Bad for the back? Hmm I have actually noticed a back ache after doing alot. Problem with doing them without feet anchored is that I can only do about 12 in 2 minutes lol. Feet anchored I can (well last time I tried, 3 months back) do 76 in 2 mins.
The Armys' situp, feet anchored and arms folded across chest. The reason your quads hurt is because the second half of the sit up is using the quads! The first half of the sit up uses rectus abdominus (six pack ones) second half uses hip flexus/quads.
Take the pain! Pain in this case.....is weakness leaving the body!! Yeehaa.
Also leg raises - lie flat on ya back, legs straight, raise your feet 6" off floor. This is start pos. Raise feet a further 6 - 12", then back to start pos to complete 1 rep. Worse thing about these is they're boring, but when your done and stand up you'll feel it!
Also re sit ups, the fit - for - the - Army pages on the Army site suggest keeping your hands flat on the floor, next to your butt, and as you raise your chest and shoulders sliding your hands down the floor, toward ya knees, all the way throughout the movement keeping your palms on the floor.