Question

When lifting weights up, use your lower back and not your legs

Answer 1

There is more muscle in your legs than there is in your arms. The reason for this is because your legs have to support you every day, so they have built up the type of muscle to be able to do that, plus whatever to carry or lift. Arms don't have to do that, so they have adapted to be able to carry smaller things.

Answer 2

Yes, there is more support