Chicago ? In a workout area smaller than a Bally’s locker room and quieter than most libraries, Elizabeth Brostoff slowly lifts weights before quickly moving to another machine.

The only noises at the Northbrook, Ill., training center are Brostoff’s grunts and her trainer’s appeals to slow down.

“Really, really slow,” said trainer Karen Heffernan, checking her stopwatch for how long Brostoff took to complete one lift. “Way too fast. That was six seconds.”

Fifteen minutes after it started, the SuperSlow workout is done. Another quick round with the machines later in the week, and Brostoff’s weekly fitness regime is complete.

Tapping into a growing market for quick, no-frills workouts, the controversial SuperSlow exercise routine has been gaining a following. Like Curves and other specialty workout facilities, it is part of an overall boom in gym memberships.

Health-club memberships increased to 36.3 million people last year from 29.5 million in 1998, a 23 percent rise, while the number of clubs grew 43 percent, to 20,200 this year from 14,100 in 1998, according to the International Health, Racquet & Sportsclub Association.

The association does not keep statistics on SuperSlow or other niche training.

Often found in suburban strip malls, SuperSlow training centers stress simpler, less social workouts than other gyms.

Participants work with a trainer to lift weights–10 seconds up, 10 seconds down–on a half-dozen or more machines. No jogging, aerobics or other strain is required.

At first, SuperSlow seems like the fitness world’s version of the seemingly permissive Atkins diet, although Brostoff’s drained expression after her workout suggests otherwise.

“I’m toast,” the 28-year-old said. “That was a good one.”

The SuperSlow concept, building strength by slowly lifting weights until muscle exhaustion, is not new. Nautilus founder Arthur Jones was an early advocate.

Ken Hutchins, who worked with Jones at Nautilus, updated the concept by creating a SuperSlow licensing and certification program and getting trademarks on the SuperSlow name in 1992.

“I was frustrated by the lack of standards and science,” said Hutchins. He operates a SuperSlow center near Orlando, Fla., trains instructors and has a separate company that make equipment for SuperSlow training.

Hutchins, who started the venture with his own money, said he brings in $200,000 annually training people and another $750,000 from the equipment business.

Today, there are 300 certified SuperSlow trainers, including large concentrations in Chicago, New York City and Santa Rosa, Calif., Hutchins said. Those trainers can bring in $30 to $45 for each half-hour workout in the Chicago market.

The controversial claim made by SuperSlow trainers–that people can stay physically fit by completing one or two of their workouts weekly–is also one of its biggest appeals.

Owners of SuperSlow gyms said their facilities attract men and women of a wide range of ages. What the clients have in common is a disappointment with other workouts, particularly their time requirements.

Like Curves, a gym chain for women that offers quick workouts on a circuit of machines, SuperSlow facilities are usually bare-bones workout centers.

“We want a room that’s kept cool,” Hutchins said. “No socializing. No mirrors. A room where it’s not treated as a bunch of fun and games.”