Is there a difference (in terms of which muscles are activated) if you pull your belly up and in as opposed to just pulling it in? In other words does the “up” do something different?
I use “up and in” because these actions help coordinate the pelvic floor elevation along with the transverses abdomius (TA) which is what we are trying to get back, since this is how the muscles should fire normally but have often stopped doing so due to childbirth, poor posture, etc.
As far as our abdominal muscles go, there is a clear difference between the transversus abdominus and the rectus abdominus and their functions.
In a nut shell, contraction of the rectus abdominus does definitely increase our intra abdominal pressure and press down on the pelvic floor. The transversus abdominus, however, tightens around our thoracolumbar region acting more like a vacuum or a compression stocking. If you perform a 2-step Kegel and tighten your sphincter muscles and then elevate your pelvic floor, you will feel your TA draw in with the elevation of your pelvic floor. Put your hand on your belly button and you will feel it draw away from your hand when you do the lift and hold. Once you become proficient at contracting your TA before overhead lifting, reaching, etc, you will feel the same connection to your pelvic floor. It will elevate as you engage your TA muscle.
Bottom line, you never want to see your belly pooch out when you contract the muscles. If you do, it’s the rectus abdominus being engaged. Instead, you always want to see your belly draw in when you contract muscles. When you do, you will know that you have activated your transversus abdominus.