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You can change muscle concentration and do the below hyper extensions for either your lower back or your hamstrings.
Excerpt of Writer’s Digest Judge's Commentary on Not Just Abs:
“ The photo illustrations are an excellent aid. The language is clear and well organized. “
Form and quad strength are super important doing the sissy squats below to avoid knee injury. Don't attempt until after you've built up your quad strength substantially. They have to be strong enough to control the whole movement with no resting point between reps, without putting any pressure on the knees. A lot of people do this wrong, but done correctly - this is a great way to finish off your quad workout. What a nice burn!