Yeah the lever of your body changes. You can't do a glute-ham raise on a roman chair (back extension chair). I've tried!
Glute-ham machine you bend at the knees. Roman chair you bend at the waist/hips.
I don't know why the guy on the roman chair is doing situps or whatever he's doing. But picture him face down. He'd be bending at the hips/waist. You'll notice that the upper thighs would be touching in one picture and the lower thighs are touching in the other.
The girl in the other picture is bending at her knees and comes up with her torso perpendicular to the ground. This is a ton of hamstring and glute work. I'm not sure how much hamstring is in the roman chair, but i'm sure it's far less involvement than the glute-ham raise. This machine is godly as an accessory lift at Westside. Sadly i haven't actually SEEN one at any gym yet, but i do ghetto ones on the floor with my feet anchored by a bench. If i remember correctly the big advantage of the lift is that you work the hamstrings at both insertion points: at the knee and in the hips, which is supposedly a rare occurrence and much preferable for some scientific babbly reason that powerlifters like to talk about.
