When i was boxing for approx 2 years i got down to 76kg, leanest i'd ever been. Trained 6 day's a week, exercising core most every workout with bridges, medicine ball, leg raises sit-ups all sorts of shit. I had more definition i my ab's prior to boxing, at 90kg when i was squatting, deading, pressing every other day with no direct ab work. Make of it what you like.