Well, there's really no such thing as "lower abs." They are quite long muscles that run all the way up to your ribs.
One of the best ways to work this set of muscles, as well as your back (both of which make up your "core") is to do Pilates. Kathy Smith has a great video called Pilates for Abs. I also like the Gaiam video Pilates for Weight Loss. Kathy has good, shorter ab workouts on her Lift Weights to Lose Weight videos (both the original and Vol. 2). I also like Tamilee Webb and I've heard her ab workout is good, too.
If you are more of a do-it-yourselfer, go to
www.exrx.net, click on Exercise Instruction, then on Exercise and Muscle Directory. You'll see information and demos of lots of different ab exercises under the Waist section.
Remember that if you work one set of muscles, you should always work the opposing muscles. So, if you work the abs, you also need to work the lower back.
Lower Abdominal
