NAVY Seals are legendary for their tiptop physical condition, but have you ever wondered how they stay fighting fit out in the field?

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Darcy Padilla for The New York Times

Kurt Dasbach, the creator of Inkaflexx straps, with Kathy Lee Bickham at Equinox in Darien, Conn.

Aaron Baldwin, 43, who retired in December as a master chief in the Seals, used to make barbells out of nothing more than plastic milk jugs, fresh concrete and a sturdy tree branch.

?e? make one weight and use it until we had to move and start over,?Mr. Baldwin said.

Things changed in 2002, when a Navy Seal turned entrepreneur sent Mr. Baldwin a test model of the TRX system, a suspension gadget made of a pair of straps with handles joined by a metal clasp ring. To set it up, he only had to wrap the straps around a freestanding pole or over a thick branch. Strength training became as simple as placing his feet in stirrups to suspend them off the ground, then performing dozens of exercises like knee tucks or pushups.

After 45 minutes of so-called suspension training, Mr. Baldwin exhausted his body from shoulders to calves using just the 170 pounds of his weight. Better yet, the two-pound straps rolled up to the size of a military bag lunch.

In the last year suspension training has entered the mainstream after two kinds of straps landed on the market: TRX and Inkaflexx. They have attracted the attention of personal trainers and group fitness directors as strengthening tools that also improve balance and flexibility. Suspension workouts consist of either hanging the legs or leaning back while gripping the straps and then performing a variety of moves.

The beauty of suspension training, its advocates say, is that you can? help engaging your core to steady yourself. On the other hand, critics warn that the instability of suspension straps can result in injury, especially if you have a history of joint or back injuries, or inadequate core strength.

Personal trainers use TRX equipment at over 1,000 gyms nationwide, according to Fitness Anywhere, its maker. Roughly 10,000 $150 sets of straps have been shipped.

Group classes for suspension devotees have begun to crop up nationwide. At Crunch, a class called BodyWeb With TRX is in full swing at two San Francisco outposts, and so is a pilot class featuring Inkaflexx equipment at Equinox in Darien, Conn. By mid-March, Equinox plans to offer TRX or Inkaflexx classes in Boston, Los Angeles and a new Manhattan club on Park Avenue. This summer, Life Time Fitness will add suspension training classes at a handful of its 60 locations in 16 states.

?t? like yoga on ropes because it takes a lot of balance,?said Mark Undercoffler, 32, a public relations executive in San Francisco, who has attended the Crunch BodyWeb class for three months. ?he TRX works every part of your body in 50 minutes, especially your core. It? the quickest way to get a cardio and muscle workout in less than an hour. I sweat as much in BodyWeb as I do in spin class.?/p>

The BodyWeb With TRX class ?which involves lunges, chest presses and one-legged squats to high-energy dance music ?is so fast paced that some say it amounts to a cardiovascular workout. At Equinox a mellower class called Inka focuses more on flexibility. To Andean flute music, participants are led through a series of stretches and a handful of strength-training moves.

Suspension training is having a moment partly because some trainers and clients are bored with the ubiquitous balance equipment like stability balls. Interest in suspension straps is also high because a theory called functional training has been making slow but steady inroads in the fitness business. It advocates strengthening muscles synergistically, rather than in isolation.

?ith so much emphasis put on core and functional training, the timing is right?for suspension training, said Kathie Davis, the executive director of IDEA Health and Fitness Association, a trade group. ?t has staying power because it has good education and programming behind it. Usually the trends that come and go are the ones that don? have good educators putting together interesting programs to go with the equipment.?