My introductory Japanese classes are so far in the past now that all my memories feel like a blur. I do have a vague feeling that for whatever reason they never taught us the body parts in a single lesson. Maybe I was expecting something along the lines of my high school Spanish class where we had to label a poster or at least fill in the blanks around a mannequin on handouts and tests. We probably got bits and pieces here and there – お腹がすいている, 喉が渇いている, etc. – but never a full lesson with all the parts…I think.
So maybe that’s why I thought that 背中 (せなか) meant back for so long. I mean, I guess it does, but if you’re talking back pain, that’s 腰 (こし). 背中 feels more like that area around your shoulder bones, almost. The two basically mean upper back and lower back, but if you’re talking in general, 腰 might be the word you’re looking for.
If you’ve been sleeping on a too-thin futon for too long, the phrase you’re looking for is – 腰が痛い.
(Past body part entries: read about boobs here and here – both bring in fans from various search engines – and fingers here.)
In my university linguistics class, we spent a whole hour discussing 腰. I’m not surprised the gloss over it at the lower levels.