Check out Women's Health's Fitness Faceoff every week to learn which moves will best help you reach your fit goals. Here's this week's faceoff:

The real secret to a sexy stomach is carving out those lines to frame your budding six pack (a.k.a. your obliques).

And while you're already well aware that planks help tone your entire core, there are also some variations that focus on working your side abs: plank knee taps and plank sweeps. But between the two, which one challenges your obliques the most? 

Don’t be fooled by appearances here: The plank knee taps might look more advanced, but it’s actually the plank sweeps that’ll carve killer obliques best. “Your abs are maximally contracted the entire exercise so they don’t get a break, and you have to resist the weight of your leg being shifted underneath you,” explains Marc Perry, CSCS, founder of BuiltLean. “Plus, you have to do a full twist to have the knee hit the opposite elbow, which works the obliques fully.”

.Both moves target the entire core musculature—internal and external obliques, rectus abdominis, and transverse abdominis—but in the other move, moving from plank to downdog actually relieves the tension from your abs and shifts it more toward the shoulders and arms. In addition, while reaching for the opposite leg creates some rotation, it doesn't challenge the obliques quite as much, Perry adds.

Plank Knee Taps: Get into plank position. Shift your weight back to the balls of your feet, coming into downward dog, as you reach your right hand to touch your left knee. Return to the starting position and repeat, reaching your left hand to touch your right knee. That's one rep.

Sweep Plank: Assume a pushup position with your arms completely straight. Your body should form a straight line. Slowly raise your left knee to your right elbow, and then sweep it over to your right elbow. Hold, and return to the starting position. Repeat with the right leg. That's one rep.