps (underneath) straightens it again.

Types of Muscles

There are three main types of muscles in your body:

Try This: Feel Your Muscles Work

Here's an experiment to feel your muscles in action.

  1. Hold your arm out, palm facing up.
  2. Put your other hand on your bicep — the muscle on top of your upper arm.
  3. Bend your elbow, bringing your hand towards your shoulder. Feel the bicep bunch up and go hard. That is it contracting.
  4. Straighten your arm again. The bicep goes soft. It did not push your arm straight — it simply let go, and the triceps underneath did the pulling.
  5. Bend and straighten a few times slowly, keeping your hand on the bicep. Feel it take turns: hard, soft, hard, soft. That is what working in a pair looks like from the outside.

Your bicep pulls your arm bent. Which muscle pulls it straight again?

Next time you chew, you are using the strongest muscle in your body, pound for pound — your jaw's masseter. And the one muscle that has never once stopped working, not even while you slept, has been beating inside your chest since before you were born.