You cannot train lower abs alone.
According to Dr. Fred Hatfield, PhD:
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It's a myth that you can "isolate" the upper from the lower abs -- electromyographic studies show that the minute you apply resistance, both your upper and lower abdominals kick into action together
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If you look at the abdominal muscle's insertion and origin, it's easy to see why this is so.
Do crunches, you work the upper and lower abs; do leg raises, you work the upper and lower abs.
It's called non-contiguous innervation.