INSIDE an unmarked warehouse here, not far from a depressing stretch of fast-food joints and the Southern X-Posure strip club, Robert MacDonald — nickname: Maximus — is torturing a group of people.

Or at least that’s how it looks. One man, howling in agony a second ago, has collapsed in a pool of sweat. A woman wipes away tears. A few of the rest are limping.

Maximus is not sympathetic. After all, they had been warned. It’s right there on the Web site: “You were free to choose and you did. Now lie in it.”

This is Gym Jones, a no-frills private club that caters to extreme fitness buffs, professional athletes, the military’s special operations and — on the opposite end of the pampered scale but only slightly less secretive — movie stars. (Jude Law didn’t get those contoured pectorals in “Repo Men” by accident.)

Yes, the name is an overt nod to Jim Jones, the sect leader who steered more than 900 people to suicide in 1978. No, the couple that owns the gym, Lisa and Mark Twight, don’t see anything obnoxious about that. “We knew some people would call us a cult,” Ms. Twight said, “so we decided to own the joke.”

The zealous devotion clients have toward the gym and its fitness philosophy, which turns as much on psychology as it does on physicality, can indeed be a little frightening. Picture Scientologists, except with really big biceps.

One prominent fan is Zack Snyder, the Warner Brothers director whose testosterone-turbocharged “300” became a global smash in 2007. It was Gym Jones, using a mix of power lifting and military-style calisthenics, that whipped Gerard Butler, the star of that film, into loincloth shape.

Superman himself has recently been training here. Henry Cavill, hired to play the title character in Mr. Snyder’s coming “Man of Steel,” arrived with eight-pack abs, or close to it, from training for another film. But Warner brought on Mr. Twight to mold the British actor into a true superhero; the two have been working together since April. “It’s not just about muscles,” Mr. Snyder said. “In fact, that’s the least of it. They have a way of making you find things in yourself, and it’s fantastic.”

That’s easy for him to say. Mr. Snyder’s days don’t include a 5,000-calorie diet and more than three hours of pumping iron, like Mr. Cavill’s. “Mark gave me, or perhaps I could say allowed me, to discover my love of pushing myself beyond my notions of limits,” Mr. Cavill wrote in an e-mail. “He also gave me not an arrogance but a humble and healthy belief in my physical abilities, which I never before possessed.”

Trainers of this elite caliber tend to dismiss celebrities as dabblers who want results without the work. And don’t get the serious fitness crowd started on the use of steroids in some corners of moviedom.

Gym Jones is no exception. But the Twights and Mr. MacDonald, the gym’s manager, have also learned to open their minds. “Take Jude Law,” Mr. MacDonald said. “At first I wasn’t interested in Jude Law at all. He smoked, drank and was out of shape. But he surprised me and worked really hard. He proved me wrong.” (Mr. Law declined comment.)

A willingness to take on famous clients has actually been problematic for Gym Jones. The studio cash is nice, and the “300” notoriety was rewarding; a version of a 300-rep workout designed for the cast as a graduation test has gone viral and was even plugged by Men’s Health. But the Twights prefer privacy. They aren’t angling for their own line of protein powders or a reality show, and accept only 30 to 40 clients at a time. If you are hearing about them through their work with stars, a tiny part of the gym, your chances of getting in are pretty much zero.

The Twights generally require an interview or a referral from a current Gym Jones client, the completion of a written application that’s more of a fitness SAT than anything and, if you pass that step, a workout with Mr. MacDonald, a world champion mixed-martial-arts fighter. “If I’m surrounded by substandard people, I’m not going to work that hard myself,” Mr. MacDonald said. Again, it’s right there on that full-of-itself Web site: “We choose clients. Clients don’t choose us.”

Gym Jones has another reason to guard its privacy: its military customers like it that way. Although the Twights refuse to talk much about this side of their business, which occurs inside the gym and in the nearby mountains, it appears to be considerable and to involve people who are supposed to be invisible. Six of Mr. Twight’s former students, for instance, were among the 30 Americans — most of them Navy Seals, including members of the team that killed Osama bin Laden — who died in Afghanistan in August when their helicopter was shot down.

But don’t push for more details: “ ‘No’ is a complete sentence,” Ms. Twight said. “I don’t need to give a reason.”

GYM JONES, of course, is not the only fitness business thriving on tough love. From the boot camp programs that have become trendy in big cities to the screeching trainers on “The Biggest Loser” to CrossFit, a chain that has gained attention for promoting risky exercise, Americans seem to want their workouts served with hiss and spit.

Theories abound as to why. Fads tend to oscillate: the karate- and steroid-fueled bodybuilding of the 1970s (which notably accompanied the rise of women in the workplace) gave way to Jane Fonda’s aerobics in the ’80s and spinning and cardio hell in the ’90s. Next came yoga and Pilates. More recently, economic and political upheaval, not to mention two grinding wars, have tipped people toward strength training and militaristic exercise.
 

Theater is a big part of Gym Jones, which the Twights founded in 2003 in a garage with no air-conditioning and no heat. (The couple moved to Utah from Colorado in 2001 to operate a climbing-equipment company and later started Gym Jones as a side project. Eventually, the Twights decided to go full time with Gym Jones.)

Everything about the gym’s current configuration screams hard core, from the Web site (“Don’t complain if the work is too hard, or if you pass out, drop a barbell on your head, a kettle bell on your toes”) to cold décor: cinderblock walls, black rubber floor mats, fluorescent lights, no mirrors or windows. Outside magazine described the gym as “part martial-arts dojo, part smash lab, part medieval dungeon.”

Gym Jones calls clients “disciples” and prominently displays a quote from “Fight Club,” the 1999 film starring Brad Pitt. It reads in part: “Quit your job. Start a fight. Prove you’re alive.”

But once you’re past all that, the mood at the gym is surprisingly warm. Mr. MacDonald, 33, has a daunting physical presence (at 6-foot-3, he can dead-lift 550 pounds) and blunt speaking style, but he also once taught kindergarten. The pixie-ish Ms. Twight, a 50-year-old jujitsu practitioner, has a quick, infectious laugh. A celebrated mountain climber, Mr. Twight, 50, is direct and aggressive but also quite polite and generous with his time.

And despite his stoic manner, Mr. Twight was clearly emotional over the deaths of his former students in Afghanistan. “Different from the many fatalities that occurred throughout my climbing career, but the impact is still quite strong,” he wrote in an e-mail.

The Gym Jones Way notably does not involve a bodybuilding-centered program of progressive overload and over-feeding. Pushing people so hard that they riskrhabdomyolysis — a condition brought on when severely damaged muscle tissue releases toxins into the bloodstream —“is a tremendous failure,” Ms. Twight said. She added that the gym encourages its trainers to get to know their subjects personally, whether than means going shopping at the grocery store after a workout or lending a sympathetic ear.

“It’s like sex,” she said. “You can’t just get up and leave afterwards. This is a relationship.”

It’s possible that a reporter’s visit has prompted this convivial tone, but Mr. Snyder and other clients — sorry, “disciples” — insist otherwise. “They don’t belittle you,” said Mr. Snyder, who has trained with Mr. Twight over the years. Carolyn Parker, a climbing guide who lives in Albuquerque, where she goes by the nickname Blitzkrieg Barbie, also emphasized the lack of drill sergeant behavior. “They’re more interested in training the mind than in physical fanaticism,” she said of the Twights.

Working with Gym Jones involves completing excruciating tasks. They use some basic machines — rowers, Airdyne stationary bikes — but mostly rely on free weights, kettle bells and low-tech equipment like weighted sleds. The Jones Crawl involves three rounds of dead-lifting 115 percent of your body weight 10 times and then jumping on and off a box 25 times as fast as you can. “Recovery” at the end of a workout might involve a 30-minute run and an ice 

At the end of the day, working out is just picking stuff up and putting it down. The complicated part, at least at Gym Jones, is the mental workout: constant awareness of what helps or hurts progress, including maintaining proper nutrition and executing recovery accurately. “Breaking someone in a workout is pointless without post-training discussion and analysis,” Mr. Twight said.

FITNESS companies inspired by Gym Jones are popping up around the country, mostly opened by people who have apprenticed with Mr. MacDonald and the Twights. Ms. Parker owns Athena Fit in Albuquerque; one in St. Louis is called Project Deliverance.

The Twights say they have no plans to expand beyond Utah, but they have increased the number of seminars they hold, which cost $1,500 to $2,000 a person and range from a two-day Fundamentals course to the five-day Advanced Program Design and Theory. Sessions with a trainer cost from $100 to $300 an hour; clients come either for a few days or on a regular basis.

The Twights now offer Web-based memberships starting at $500 a year; participants log into GymJones.com, which has more than 70 videos and 17 training plans, among other materials.

It was the advanced program that Mr. MacDonald, barefoot and in cargo shorts, was teaching in early August, the one that culminated in tears and sweat. But that was only a small part of the course. The eight participants spent much of their time in front of a white board at the back of the gym.

There, they sat in folding chairs and listened to Mr. MacDonald give a lecture. The class members, some of whom had traveled from as far as London and New York, took notes and followed along with a 135-page study guide.

“You can never get rid of endurance work,” Mr. MacDonald said, stabbing at the board with a marker. “If you need to get rid of something in your workout, it needs to be the weight room.”

He put the cap on the marker. “And always — always — be prepared to go to a dark place,” he said. “Now, who’s ready to work out?”