Bodybuilders have long believed in separating abdominal work into exercises that focus mainly on the upper abs, such as crunches, and exercises that focus more on the lower abs, such as hanging leg raises and reverse crunches. Some research has made the claim that it is impossible to focus on just the upper or lower abs and that both areas are used equally whether you perform crunches or reverse crunches. Yet bodybuilders know this is false, as they can feel the ab area that is being focused on when training, and they have seen the results from doing this for decades. Now new research suggests that bodybuilders were right all along.
Researchers from the University of São Paulo in Brazil had male subjects perform the crunch and the reverse crunch while they measured the muscle activity of the upper rectus abdominis (upper abs) and the lower rectus abdominis muscle fibers.
They reported in a 2011 issue of the Journal of Sports Science and Medicine that, based on the muscle-activity data, the upper abs are definitely used more than the lower-ab muscle fibers during the crunch and the lower-ab muscle fibers are used more than the upper-ab muscle fibers during the reverse crunch. They also supported this finding with the fact that research in human cadavers shows that the different portions of the abs are innervated by different nerves.
It is possible to focus on the upper abs or the lower abs by doing exercises that emphasize each area.
To build well-balanced abs from top to bottom, incorporate a variety of exercises that focus on the upper abs and exercises that focus on the lower abs. Exercises that involve flexing the spine and moving the shoulders forward, such as crunches, place more focus on the upper portion of the abs. Exercises that involve flexing the spine by moving the hips and legs forward, such as reverse crunches and leg raises, place more emphasis on the lower abs. Since the lower abs tend to be the weaker area for most bodybuilders, consider starting your ab workouts with a lower-ab exercise while your abs are their strongest and not yet fatigued.
REFERENCE P.H. Marchetti et al., J. Sports Sci. Med., 10(3):322–27, 2011.