Eli5: why is it so easy to hurt ourselves when lifting with our back compared to our legs?
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Leverage.
The muscles in our back are much smaller compared to our leg muscles. There may be more, but they don’t work together as well as the few in our leg do. Easier to set the lifting load onto a larger muscle that’s used to “lifting” something up, as opposed to our backs that mostly work sideways/twisting
Our spines are essentially bone disks sliding on jelly cushions. And all that has to be somehow operated by muscles that run right along it. So little leverage, so many points of failure.
Compare that with legs, which have few robust joints and large muscles attached some way away from those joints, giving a degree of leverage.
Backs are better at handing stress straight up and down than side to side. It’s like a tower of blocks tied together with some string. [Does decent in compression, but try and bend it, it’ll fail way sooner.](https://imgur.com/a1LqGWM)
Really bad leverage.
You know those little ‘knobs’ that stick out? Thats where the back muscles attach. And those muscles are very strong, but it’s like pulling on the wrong side of a lever.
Your arms and shoulders are a long distance from the pivot point (hip) and so their stress is magnified. The muscles pull on those much, much smaller levers to try and tilt you upright.
Furthermore you want to pull “up” but your back muscles pull towards the base of the spine. That’s the wrong direction. They pull back, basically compressing the spine, and from the ‘knobs’ tilting them up a bit.
A 50lb load lifted “from the waist” can produce ~800’lbs of compression on the spine.
Now, if you lift with the legs, your arms & ligaments pull up. Your shoulders don’t torque your back. Your back muscles mostly try to stay in alignment only. Since your moving up, and the back is “up” it only has to lift that 50lbs of extra weight. Not nearly as bad.
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