Fun Anatomy Fact: Your “six pack” (if you have one...) is actually one long muscle. Called the rectus abdominus in the doctor books, it runs from your chest bone to your pelvis. It looks like six separate muscles because tendons run horizontally across it, cutting striations into the muscle. That muscle actually tapers as it runs downwards – it's wider near your ribs than it is by your hips.

The point of all this is: you do not really have a lot of muscle to work with in your lower abs. But with a very precise exercise, you can specifically target the lower, smaller portion of your rectus abdominus. That's the thesis of a recent column written by Brad Schoenfeld, Ph.D, C.S.C.S., an internationally renowned fitness expert and widely regarded as one of the leading authorities on muscle development and fat loss.

The best exercises for muscle growth are ones that cause a muscle to shorten and lengthen, according to Schoenfeld. Isometric exercises like planks do build strength and size, but they keep the muscle contraction static. There is something about a dynamic contraction that has a bigger impact on muscle growth, he says.

The trick is to tilt your pelvis in toward your belly button. Instead of lifting and lowering your legs, focus on tucking your pelvis in toward your belly button and back to neutral. This forces the lower portion of your rectus abdominus to shorten and lengthen while lifting the weight of your lower body.

Here is the exercise that turns theory into practice: Lie face up on the floor or a bench. Bend your hips and knees 90 degrees. Tilt your pelvis forward toward your belly button. Return your pelvis to a neutral position. That’s 1 rep.

Looking for something more hardcore? Try a hanging knee raise that focuses on tilting your pelvis. Grab a pull-up bar with a shoulder-width, overhand grip and hang from the bar with your knees slightly bent and feet together. Tilt your pelvis in toward your belly button. Return your pelvis to a neutral position. That’s 1 rep.