Is this a good idea? I'm just not completely sure I understand Stiff Legged Deadlifts enough to do them with proper form. I was just wondering if Back Extensions would work the same muscles as much?
No. Watch this.
They do work the same muscles, but not to the same extent. By this I mean yes, they both work the lower back and hammies, but the extensions will primarily work the lower back with very little hamstring involvement, while the stiff-legged deads will work lower back and hamstrings equally. In my opinion, leave the back extensions for the gals and do the stiff-legged deads, romanian deadlifts or good mornings. FYI Romanian and Stiff-legged are almost identical.
EDIT: Good Morning Link http://www.youtube.com/watch?v=S4H2EZC1UCY
if you have access to a glute-ham developer, do some glute-ham raises.
you won't build big mammies on one of those