I know a number of retired or ex military guys with bad backs and knees. I'd like to think there is a study of that out there.
While I've packed heavy packs before, but I don't anymore. I like to keep it around 80-90lbs as a max or less. I don't mind making another trip as needed. Its not worth getting hurt, like you said. I limit my weight to what I can hoist to my leg and slip a shoulder through the strap. More than that if I fall down, I can't get back up.

. Getting older and not as tough as I used to be either.