Lower ab definition can seem impossible to achieve. Some of this is because we store fat in our lower stomach more readily than in our upper stomach. But even if you're pretty lean, it's common to see good definition in the upper abs, and little in the lower. The problem is that most abdominal exercises only hit the upper abs, leaving the lower abs underdeveloped. The good news is that as long as you exercise strategically, you can strengthen and tone your lower abs.
The American Council on Exercise decided that it was time to figure out which ab exercises work, and which are a waste of time. So they commissioned a study and published the results in the May/June 2001 issue of "ACE Fitness Matters." The study showed that the bicycle crunch was the best, activating two and a half times the amount of abdominal muscle of the traditional crunch, and beat most ab exercises that required equipment by almost as much. To do a bicycle crunch, lie on your back. Raise your upper body into a crunch position. Raise your legs up, bent at the knee, and pump them in and out almost as if you were pedaling a bike. Rotate your upper torso so that you twist each shoulder toward the opposite knee. The rocking motion really makes your lower abs work.
The problem with a lot of lower-abdominal exercises--or at least with the way they're usually performed--is that they tend to tire the front hip flexors long before they wear out your lower abs. To perform a reverse crunch that will really strengthen those lower abdominals, you need to bend your knees to a 90-degree angle, and never drop them past the point where your shins are parallel to the floor. That way, your front hip flexors aren't struggling to support your legs. Place your hands palm down on the floor to either side for balance. Roll back onto your mid-to-upper back and crunch your stomach. Your torso should look like a crunch, but with your shoulders on the ground instead of your feet. Squeeze hard at the top, then let your butt touch the floor again.
Have you ever noticed that runners have really flat stomachs? It's not just because they're skinny, but because running builds the lower abdominals like almost no other exercise. In fact, a study published in the December 2009 issue of "Applied Physiology, Nutrition and Metabolism" concluded that running activates the abdominals well enough to be considered a trunk exercise. So running will not only make your abs more visible, but will tone and strengthen them as well.