You can hang out instead of going to a gym

Dangling like a monkey from two industrial nylon straps slung over a workout cage, former professional volleyball player Chad Convis is demonstrating how to do a dazzling array of exercises on a newfangled contraption called the TRX System.

Keeping a firm grip on the neoprene-padded handles, the TRX rep leans backward on his heels at a 45-degree angle and does pull-ups, using his own weight like a commando scaling a building.

He then stands up, turns around and leans forward into push-ups. Working up a sweat, he moves on to curls, crunches and leg presses.

Seeing the strapping 6-foot-5, 205-pounder move around his two-car garage like Spider-Man is fairly impressive, but the gear itself, while sturdy, looks like something you could stitch together in a one-hour home economics class.

The device -- several yards of nylon webbing, two heavy-duty adjustable cam buckles and a carabiner -- can be attached to any sturdy horizontal support, such as a piece of gym equipment, a beam or even a tree branch, as long as there’s plenty of space around it for movement. It can also be bolted to a stud in the wall or attached to a strong door, and the whole assembly easily fits into a small mesh travel bag.

Simple though it looks, the TRX -- for Total Body Resistance Exercise -- has rapidly gained a core of devoted followers. With virtually no advertising, the device has won over trainers who are using it with individual clients and in group classes, and with professional basketball, baseball and football players, such as NFL quarterback Drew Brees. About half of the roughly 20,000 units sold in 2006 have been purchased by law enforcement groups and the U.S. military, for both elite and mainline units.

The device, which starts at $149.95, is based on the principle of loaded instability -- pushing, pulling or lifting one’s own body weight while trying to balance the body against the unstable straps. Resistance can be increased or decreased by simply adjusting one’s position.

“It’s really an integration of strength and balance into a single format,” says Randy Hetrick, a former Navy SEAL squadron commander who invented the apparatus with help from his fellow SEALS. “SEALS are pretty innovative cats,” he says.

The back story on the TRX is straight out of a Hollywood action film. While deploying to remote areas, Hetrick and his team were spending a lot of hours in cramped operating bases -- safe houses, ships and subs. Needing a way to stay in shape, they began experimenting with ways to leverage their body weight using deconstructed parachute gear. “The dirty little secret is that all SEALS know how to sew,” he says.

After leaving the SEALS in 2001, Hetrick continued to develop the system and launched it in December 2005, marketing it exclusively at trade shows. “Trainers were the early adopters, and they talk,” says Hetrick.

David Donatucci, director of the International Performance Institute, an athletic training center in Bradenburg, Fla., has found the system helpful for golfers.

“Suspended in the air, you have to maintain a better posture,” he says. “If you have slouchy posture or swayback, it forces you to correct that,” he says.

He particularly recommends it for athletes spending long hours in hotel rooms.

Anthony Carey says he uses the TRX at home and in his work as a corrective exercise specialist with Function First in San Diego, which provides conditioning and fitness services for clients with musculoskeletal and postural problems, such as back pain and degenerative hip disease.

“Because it hangs from above,” he says, “clients can use it to do assisted squats and lunges. It allows movement but still provides an external source of stability.”

The length of the routine and types of exercises recommended will depend on the goals and fitness level of the user, but a 30-minute workout, which might include half a dozen exercises, will work up a sweat.

Still, not everyone is enamored with the straps. Some fitness professionals say the system may be inappropriate for non-athletes or weekend warriors who tend to overdo.

“Suspension training is very tough on the joints, especially the shoulder,” says Neal Pire, facility director for Parisi Sports Clubs, USA, a health club and training facility in Fair Lawn, N.J.

For example, “if you’re in a push-up position, your hands are in the straps and you’re either holding the pushup position, or actually doing the pushup on the straps, suspended in the air,” he says. Either way, “your shoulder girdles are really being stimulated, because they’re unstable,” which could lead to strained muscles and joints.

One doesn’t need to be a fitness guru to anticipate another problem: users who affix the TRX to a weak support, such as a termite-eaten door, or who lose their grip on the handles could do a face-plant on the floor.

Hetrick bristles at the notion that the device is potentially dangerous, but then Hetrick is a former Navy SEAL. He thinks that scaling a freighter in the Balkans with 100 pounds of gear on his back is a light workout.

“It’s no more dangerous than yoga or any kind of resistance training,” he says, “and I believe much safer because of your ability to self-support and choose your right level.”

Hetrick or his marketing partners are so protective of their product that their website includes a “reprint” of a glowing New York Times story about the product with all of the writer’s concerns clumsily excised.

Hetrick points out that he’s yet to receive any reports about injuries or equipment failure but allows that it’s possible.

“Some people are just knuckleheads,” he says, “and you could in theory hurt yourself bending over to pick up your shoes.”

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janet.cromley@latimes.com