Undulating her hips hypnotically, Allison Miller can almost make you think the hoop spinning around her waist is suspended in air, defying gravity.

Round and round it goes — effortlessly for hoopers, it seems. And the six students following Miller, 21, at her hooping-for-fitness class in Carmichael, Calif., try to keep pace, though occasionally the multicolored sparkly hoops clang to the wood floor.

After a few minutes, a steady rhythm develops. The students, all women ages 21 to 50, get into the groove and start swaying to the reggae issuing from Miller's iPod.

But now comes the hard part: "the stall." It's a move not meant for beginning hoopers, those alternative exercisers who use this erstwhile child's toy for cardiovascular and core fitness.

The idea behind the stall is to create the illusion that the hoop stays in the same body position. This involves no small amount of timing, balance and coordination. The hooper must spin her body to match the hoop's rotation while transferring the hoop from hand to hand with arms extending below the waist.

Good luck with that, ladies.

Turns out, though, that everyone could do the move, albeit not as fluidly or swiftly as Miller. As a finale, she shows them how to step out and back in to the hoop without losing momentum.

"The key is to keep the hoop at the flattest possible plane," Miller chirps to the class.

Easy for her. Students fumble and drop hoops, then dissolve into puddles of laughter.

By the end of this particular hourlong class, some will have figured out this new move and some will have struggled. All, however, will be red-faced and sweaty from a surprisingly vigorous workout. And they'll experience something many couldn't find in other exercise programs.

"Fun," says Marcia Alexander, 49, of Sacramento. "It's the one time of the week I know I'm going to laugh for an hour."

You would expect nothing less from an activity spawned by a late 1950s craze.

Hooping — don't call it "Hula-Hooping," by the way, or you'll run afoul of Wham-O Inc., which holds the trademark — languished for decades after the initial flurry but started a comeback about five years ago in dance clubs. It is a feature of Burning Man, the annual bacchanal in the Nevada desert, and long has been a fixture at String Cheese Incident concerts and outdoor music festivals such as High Sierra. Now it's finding its way into health clubs, college curriculums and community recreation centers, such as Carmichael's.

The new hooping features slightly heavier, bigger hoops to accommodate adults. And the goal is not necessarily to stay stationary and keep the hoop going as long as possible but to master moves and tricks and keep the heart pounding.

Miller, a senior at the University of California, Davis, majoring in French, has been a certified hooping instructor since 2007. She's the only one with her own Web site.

"In the last six to eight months, I've noticed a big change in interest," she says. "In June, I had 18 new students. Normally, I'll only get eight to 10 new ones each class."

If celebrity cachet is any gauge, hooping is about to go big. In October, first lady Michelle Obama hooped with schoolchildren on the White House South Lawn during a childhood fitness summit. And actress-singer Beyonc Knowles is said to be a hooper. The activity also has gone virtual: Super Hula Hoop is one of the more popular sports in Nintendo's Wii Fit computer gaming system.

This month, the American Council on Exercise, a fitness trade industry group, listed hooping as of the "one of the top fitness trends for 2010."

Miller says the appeal of hooping, at least to her, is that it's relatively easy (until the more intricate moves come in) and it's liberating because, in Western society, "we aren't really encouraged to move our hips in this way."

"Rarely do I have someone who just cannot hoop," Miller says. "Everyone at least gets 10 to 15 revolutions by the end of the first class. But it takes time, obviously, to build up your skill."

But just how good a workout is it?

The exercise council, working with researchers from the University of Wisconsin, La Crosse, recently studied how many calories are burned in various Wii Fit games. Hooping got one of the highest scores at 111 calories burned in a 30-minute session. (That's still below a moderately paced walk, which burns 160 calories in 30 minutes.)

Once actual hoopers — as opposed to the virtual Wii kind — advance to some of the elaborate moves Miller can perform, the exercise intensifies. At one point during the class, she was spinning like a dervish, snaking in and out of the hoop and using her arms, legs, hips, trunk and even her neck.