heavy work

Some units require more movement than others. ER tends to require a lot of transfers. In ICU nurses must escort their patients to procedures such as CT Scans, etc. Other untis not so much.

One must be in good physical shape to be a nurse and take care of their backs.

Nursing is physical work. In the ICU, we reposition patients a minimum of every 2 hours, usually with assistance. But consider that the average patient is not 70 lbs. It is common for nurses to injure their backs. Some hospitals have "lift teams" who help with "heavy lifting" but they are usually not on at night - keep that in mind.

I would guess that ortho would have high back risk, ER too, so keep that in mind. Any time you have a patient at risk for falls (most), you are at risk for a back injury.

Pay attention to any training for patient transfers, positioning etc. Take every precaution you can to protect your back. Training and precaution will help.