By back extension “machine” do you mean the Nautilus-type weight stack machine or a regular back extension bench?
If you mean a back extension bench the answer is obviously yes: this exercise is designed to be used on a back extension bench or a glute-ham bench.
It might work on a back hyper machine if you sit sideways on the bench. You might have a problem rotating though: you will “stick” to the padded cushion! Other than that it might be an alternative worth exploring.
Abs need to be worked like any other muscle - with progressive resistance.
Just went through 1 cycle with the dreaded 5 exercises. Nice…Feeling it everywhere and looking forward to doing 2 cycles next time around !!!
There are exercises which will stregthen the IP complex without damaging the lumbar section. You just cannot argue in favor of an exercise which is so very dangerous to same. I will be happy to explain the anatomy and kinesiology of why, but already did so in a post last week. Also, an important component of leg lifts which is (usually) missed is range-of-motion. In other words, if you want a serious lower back injury, go right ahead and do them.
Swale I’m not trying to start an argument but always consider that there are more factors involved. This sounds much like the same line of reasoning as when someone says that the olympic lifts are dangerous, that squats are bad for the knees and that deadlifts are hazardous to the lower back. This is all I’m going to say on the subject.
No, anyone who says that (avoid dead lifts, squats) does not know what they are talking about. And anyone who chooses to ignore the reasoning behind avoiding leg lift movements does so at the peril of themselves and their trainees. I hope you at least looked up my previous post where I show why this is true. Either that, or YOU will be in the same company as those docs who tell people not to squat, deadlift, etc. BTW, I do not prescribe Olympic lifting for the average Joe. There are ways to produce better results, with the same effort, which ARE safer (and more productive).
I have. Brutal! Almost killed me. Gotta love it. 
–Ike