I have lately been using the 45 degree back hyperextension machines to supplement my deadlift and squat. I started adding weight a while back. The exercise had a good solid feeling, so I continued to use it. The other day I loaded up a 100 lbs mini barbell onto my neck, stepped into the machine, and just before I storted knocking off reps, I heard someone say "that kid is going to snap his back."
I scowled at the guy and continued. Half the shit people say to me in the gym is positive, but a lot of it is people trying to fuck with me so I generally ignore that stuff.
Afterwards, the guy came over and said "I'm not trying to tell you how to do your workout, but you could seriously hurt yourself doing that." I told him that I didn't feel any back pain at all and that I had been doing these for a while. He said I might be OK for now because I'm young but after a year or so, the damage would take it's toll.
He said he was 42 and that he had 11 or so dislocated disks in his back over the years and had been there and knew what he was talking about.
I don't see how these things can possibly be dangerous provided that your spine is not flexed during the completion of the movement. My spine is either neutral or hyperextended the whole time.
Anyway, I won't be able to do these much longer as my gym's mini barbell set only goes up to 110 lbs and it's impractical to do these with an olympic barbell.