Pushing muscles

As most people know, muscles can only pull and relax, they can't push. When called upon, the cells in a muscle contract, which shortens the muscle and makes a body part move. So when you bend your arm, your biceps contract and shorten, pulling the underarm at the elbow and bending it. At the same time, the triceps relaxes. By next contracting this muscle, the arm stretches again.
Muscles can only pull, they can't push.
But think about this.

I stretch out my arms to the left and right, positioning my hands as far away as I can, and I push my fingertips even further away, as far as I can. I push and push, and yes, my arms reach out even more...

And I wonder.

How does my body do this, if muscles can only pull? Where does the push come from? Which muscles pull in order to stretch out this way? How can muscles make my arms seem longer?
I can't explain it.

< back