We've long heard about the merits of building a stronger core—it solves back problems! It makes you run faster!—which is why gym acolytes have been grunting it out in plank position ever since. But if you're tired of watching the seconds tick by on your stopwatch, look past the stretch mat and head to the kettlebell rack. This humble piece of iron can drastically improve your midsection when you use it in core-building moves like the suitcase carry (see below). "Think of it as the new plank," says Don Saladino, who co-owns Drive495 fitness club in New York City and has trained Ryan Reynolds and Ryan Gosling. "To maintain upright posture as you walk, you have to engage all the muscles of your middle."

The evidence isn't anecdotal: Research from the University of Wisconsin, La Crosse, shows that two kettlebell workouts a week can boost core strength by 70 percent. And there are satisfying superficial benefits, too. A study found that kettlebell training burns more than 800 calories an hour, helping shed your body's top layer of fat to reveal the definition underneath. It makes sense that the now-ubiquitous gym tool's homeland, Russia, requires a kettlebell workout—not the push-up or pull-up—as a standard physical test for army recruits.

How It Works
Do these four exercises as a circuit, performing all the reps of one exercise, then moving immediately to the next. Rest two minutes, then repeat the circuit for a total of five sets.

1 / Single Leg Dead Lift (see above)
Stand with left foot in front of right and a kettlebell on the floor about two feet in front of you. Bend left knee slightly and lift right leg as you lower torso toward the floor, keeping back straight so that body forms a T. Grab kettlebell and push hips forward as you return to standing. That's 1 rep. Do 10 reps, then switch sides.

2 / Single Arm Swing
Stand with feet slightly wider than shoulder-width apart, hips back, knees bent slightly, back straight, and hold the kettlebell with left hand. Hike bell between your legs. In one explosive movement, straighten legs as you push hips forward and swing arm to shoulder height in front of you. Let bell fall back between legs and repeat. Do 10 reps, then switch sides.

3 / Suitcase Carry
Stand tall and hold a kettlebell in left hand at your side without leaning toward it. Walk 50 yards, then turn and walk back to start. Switch sides and repeat.

4 / Kneeling Bottoms-Up Press
Kneel with right leg in front, left leg back. Hold a kettlebell vertically by the handle in right hand, right arm bent with wrist near right shoulder. Keep biceps close to ear as you press the bell directly overhead. Return to start. Do 5 reps, then switch sides.