Seated knee-up
Sit on the edge of a stable chair or bench, grasping it at your sides to stabilize your body. Lean back slightly and extend your legs down and out in front of you. With your legs together and toes pointed, bring your knees in to your chest in a smooth motion as you curl your upper body slightly forward at the same time.

Crunch
Lie on the floor with your knees bent at about 60 degrees, feet flat on the floor about shoulder-width apart. With your hands lightly gripped behind your head (not pulling), curl forward to bring your shoulders blades just off the floor.

Twisting crunch
From the lying crunch position, hands loosely placed behind your ears, cross your right elbow toward the outside of your left knee in a smooth motion. You don't need to actually touch your elbow to your knee. Alternate sides after each rep.