1What a muscle can actually do
A muscle can squeeze short — and that's its only move
You only need two ideas. Watch each one:
It pulls, it can't push
A muscle squeezes shorter to pull. 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
Your upper arm carries two muscles, tied on opposite sides of the elbow. They both do the same move — a pull — but their jobs are opposite:
Pulls the elbow BENT
Tied on the front side. When it squeezes short, it tugs the forearm up and the elbow folds closed.
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.
4Now take one muscle away
What if the arm had only ONE muscle? 💪
Two muscles seems like a lot for one elbow. Let's remove the back one and see if the front muscle can do the whole job by itself — bend the arm AND straighten it back. Guess first, then run it.
Guess before you take a muscle away
You give the arm just one strong muscle and squeeze it to bend the elbow. Can that same muscle straighten it back out?