What's a good substitution?

[LT Hebert]

I'm currently doing the Human Inferno, and it calls for single-leg back extensions. I don't have a machine, so I was wondering what would be a good substitution? I did regular extensions on a Swiss ball, but I was wondering if anyone had any other ideas.

[galya]

How about one legged good mornings with the back foot elevated on a bench (as in a Bg squat)?

[Ogedei]

What the hell kinda sadist are you!?!?!

J/k.

Anytime I see the leg elevated on the bench doing anything like Bul SS I get all weird

Og.

[igunick]

If you have powerrack, load up a barbell on the pins and put your feet under it. That way you can do back extensions with a bench and the rack.

[Lisa~]

I'd use a single-leg RDL or single-leg good morning, but unsupported.

Galya's suggestion actually matches the prescribed lift more closely, but I think the unsupported single-leg lifts are just superior.