Those are "las lumbares".
We say lower third in spanish like this-el tercio inferior.
menor
I sleep on my back in Spanish is: Yo duermo de espaldas.
Devuelvaselo
Me retracto
we say: necesito que vuelvas
If you mean your back as in the part of your anatomy, the spanish word is "espalda" (S-paul-dah)
Devuelve eso
no a mis espaldas
Asiento trasero
I'm no expert but I'd say arthritis may be your problem, however I'd talk to your doctor before you make any stops at the drug store.
tiene su espalda