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Low body weight but no muscle definition in core?

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So I'm 5'5" and at 115 pounds, but for whatever reason the muscle definition in my abs is not there. I know I have muscles there, because I can hold a three minute plank with good form and do boatloads of crunches/sit-ups/ and especially russian twists without fatiguing, You would think I would have a low enough body weight to see some results, but not really. Do you think I just really need to strength train that core more or lose a few pounds? Thank you!

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