[yitmy]

Excellent point- abdominal muscles to work them and know what you are doing is critical to wear you feel it, and increased targetting of the tissue. Thus saying by lifting your legs keeping your upper trunk fixed you work your lower abs, the opposite for the upper abs. Laying on a bench and lowering your legs down (usually when straight) while maintaining your back in neutral posture works the transverse abdominis. Any type of rotation whether it is with the upper or lower trunk fixed would work your obliques. Yes, with the lower trunk fixed you work your upper obliques and the opposite for the other. Can you train the fibers that way for the obliques? The jury is out, but we all know that varying the routine may be a way of confusing the muscle and increasing recruitment and decreasing staleness of the routine. "Variety is the spice of life..... and the life blood of making the routine work in unexpected ways".

I agree with 10-20 reps due to abs being endurance based musculature, so to build the tissue up, more reps seems to feel better. I tend to use a clock/timer and just focus on eeking out the best reps in the best form in a certain time frame say 1 minute. Then I may increase or add a little "spice" to the routine.

After any abdominal routine I highly recomend you perform a trunk stretch by gently twisting your arms and head in the opposite direction of your legs. By doing surch for a couple reps would decrease any back pain you may get by targetting the abs. On some occasions you may be targetting the abs but by keeping your legs fixed and curling down ie... (cables) you could be drawing in your Iliopsoas. This would cause some back discomfort, so Quadricep stretches or Cat press (push ups with pelvis on the mat) would help stretch this area.
-M