Question: Can You Exercise the Upper and Lower Abs Separately?
Answer: Although it may feel as though you can "isolate" the upper and lower abs (the rectus abdominis), you actually can't contract one section independent of the other. Research that looked at the muscle activation while performing a basic crunch exercise found that you may feel more contraction in the upper abdomen because the muscle fibers in the upper part of the rectus abdominis shorten more than in the lower part of the muscle, but the entire muscle is, in fact, activated during the movement.
When you perform a reverse crunch (lifting the hips), the opposite happens; you feel more contraction in the lower part of the rectus abdominis even though the muscle fibers in the upper rectus abdominis are also contracting.
Additionally, when you stabilize the hips and only lift the torso during the crunch, there is greater muscle involvement in the upper rectus abdominis as well as the internal obliques. When you perform reverse crunches, there is more activation the lower part of the rectus abdominis and more involvement of the external obliques.
Question: Can I Lose Belly Fat by Doing Lots of Crunches?
Answer: Unfortunately, no. Simply doing lots of crunches won't reduce belly fat. The belief that you can lose body fat in a specific area by exercising the muscles just beneath that fat is called "spot reduction." Spot reduction is a myth that has been disproved time and time again.
In 1984, researchers at the University of Massachusetts concluded that abdominal exercises do not decrease the amount or size of belly fat. In this study men did the equivalent of 5,000 sit-ups for 27 days. The researchers measured body fat in the abdomen, buttocks and upper back during the study. If spot reduction worked, the men should have lost fat only in the abdominal area because the buttocks and upper back are not worked during sit-ups. However there was no change in the thickness of abdominal fat or waist size. Biopsies showed there was no significant change in the diameter of abdominal fat cells either.
What ab exercises can do is tone and firm the muscles, but to reduce abdominal fat takes more than just ab exercise. You need a balanced fitness routine that includes cardiovascular exercise, resistance training, and a sensible diet.