Most people think of hula hoops as a retro fad that came and went during the "Leave It to Beaver" era. But for the past couple of years, a pumped-up version of the toy has been making the rounds on the fitness circuit. Called the Heavy Hoop, it's a foam-covered, three-pound ring designed to provide the resistance necessary for a heart-pumping, feel-the-burn aerobic workout. And according to a 2001 study by Dallas's Cooper Institute research center, its vigorous twirling not only thoroughly engages your core muscles, it also can burn more calories than a high-impact aerobics class, an intense bike ride or a comparable run on the treadmill.

"It's much harder than normal aerobics," says Lisa Dunay, who regularly works out with the Heavy Hoop at Bally's Total Fitness's L Street "Cardio Hoops" class. "Plus, I like the music; everything is choreographed, and it has a great dance aspect to it." (The hoop is also available for home purchase, along with a variety of instructional videos.)

What to Expect: Even if you were an ace hula-hooper in third grade, you'll find that the new model, with its extra weight and heft, requires that you employ skill and concentration to rotate it around your waist. After the warm-up there's no more kiddie stuff: You'll add arm movements, lunges and squats while keeping your hips swaying. (If you're worried about catching a case of the giggles, don't: The workout is so intense, there's no energy left over to crack up.) The floor exercises that follow are equal parts Pilates mat work and military calisthenics: stomach crunches to tighten abs while working the shoulders and arms (you'll hold the hoop above the body to add resistance) and lightning-fast legwork, with jumps in and out of the hoop (which lies on the floor), to improve speed and agility. Finally, a cool-down period incorporates yoga positions, including a downward-facing-dog-type stretch that uses the hoop for support.

What to Bring: Exercise sneakers and well-fitting exercise clothing. Women with sensitive skin may want to throw on a T-shirt over that sports bra -- the Heavy Hoop can cause irritation or even bruising after so many skin-pounding rotations.

Cost: Heavy Hoops are available in class to Bally's members or to George Washington University students (who will find them at their gym). You can purchase your own for about $80.

Lauren P. Kennedy

How to Shimmy and Shake

Bally's Total Fitness. 2000 L St. NW. 202-331-7788. www.ballyfitness.com. Vivian Thurman teaches "Cardio Hoops" Mondays at 6:30 p.m. and Fridays at 12:15 p.m.

www.heavyhoop.com. To work out at home, you can buy the Heavy Hoop and introductory video for $79.95 (plus $14.50 shipping and handling). You'll also find a range of additional exercise videotapes, each $15.95.

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This is not a plaything: The Heavy Hoop joins the long roster of props being used in creative, exercise-focused ways.