Contrary to popular belief, your lower abs aren't a separate muscle group. They're actually part of a larger muscle called the rectus abdominus. The rectus abdominus is made up of sections that form what we understand as a "six pack." Leg lifts work the lower abs because lifting your legs creates more resistance since they're farther away from your center. So, you can actually tone your stomach without a single sit-up. Cool, huh?
To accomplish the leg lift, start by lying on your back on the floor with your arms at your sides. Using your ab muscles, pull your legs straight up in the air, lifting your butt slightly off of the floor. Slowly lower your legs back down to the ground. This counts as one rep. Aim for five to10 reps, but do less if you find yourself breaking form. For this exercise, quality is more important than quantity.