I have been known to thrown my lower back out every once in a while, so I am quite sensitive to this issue.
Here is the thing:
When the pressure is applied correctly, aligned directly with the spine, things are fine. When you apply lateral pressure across the (lower) spine - then you are asking for trouble.
Lets put it this way. If a squat is DONE CORRECTLY - then the pressure is actually pushing right down into the spine, you lower back and glutes help pull against the back part of the spine, and the pressure is one that simply compresses your lower back. That does not hurt me all that much.
Now, if I do leg presses, this is different.
Now, hold on, there ARE different leg press machines. The one I am thinking of is such that you sit in it, and your hip is bent. Like, say you are sitting on the floor, your legs make an angle with your torso. Aboput 90 degrees.
Now, most leg press machines hold you at less of an angle.
Some hold you at a zero angle. You are laying down stirght. Thats almost like a squat. Thats fine.
But when you use the machine that puts an angle into where your legs meet your hips - then that can cause problems.
See what I mean?
The biggest problem with this is that, well, first, that angle itself is placing a laterla stress across your lower back. By lateral, I mean, a stress which is NOT up/down on it, but pusehs it in a front/back manner.
What is WORSE is when guys sit at these machines and do NOT keep their ASS pressed ALL the way into the seat. They sorta "recline" a bit. That seems to "feel" more intense. You are calling your back and abs a bit into play, BUT you are placing a stres on your lower spine!
Got it?
So, what I do is:
Squat.
Then if I do use a leg press machine, I either use one which keeps me stright (almost like a squat), or I make sure to use one which place my butt and legs in such a position that I feel like amost NO stress pushing against my lower spine.
And that works for me.
And yes, I know my typoing sucks. So, just read slower. LOL
Hope that helps.