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If you've eyed a cast-iron kettlebell at your gym and couldn't imagine a use for it beyond doorstop, keep reading.

Researchers found that people who did 20-minute kettlebell workouts torched almost 300 calories—and that's just for starters. When you factor in the muscle-sculpting impact (the calories burned after you exercise as your body repairs its muscle fibers), the total expenditure could increase by up to 50 percent.

Kettlebells also make traditional dumbbell moves more challenging. Their weight isn't evenly distributed, so your stabilizer muscles have to work harder. Add definition to your shoulders and back; tone your butt, core, and arms; and shed a few dress sizes with this circuit from Sean Armstead, owner of Phenomenal Fitness in Chicago. (Another muscle-sculpting fitness tool? The medicine ball. Try this Lean-Body Medicine Ball Workout from Anna Kournikova.)

The Workout
Starting with the first move, complete each exercise back-to-back without resting. Rest for one to two minutes, then repeat for a total of two or three circuits. Follow this routine two or three days a week, using a 10- to 15-pound kettlebell.

Around-the-Body Pass
Hold the kettlebell with both hands in front of your torso and stand with your feet hip-width apart. Release the kettlebell into your right hand and move both arms behind your back. Grab the bell with your left hand and bring it back to the front (completing a full circle around your body). That's one rep. Do 10, then switch directions and repeat without stopping to rest.

Quick tip: Keep your core engaged and avoid moving your hips throughout the entire move. (Want to give your abdominals some extra attention? Sculpt your core with these 4 Amazing Moves for Your Abs.)

Take the kettlebell in your right hand, squat slightly, and bend forward from the hips until your torso is almost parallel with the floor. Let the kettlebell hang at arm's length. Keep your torso still as you pull the kettlebell to the side of your chest, elbow close to your side. Lower back to start. That's one rep. Do 10 to 12, then repeat on the other side.

Dead Lift
Stand with your feet hip-width apart, the kettlebell on the floor between your feet. Squat down and grab the handle with both hands, keeping your back flat. Brace your abs, squeeze your glutes, and slowly push down into your heels as you stand up, keeping your arms extended. That's one rep; do 10 to 12.

Figure 8
Stand with your feet wider than hip-width apart, knees bent into a quarter-squat position, back straight, and chest up. Hold the kettlebell behind your left leg with one arm on each side of your leg. Grab the bell with your left hand and swing it in front of your left leg, between your legs, and behind your right leg. Grab it with your right hand and swing it in front of your right leg, between your legs, then behind your left leg. That's one rep. Continue for 10 reps.

Quick Tip: The movement should be slow and controlled but fluid.

Half Get-Up
Lie face-up on the floor, legs straight, holding the kettlebell in your right hand straight above your shoulder. Bend your left knee, place your foot on the floor, and prop yourself up on your left arm. Keep the weight directly in line with your shoulder and sit up until your back is straight. Reverse the movement to return to start. That's one rep. Do five, then repeat on the other side.

Swing
Grab a kettlebell with both hands and stand with your feet wider than hip-width apart. Squat down until your thighs are nearly parallel to the floor. Immediately stand and swing the kettlebell up to shoulder height. As the kettlebell begins to arc back down, bend your knees and squat, swinging the kettlebell between your legs. That's one rep. Do 15 to 20.

Quick Tip: If you have any back problems, do this move without using a weight. (Improve your posture and stave off pain by strengthening your upper and lower back muscles with this 15-Minute Back Workout.)

Front Squat
Stand with your feet hip-width apart and hold the kettlebell with both hands in front of your chest, elbows bent and close to your body. Keeping your back straight and chest up, push your hips back and bend your knees until they form 90-degree angles. Press down into your heels and return to standing. That's one rep. Do 10 to 15.

Quick Tip: Keep your knees over (not in front of) your toes at the bottom position of the squat.

Windmill
Stand with your feet wider than hip-width apart, weight in your left hand. Raise it next to your left shoulder, then press it overhead. Rotate your chest to the left and look up at the kettlebell as you try to touch your right hand to your right foot. Pause, then return to start, keeping your left arm extended. Do five to 10 reps before lowering the weight, then repeat on the other side.

Quick Tip: Practice without the weight until you nail the form. (Training without weights can help you fit in a workout anytime, anywhere. Try our Ultimate Bodyweight Workout.)