Primary function of the recus abdominus is eccentric
deceleration of the trunk into extension (thus preventing movement
- not creating movement) - you should first look at the "core" as
preventing movement, over trying to create movement.
There is so much bullsh**t out there that I can understand why
people are confused about core training. Sometimes it makes me
laugh to here some of the crap people are teaching.
Look at many of the popular rotation movement patterns that they
use to train the "core" - most of the movement is generated from
the lumbar spine - yet the lumber spine only has 13 degrees of
rotation - rotation movement should be generated from the thoracic
spine not the lumbar spine.