The rectus abdominus muscle is sometimes considered two separate muscles even though it isn't. Exercises for the lower abs and exercises for the middle and upper abs work these sections more than the other, though not exclusively. It is also possible to work the entire rectus abdominus evenly through exercises for both the upper and lower abs.
Upper Versus Lower Abs
The upper, middle and lower abs are all part of the same muscle but some exercises emphasize the top part or the lower part more. This happens when one end of the rectus abdominus shortens to perform an exercise, says Len Kravitz, Ph.D., at Unm.edu. For example, when you lift the pelvis off the floor as in a reverse crunch, you contract the lower abs and external obliques. Conversely, when you lift your shoulders off the floor, you contract the upper part of the abs and internal obliques. Most exercises only involve one or the other and you can use these to target the upper and middle or the lower abs, though you cannot completely isolate muscles fibers from the same muscle.
Reverse Decline Crunches
Reverse decline crunches emphasize the lower abdominals. This exercises uses a decline bench, which is also called an incline bench. To perform this exercise, lie on your back with your head at the top of the bench. Hold the top of the bench with your elbows bent and extend your legs straight. Then, pull your knees into your chest and roll your pelvis off the bench. Keep your middle back and shoulders on the bench. Reverse the motion.
Overhead Abdominal Crunches
The overhead abdominal crunches emphasize the middle and upper abs. Your arms begin overhead on the floor, which is why the exercise is called overhead abdominal crunches. To perform overhead abdominal crunches, lie on your back with your knees bent and your arms straight on the floor, palms up. Next, raise your arms, head and shoulders toward the ceiling. Bring your arms forward toward your knees until the arms are perpendicular to your torso. Reverse the motion until you are flat on the floor.
The double crunch exercise works both the upper and lower abdominals simultaneously. This exercise works the upper abs through spinal flexion of the torso and the lower abs through spinal flexion of the lumbar spine. To do a double crunch, lie on your back with your knees bent and your fingertips touching the sides of your head with the elbows bent. The palms face the ceiling. Next, lift your knees toward your chest as you curl your back and shoulders off the floor. Lie back down.