WHEN IT COMES TO catchy trend names, "core training" has to be right up there.

"Core" refers specifically to the body's center of power, the torso, including the back and abdomen. Who among us doesn't have back problems or want a flat stomach? Or both?

But "core" also implies something fundamental, crucial, maybe even essential. In the eternal search for the most efficient exercise program, the essentials must be included, no?

"Core training" advocates contend that muscle weakness or imbalance in the torso may contribute to a range of problems, from simple stiffness and fatigue to injuries to chronic pain.

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Although many exercises to strengthen, stretch, stabilize, balance and coordinate the torso can be done without apparatus, equipment can be helpful, to the fitness industry at least. Some are quite pricey, such as variations on the Pilates Reformer, which can run from $200 to $2,000. More affordable are big inflated balls — called Swiss balls, exercise balls or stability balls — which became popular a few years ago and have resurged on the core-conditioning wave.

Recently I tried two brands of another type of core-training equipment developed for Pilates workouts. The Fitness Circle and the Pilates Magic Circle are sprung-steel metal hoops with contoured pads for positioning between hands, legs and mat. Squeezed or stretched, both hoops provide resistance intended to develop muscular strength, flexibility and endurance without building bulk.

With a hoop between my ankles I could work on inner and outer thighs, and then shift positions and get the front and back thighs. With it on one shoulder I could use the upper arm; between the hands I could work the chest muscles. With it between my lower thighs during crunches, I could squeeze my knees together with each repetition, thus working both abs and hip muscles simultaneously. I could also use it as a head support while doing crunches, and as an aide for a hamstring stretch.

Stott's Fitness Circle feels a bit sturdier but it's $60 and, unlike the Pilates Magic Circle, doesn't have a second set of pads that add comfort when pushing against the inner surface of the hoop. Though I wonder about the durability of its loose padded-rubber casing, I'd still probably favor the slightly less-taut Magic Circle, which I bought for $38 including tax at The Pilates Studio of Seattle (206-405-3560); online I've seen it as low as $29.99 via www.staminaproducts.com.

I tried the hoops while watching three videos. If I had to stick with just one, I'd choose Stott Pilates' "Power Fitness Circle Workout" ($14.95; 800-910-0001; www.stottpilates.com) for Moira Stott's instruction, which was more detailed than in her earlier "Fitness Circle Workout" ($14.95). Karen Voight's "Body Reform Total Body Training" video ($19.95) emphasizes inner-thigh and buttocks exercise and includes some exercises without equipment; it also uses a third type of hoop I didn't try, the Resist-a-Ring ($39.90; 800-735-3315; www.karenvoight.com).

There are — of course — more "core training" products to come. Reebok's Core Board is a high-tech version of an old training aide, the wobble board, which provides an unsteady surface as a base for exercise, to develop balance and activate muscles needed for torso stability. At the YMCA in Yakima, they're used in step classes and personal-training sessions. A home version of the board, already available via www.core.reebok.com for $189, is expected in stores next month.

Also due out next month is "The Core Program" ($24.95, Bantam) by Peggy Brill, a Manhattan physical therapist who outlines a 15-minute daily program to stretch and tone the large muscle groups of the back, abdomen, hips and pelvis. Though designed with women in mind, most exercises are useful for both genders, and their goals are lofty: to strengthen, tone, balance and align; reduce the risk or relieve the symptoms of osteoarthritis, osteoporosis and urinary stress incontinence; help people learn to identify their own "core" weaknesses; and provide relief from many chronic injuries and illnesses.

Despite all that, Brill emphasizes that such an exercise program doesn't preclude our need for what we already know: aerobic exercise, weight training and a balanced diet, including plenty of water.

Maybe we could call that "the core — and more."

Molly Martin is assistant editor of Pacific Northwest magazine. Ellen Banner is a Seattle Times staff photographer.