your shoulders are supposed to hit the deck on every situp. Marine Corps situps kill my quads too, i don't know why, but i always get 100. i think the key is getting your butt as close to your feet at possible, and having your partner sit on your feet and hold your legs as tight as possible. put your arms on your stomach as close to your waist as you can get to minimize the angle between your arms and your thighs when you go up for each repitition. knock out as many as you can in the first 60 seconds, rest for a little then start up again. i think situps are the money-maker on the pft, everyone should get 100, just do a few situps everyday.