SALT LAKE CITY — 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 website: “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,” Lisa 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.

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 Snyder’s coming “Man of Steel,” arrived with eight-pack abs, or close to it, from training for another film. But Warner brought on Mark Twight to mold the British actor into a true superhero; the two have been working together since April.

“It’s not just about muscles,” 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.”

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 MacDonald, the gym’s manager, have also learned to open their minds.

“Take Jude Law,” 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.” (Law declined comment.)

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 MacDonald, a world champion mixed-martial-arts fighter.

“If I’m surrounded by substandard people, I’m not going to work that hard myself,” MacDonald said. Again, it’s right there on the website: “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 the Twights’ 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,” Lisa 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.

The Gym Jones Way notably does not involve a bodybuilding-centered program of progressive overload and over-feeding. Pushing people so hard that they risk rhabdomyolysis — a condition brought on when severely damaged muscle tissue releases toxins into the bloodstream — “is a tremendous failure,” Lisa 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 afterward. This is a relationship.”

It was the advanced program that 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 MacDonald give a lecture. The class members took notes and followed along with a 135-page study guide.

“You can never get rid of endurance work,” 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?”