Of course, if your clients have "split abs" they should ask their doctor what to do. But I have heard the same thing from many - which equates to PILATES exercises. OR - the dreaded surgery to put them back together. (Which is basically what I need.)
When doing abdominal exercises in classes, the ones that really don't do us any good are those where we are seated and draw the needs in or pilates roll ups (if they aren't totally engaged from the transverse level) or anything where they will tend to 'pop out' instead of pull in.
What's best is oblique work and transverse - ex: just laying on your back with bent knees feet touching the floor and lifting one foot off at at time just 1/4 inch off. Alternating. Side planks - low or high. Etc.
So find out if people in your class have this condition. There's a simple test they can do to themselves to see if they do or they can ask their doctor. See here for the test: http://www.befitmom.com/
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All the best!