Obviously off topic now, but as a youngster in the gym I used to use a similar machine that required me to lay down and push my legs. I worked my way up to max weights on that machine (very quickly). One day I pushed my legs out and heard this terrible crunch come from my lower spine, was barely able to hobble back home. It was near enough back to normal after 2 days or so, but scared the hell out of me.

For me it goes to show that humans are more fragile than we often like to think.