I'm sure I'm not saying anything new to fellow SZ'ers in that it's also very important HOW you lift something. Carrying equal weight in each hand makes a big difference, as well as having good roomy handles on the stuff you are lifting, so you can get your fingers right around them.
Balance is everything, especially where the back is concerned, as well as the well known bending the knees when lifting.
I carry my Tyros4 (in it's case) in one hand, and my books, cables, music stand and base, sustain and volume pedals etc. in another similar equal size case...they weigh the same so are easy to carry.
My HD Double X-stand and bench are in two cases (with handles), and then there's my two speakers in custom cases (ditto on the handles) in the other. Three trips at most, stairs or no stairs.
It's not for everyone, especially if you are doing multiple gigs and are in a hurry.
I stopped hurrying years ago, and now that I'm retired, I hurry even less.
Ian