Re: doing sit-ups without feet stabilized?
situps are pretty hard on the back from what i understand and IME. although you may have to be a little bit older before you notice any actual discomfort.
a replacement ab-cruncher is to stay in that same position on your back and just lift your legs (fully extended) from the floor, straight up, making an L shape. then bring them down again. do it all smoothly and not too fast. it may not be as intense as situps, but it will save your lower back, like i say. you could probably also add leg-weights or something to make the abs work harder.