Abdominal Muscles are one of the hardest if not the hardest area to train because the abdominal muscles are a long thick group of muscles. To properly train them can take a variety of exercises and quite a lot of time. Some abdominal exercises target the upper abs (sit ups, crunches, etc.) and some target the lower abs (ab slings, vertical knee raise, etc.). When doing lower abs exercise by lifting the legs the upper leg muscles help out tremendously limiting the lower abdominal muscles work load. The Side, exterior, and interior Obliques also have to be trained and can take quite a few movement to train them properly (side bends, side crunches, etc.).
If you are looking to train your abdominal muscles fast, conveniently, and thourally then I have some good news for you it is called the Lean Abs Machine.
The Lean Abs Machine fully trains the Abdominals in the standing position. It is similiar to the ab wheel movement but it is better, more convenient and a much more enjoyable exercise to perform. The differences are: the pivot is the ball of your feet and not your knees which adds additional leverage making is unnecessary to have to lay out paralell to the ground reducing knee pain and back pressure; your arms are allowed to come in closer to your body on the return which incorporates more abdominal activity; the exercise is finalized with a standing crunch which can be felt all the way down into the lower abdominals.
Standing crunches have no breakover point and are not limited! The obliques are trained the same way just by turning to the side.
Since I started using the Lean Abs Machine I no longer do any other abdominal exercises and my core is in the best shape of my life. My abs are stronger and more profound and defined!
You may see it at http://www.LeanAbs.com
Try it and you will like it I gaurantee it!