Here's a helpful trick when performing reverse curls: Hug a pillow between your calves and hamstrings. This activates the backs of your thighs so you can't use the hip flexors (the opposing muscle group) to create momentum and pull your knees forward.
-- Karen Voight
1. Lie face up on a mat or a firm padded surface. Hold a large pillow behind your legs and think of squeezing your heels to your buttocks. Place your hands behind your head. Inhale.
2. On an exhalation, contract your abs to lift your hips off the floor and curl your upper body toward your thighs. Keep your elbows open and don't pull your head forward with your hands. Lower your hips and upper body and repeat as many times as you can. Even though the range of motion is just a few inches, this is a challenging exercise.
Voight is the creator of a line of fitness DVDs, including "Sleek Essentials."