The individual joints are not designed for severe loads. Unlike the bones in the legs and feet, it does not suffer regular hard acceleration. Just hurrying up a flight of stairs subjects your feet to several gravities of acceleration.
Unlike the shoulders and arms it's not made to support loads at a distance, either. Subject a spinal joint to the kind of load your wrist supports when you do a one-handed pull-up and you'll destroy it.
The spine at a whole can take fairly large loads--but like a chain, the weakest link will fail first.
A spine that could handle large loads would be bulky and unwieldy, and not very flexible. Compare the carrying capacity of a cat with that of an ox, correcting for size differences. The cat has a very supple spine but it's not as strong, pound for pound, as that of an ox. But the ox is not known for its flexibility.
The human spine is a compromise; and it's as good as it's going to get.
I mention all this because my spine hurts bad enough that I called off work tonight.
When I was 21, I spent some time parting out a VW Beetle. I brought the engine home to work on it, intending to use the frame of the car as the basis for a project I wanted to work on. I had help loading the assembled engine into my car, but once home, had no help to get it out of the car.
I very carefully propped the thing up on some boxes, then slid my arms under the cylinders and lifted it, and carried it the length of the garage to my work area. When I set it down, there was a slight twinge in the small of my back. I had lifted it as safely as possible, but the load was a bit too much for me.
Fast-forward to October of 2004, some 16 years later; I foolishly tried to push my 1986 Fiero off the car dolly it had ridden from Cedar Rapids to Crete.
Since then, my back has periodically hurt. It gets stiff, mainly, but trying to move hurts; and if I don't sleep right, it hurts more. Mostly I have managed to get by, even with my very highly physical job, but sometimes I just can't do anything with it. Tonight was just such a night.
The "weakest link" in my back was the minor injury I sustained in 1988, by carrying a 200+ pound engine 20 feet. I exerted too much force, in too awkward a position; my 37-year-old back couldn't take it.
When it's really bad, I feel it all the way down my left leg, all the way to the knee--a problem with a sciatic nerve, no doubt--which makes walking an exercise in torture.
The moral of this story is that no matter how old you are, you must always use proper lifting technique--and that includes setting the load down! I was as careful as I could be in 1988--I was no dummy, and that engine was the heaviest thing I'd ever lifted, bar none!--but it still left me with the forebear of a problematic back injury. A few seconds is all it takes to screw you up; when lifting heavy loads you must always do it carefully. A few seconds' extra time can save you a lifetime of pain pills.
I think my current pain comes from the work I did on the parts car yesterday. I had what I thought was a brilliant idea: if I could get at the accessories of the thing's engine, I ought to be able to use the impact wrench to turn it over. If the battery was in, the engine should start, right?
Well, theoretically, if I could get the engine to turn over fast enough, yes, it would start. The problem is getting the engine to spin fast enough. You can't do it by hand; there's no way to hand-crank a car with a modern transverse engine. If I knew that the car would run, I could just let it coast backwards down the driveway and then pop the clutch--but if it didn't start, I'd have to push it back up with another vehicle.
I even took the right front tire off in order to get at the crank pulley--but all the impact wrench did was tighten the bolt. Argh etc.
To make matters even more annoying, it seems that I failed to be careful about what I was doing, and twisted or otherwise injured my back. It's one of those things where you don't really notice it when it happens, but a few hours or days later, AAAIGH!!!
I had a moment like that on Saturday evening, come to think of it. I was pulling the van into the driveway after I'd taken Mom shopping, and I leaned forward to get a look at the position of the front end, and SPOING! That might have been it, too.
Anyway, I've got Tuesday off, so that gives me a bit more than 44 hours, at this writing, to get over this to the point I can work again.
It sucks to get old. But of course the alternative is even worse.