How can pulling muscles make your arm push?

1What a muscle can actually do

A muscle makes a pulling force

You only need two ideas. Watch each one:

It pulls, it can't push

In this picture, a muscle shortens as it pulls. Then it goes soft again — but going soft isn't a push. A loose, floppy muscle can't shove anything, the same way a slack rope can't push.

It's tied across a hinge

A muscle is tied across a joint like a rope on a door hinge. When the rope gets shorter, it tugs the door. A rope can pull the hinge one way — but it can never push it back the other way.

2Two muscles, one elbow

The front puller vs the back puller

Here are two of the muscle groups that help move your elbow. Their attachments let their pulls turn it in opposite directions:

Front muscle

Pulls the elbow BENT

Tied on the front side. When it squeezes short, it tugs the forearm up and the elbow folds closed.

Back muscle

Pulls the elbow STRAIGHT

Tied on the back side. When it squeezes short, it tugs the forearm down and the elbow opens out.

3Your turn — work the pair

Slide between the two muscles and bend the arm yourself

Slide one way to squeeze the front muscle, the other way to squeeze the back muscle. Watch the squeezing one bunch up tight while its partner goes soft — and the elbow follows.

front muscle pulls it bent back muscle pulls it straight

Who's pulling now: front muscle squeezing

◀ back pulls STRAIGHTfront pulls BENT ▶

4Now move an attachment

Two muscles — but on the same side?

Here is an imaginary model arm. It has two pulling bands, but both attach on the same side of its hinge. Nothing else pushes it, and we ignore gravity. Predict what happens when they take turns.

Predict before testing the new attachment

The coral band bends the arm. If the violet band also attaches on that same side, can it pull the arm straight again?