The lower abs are part of the rectus abdominis. This is the main abdominal muscle. It's what you see when someone sports six-pack abs. The rectus abdominis is responsible for flexion of the spine, pulling your rib cage toward your pelvic bone, as in a crunch. Because the rectus abdominis is a long sheath of muscle, ab exercises that pull the legs toward the torso target the lower part of the muscle, the lower abs.
A reverse crunch is the most basic crunch for targeting the lower part of the abs. It is simply a crunch done by moving the lower part of your body as opposed to the upper part. Start by lying on a mat on your back. Keep your head and shoulders against the floor. Extend your arms out along your sides. Pull your feet off the ground with your knees bent. Your upper thigh should be angled slightly toward the floor. From this starting position, pull your knees toward your chest. Your hips should lift off the floor. Hold this position for a count, then lower your hips slowly and with control.
Decline Reverse Crunch
Performing a reverse crunch on a decline bench makes the exercise much more difficult because you are lifting your legs against gravity. The steeper the angle of the bench, the more advanced the exercise. Lie on your back on a decline bench, your feet near the floor. Hold on to the top of the bench with both hands. For the most advanced version of the decline reverse crunch, position your legs out straight, almost parallel to the ground. For an easier modification, start with your knees bent. Slowly pull your legs into toward your chest, pulling your hips off the bench. Hold this position for a count before lowering your legs slowly and repeating.
For this lower ab exercise, you aren't actively crunching your midsection. However, the lower abs are working hard to stabilize your torso while you perform the scissor kick. Lie on your back on a mat. Position your arms along your sides. You can place your hands, palms down, under your buttocks. For some, this is more comfortable for the lower back. Keeping your head and shoulders on the mat, pull your legs off the floor. Start with your legs straight up in the air, then slowly lower them toward the floor, keeping them straight. Stop the lowering once you feel your low back pulling off the floor. Ideally, you want your legs suspended just a few inches above the ground, but they may need to be higher for the low back to maintain contact with the floor. Now, start to scissor the legs back and forth in the air. Your feet should cross over each other and then spread apart, just like a pair of scissors. Continue the motion for 30 seconds.