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.
"We'd 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't 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.
"It's 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. "The TRX works every part of your body in 50 minutes, especially your core. It's 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."
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.
"With 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. "It has staying power because it has good education and programming behind it. Usually the trends that come and go are the ones that don't have good educators putting together interesting programs to go with the equipment."
Randy Hetrick, the inventor of TRX straps, and his company, Fitness Anywhere, have developed over 300 exercises and taught 200 personal trainers and instructors in daylong seminars. After taking one, Susane Pata, a group fitness director at Crunch, designed the BodyWeb class.
Not everyone agrees that suspension training is appropriate for the masses.
Fabio Comana, a research scientist at the nonprofit American Council on Exercise, said that it might be valuable for well-conditioned athletes and gymgoers who regularly train their core, the muscles closest to their spine. But at best it is inappropriate for people who haven't built up their core and at worst is potentially dangerous for them. "A segment of the population doesn't have joint integrity and the ability to stabilize their entire body," he said. So instead of using their core, they use the wrong muscles, aggravating their risk for injury.
Mr. Comana, who has a master's degree in exercise physiology, added, "I don't mean to doubt it, because I do like it, and I've been using it for the last year." But he counts himself among the experienced athletes who stand to benefit from the straps.
Walter Thompson, a professor of kinesiology at Georgia State University in Atlanta, who reviewed instructional videos of TRX exercises, sees benefits but also risk. "My impression is that this would be effective for a small group of highly fit men and women," Dr. Thompson said. "But I see potential for muscular, skeletal and joint injuries. Particularly hyperextension of wrists, elbows, shoulders, ankles and knees."
Fraser Quelch, the director of education at Fitness Anywhere, disagreed. To avoid injury, he suggested that deconditioned users perform moves with one leg behind the other, reducing instability.
Advocates of suspension training also say that adjusting body position can make movements easier. For example, standing at 90 degrees and holding TRX straps keeps upper-body exercises manageable.
Mr. Baldwin, who spent five years in the service teaching recruits conditioning and combat skills, said, "It's easy to do a pushup with your hands against a wall, and it's a lot harder to do one on the ground." Suspension training, he added, "allows you to do exercises at every angle between the two."
Kurt Dasbach, the creator of Inkaflexx, argues that its trapezelike design - straps hanging from a wall joined by a bar - makes it inherently a bit more stable. "Inkaflexx is secured to the wall or ceiling by two anchors, so you're not countering instability in every direction," said Mr. Dasbach, a personal trainer at Equinox. "As a result, it makes a lot of the movements accessible for many people because it offers more stability than the TRX."
In a decade of working with Michael Carson, a personal trainer at Pro Gym in Brentwood, Calif., Jennifer Roth has tried dozens of new new things like stability balls and resistance cords. But Ms. Roth, 42, a carpet designer, said she likes the suspension strap best. "There's always something new and more advanced you can bring to it, whether that is trying a new move or simply making it more difficult by changing your body angle," said Ms. Roth, a college gymnast and swimmer. She has used TRX twice a week since July and credits it for leaner muscles and increased strength in her obliques.
The two systems on the market differ in design, not least because they are products of starkly different hothouses. Mr. Hetrick was a Navy Seal squadron commander in the late 1990s, when he created the first prototype of TRX out of parachute webbing and a carabiner ring.
"We were deploying throughout Bosnia and Southeast Asia in submarines, ships, warehouses and safe houses, all of these space-constrained environments where it's hard to do well-rounded training," he said.
In 2005, four years after he left the service, Mr. Hetrick began marketing his product to the fitness business. Coincidentally, Mr. Dasbach created a similar apparatus inspired by his years as a professional soccer player in Chile, where a coach tied ropes to a wall to help players stretch.
Marke Rubenstein, 53, an advertising executive in Stamford, Conn., was intimidated when she first saw the Inkaflexx straps. "I'm not very athletic and I'm not great in various difficult yoga poses, but I feel very comfortable with this," she said after six visits to Mr. Dasbach's class. "It's challenging but not too difficult, and I can always modify the straps to make them work for me."