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.
Me retracto
Devuelvaselo
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
Asiento trasero
no a mis espaldas
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.
Cuándo vas a regresar?