S.H.
Maybe call "OSHA" or check their website.
http://www.osha.gov/index.html
I think, it is the responsibility of your BOSS (ie: the business owner), to provide proper "lifting" assistance.
If anyone gets a back injury... well, you know where that can go, and all the red tape and blah blah blah.
Try telling your Boss again. Research OSHA, and perhaps wear a back-belt? But your boss should provide that too, I believe.
All the best,
Susan