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The best tool for fab abs: Stability ball

Fab-ab move: Plank Tuck with a Twist

Here's why: "This exercise works your core like no other," McGee says. "You have to engage your entire transverse abdominals—the innermost layer of muscle that acts like a girdle—just to maintain your balance on the ball." Pulling the ball in toward your chest pinpoints your hard-to-reach lower-ab muscles, and the twisting movement engages both your obliques (side-ab muscles) and rectus abdominus (the major ab muscle responsible for the hallowed six-pack). "By balancing on a large, unstable surface, you can't cheat," McGee explains. "It forces you to engage all of your ab muscles, front to back and side to side."

Next: How-to: Plank Tuck with a Twist