No upper and lower abdominals, so how come they move indepen

Modern resistance training reminds us that the main abdominal muscle that gives the lucky few with low body fat a 6- or 8-pack is a single sheet of muscle, the rectus abdominis, which gets its segmented appearance from tendons that cross the muscle. It is the rectus abdominis alongside the transverse abdominals and the internal and external obliques that support our core and enable us to turn, bend and twist the body.

My question is, if there is no such thing as upper and lower abdominals, as seemingly dated exercise routines used to get us to target individually, how come some exercises make the top part of the muscle ache more (such as crunches) and others make the lower part ache more (such as leg-lowers and reverse crunches)?

Also how can we do a belly-dancing belly roll, in which the upper, middle then lower part of the muscle are contracted to create a ripple effect? Most other muscles you can't contract just a part of - try just contracting just the upper part of your biceps, for instance.

If the rectus abdominis is a special sort of muscle in which separate parts can be activated separately as seems to occur in a belly roll, isn't there then an argument for working different parts separately as well as targeting the whole area at once?

No agenda here, just curious to hear people's opinions :flowerforyou: