Situps, or bent-knee crunches, target the rectus abdominus muscle of the abdomen. The lower abs refers to the bottom section of the rectus abdominus. Although some ab exercises do emphasize the lower abs, you cannot only work one half of a muscle. If you want to use situps to strengthen your lower abs, there are a few variations that are more effective at strengthening the rectus abdominus.
Using an exercise ball to perform a situp makes the exercise more challenging for your rectus abdominus, both the upper and lower sections. A study published in "ACE FitnessMatters" magazine in 2001 tested the muscle recruitment in the rectus abdominus during various ab exercises. The exercise ball situp recruited 39 percent more muscle activity than a situp on the floor; this was the highest amount of any situp variation. The exercise follows the same instructions as a regular situp, but you place your back on an exercise ball instead of the floor. Your feet rest on the floor, with your knees bent, and you support your head with your hands. Then, you raise your head and shoulders toward the ceiling to engage your abs.
Vertical Leg Situps
Putting the legs in a vertical position in the air instead of resting your feet on the floor increases the activation of the muscle fibers in your rectus abdominus during a situp. The vertical leg position causes a 29 percent increase in muscle activity in the entire rectus abdominus, including the lower abs, according to "ACE FitnessMatters." To perform a vertical leg situp, lie on your back with your hands supporting your neck and hold your legs perpendicular to the floor. Then, raise your head and upper back toward the ceiling just like a regular situp or crunch.
Performing a situp on a decline bench increases the range of motion of your torso and works the lower abs. You need a decline bench with a leg roller at the top to hook your feet into so that you can safely perform a decline situp. To begin, lie face up with your head on the lower end of the bench and your ankles secured under the leg roller. Your knees are bent. Place your hands in fists against your temples with your elbows bent, and then sit up toward the top of the bench.
One more way to work the lower abs with a situp is to use free weights. A weighted situp works the abs more than a regular situp due to the additional weight your abs have to lift. The weighted hand-to-leg situp exercise is especially beneficial for the lower abs because of the leg movement. Raising your legs makes the lower abs work harder to stabilize the pelvis. To perform this exercise, lie on your back with your arms extended overhead and a dumbbell in each hand, palms facing the ceiling. Straighten your legs on the floor, too, and squeeze your abs. Bring your head, right shoulder, right arm and right leg toward each other. Lower back to the starting position and then repeat with the left side.