I've done trauma ICU, ortho floor and ER so here's my two cents:
ICU: less walking, worst on back due to turning not just your pts every two hours but helping with all the other pts in the pod (usually 6 pts, 3 nurses). Then add in full baths and codes, very physically demanding.
Ortho: lots o walking, less turning, more helping out of bed.
ER: everything. Lots of walking, no turning, lots of catching people, breaking up fights, running after 5150s, etc.
Bottom line: ER is cardio, ICU weight lifting.