Transverse Abdominus

I have been doing free weights for some time now, and planks in addition to that. I've noticed a big difference in the overall look and musculature of my abs, but I am not seeing a big change in my transverse abdominus muscle. This is definitely not an issue of fat...I have very little fat there. The muscle is weak. Is there anything besides planks (I do regular & side) I can do to target it?

Thanks in advance :-)

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