They work the complete ab area, but favor the lower abs. After completing those, do a couple sets of scrunches just bringing your upper back off the floor, don't bend your neck. It is like trying to bring your face towards the ceiling. Do not pull on your neck - gently rest your hands either alongside your face or on the back of your head without exerting any force with your arms. That will favor the upper abs, and give you a complete ab workout. The ideal would be to do all three exercises when you work abs, but sometimes the leg or knee ups is not practical without proper equipment.