I also have back issues and others LOL.
When I started i couldn't find one other maker that sat down to grind and was told it won't work blah blah by a few!
try squaring your elbows in at your sides and moving your body from left to right and then reverse for the other side.
Once I get the heavy bevel grinding done,36 grit. this way and I want to clean things up with the 60 grit and the 120 belts.
I put my thumb in the middle of the blade, HOT HOt Hot! "This is why I came up with my rhino finger skins." and guide the blade along with that thumb surfing and guiding the blade along for each light pass until I get her done.
I use my arms during this point and this will build up or break your forearms & wrists.
Having my face right there means getting quite a grit blasting so I have used clear face shields and googles that will allow me to wear corrective glasses and respirators and a ball cap on forwards or backwards depending if its goggles or a shield.. Shooting ear muffs are a good idea to as it can get loud having your face right there with the motor wide open.
I've heard that Gil Hibben's has a video out of him grinding sitting down with where he leans forward and puts his armpits on his knees. I have fusion in my lower spine so that one doesn't work for me.
Like the rest of this, nothing replaces time grinding steel to figure out what works best for you and then refining it.
Hope this helps!