“If Dwayne Johnson and LL Cool J and Khloé Kardashian can have more than one job, I can too,” Gunnar Peterson told me one July afternoon as we sat in the waiting room of his posh Beverly Hills gym. I’d asked the famous trainer how he balances his workload, and Peterson looked at me as if I’d posed a daft question—like asking a tiger if it gets anxiety.

Peterson became the director of strength and endurance training for the Los Angeles Lakers this spring, which sounded like a full-time job to me. But Peterson, in his mid-50s, occupies a special role in the rarefied realm of L.A.’s personal trainers: He’s the go-to celebrity ass-whupper. His workouts have honed some of the most fetishized body parts in Hollywood for over 25 years, from assorted Kardashian glutes to Mike Tyson’s fearsome biceps to Matthew McConaughey’s immaculate Magic Mike abs. This means he hardly ever stops working. Sitting still made Peterson seem a little antsy, as though he’d calculated how many renowned deltoids and derrieres would sag for every minute spent gabbing with a writer instead of doing what he has been put on this round earth to do: shape famously perfect bodies.

Peterson had just come from a private training session (he still works with a stacked roster of celebrity clients; that day, he’d seen Scandal’s frequently shirt-free Scott Foley), and after our interview, he was headed to the airport for the Idea World Convention, a fitness technology summit in Las Vegas. “It’s a great conference,” Peterson enthused. We were accompanied by a publicist from GymGO, a virtual personal-training startup that also recently hired Gunnar as its “chief training officer.” Part of his duties: attending the Idea World Convention with GymGO.

Peterson’s gym has been at the same quasi-secret location for a decade, and he just signed a five-year extension on his lease. I recognized it upon entering from the Instagram posts of incredibly famous people, like Dwayne Johnson and Jennifer Lopez. I can’t think of another figure in the fitness world who can gather this kind of star wattage. Hell, besides studio execs, it’s difficult to imagine many other people in Hollywood, period, who can.

#TBT gettin' after it hard core with my family and fellow gym beast @jlo. #HardestWorkersInTheRoom #Sistah A post shared by therock (@therock) on Jun 15, 2017 at 6:55pm PDT

If you’ve never seen the gym, imagine that Bumblebee from Transformers got cursed by a witch and was turned into a tastefully arranged bundle of upscale workout equipment. Everything is yellow and black and sleekly industrial. The biggest tire I’ve ever seen in my life lay on the ground, there for clients to pick up in feats of strength. RIP, Bumblebee.

Peterson’s assistant offered me some water, which turned out to be grape-flavored Propel electrolyte water. Not coincidentally, Peterson has a partnership with Propel electrolyte water, which is why the word PROPEL is written in gigantic block letters in the parking garage of his gym. Propel electrolyte water tastes as if a prankster has carefully eye-dropped a small quantity of cough syrup into normal water, so I discarded it on a coffee table next to several Shape magazines as I waited for Peterson to finish up with a client. The waiting room is an altar to Peterson’s frenzied productivity. Every spare inch of wall space is plastered with laminated and mounted press clippings from The Wall Street Journal, Fitness, and People, plus a signed Expendables poster (Sylvester Stallone and Bruce Willis have been clients). “The overall aesthetic recalls one of those Italian restaurants where the walls are filled with thank you messages from famous diners,” The Independent declared, correctly, in 2011. On the table next to me, a stack of books authored by Peterson—The Workout and G-Force (which are different editions of the same book)—stood beside a package for a dietary supplement called “14 to Lean,” decorated with his smiling face. Next to that sat a photograph of Peterson with Tom Brady at a baseball game, inscribed with an affectionate message from the quarterback: “Cup my nuts and say my name!”

Like Brady, Peterson has television-ready anodyne good looks, and even though Brady is the professional athlete, I’d be more intimidated to work out with Peterson, who is the concept of the Protestant work ethic incarnated as a shredded middle-aged man. He has the Nordic face of a man who would definitely survive a Robinson Crusoe situation. He would subsist on tree bark, and herring caught with nets woven from the tree bark. He’d do pull-ups on the stranded wreckage of a plane wing, casually—too casually.

He’s an entrepreneur with an empire built on muscle, and like wellness figureheads before and (mostly) after him, he has earned a fortune by convincing people that he holds the key to making bodies better. To that end, he has been endorsed by just about everyone in Hollywood rumored to have individually insured body parts, plus a varied roster of professional athletes, from Pete Sampras to Kevin Love.

I wanted to know what he seems to know—how to get a better body—but I’m also wary of what he’s selling: the promise of a better body. Before meeting him, I wondered if Peterson would be a veiny Deepak Chopra, or maybe an aggro Tracy Anderson. I couldn’t quite figure out his gimmick, why he had risen from a self-promoting fitness buff to one of the longest-lasting and most trusted trainers in the industry. Over the past decade, a cultural fixation on wellness has elevated all sorts of figureheads, from GOOP-approved trainer Tracy Anderson, who has advised women to lift light weights and recommends two-hour daily workouts, to Silicon Valley investor Dave Asprey, who recommends “hacking” biology by following a diet involving butter-infused coffee. Peterson’s brand of fitness evangelism lacks an obvious hook, and yet he has risen to—and maintained—an uncommonly prestigious position in an increasingly crowded field.

Although he is now obsessed with efficiency and diligence, Peterson began his personal training career in haphazard fashion. He lived a jock-proximate life at Duke University but did not play varsity sports, and early on he decided to focus on where he could excel among his fraternity brothers and athlete pals. “It was kind of obvious that I wasn’t gonna be the strongest guy,” Peterson told me, “so my thought was, at least I should have the best form.”

He graduated in 1985 with degrees in physical fitness, psychology, and nutrition, and hung out in gyms in North Carolina as he tried to figure out his next step. His willingness to help fellow wannabe hardbodies morphed into a business opportunity. “A guy asked me if he could train with me, and I said sure,” Peterson said. “And then he asked what I charged.” He was not one of the gym’s personal trainers, but he agreed to the scheme anyway. “I started training him, and sort of cat-and-mousing the gym,” Peterson explained, easing back on his couch.

He didn’t stay a rogue trainer for long, taking his combination of shrewd opportunism and work-hewn knowledge out west. Peterson moved to Los Angeles in 1987, in the heydey of spandexed aerobercise. When Peterson started out, Richard Simmons had only recently gained traction with his “Sweatin’ to the Oldies” VHS tapes, and the word “gluten” didn’t appear in casual conversation. He taught spinning for seven years as he built up a personal-training client base, including a not-yet-famous Kris Jenner. His propensity for attracting famous clients impressed a then-teenaged Khloé Kardashian, who used to accompany her mother to her daily workouts.

“I would always see these high-profile people come into the gym, like Cameron Diaz and Puff Daddy come in,” Kardashian told me, “and I thought it was really cool.”

Although it took years of building his reputation to acquire high-profile clients, Peterson was familiar with the ultra-wealthy; his ex-wife, Janet Crown, is from a monied family (her billionaire father owns stakes in the Chicago Bulls and the New York Yankees) while his brother, Tor Peterson, is a billionaire commodities trader. “Peterson, in other words, is just what celebrities want—someone to talk to who could not care less about talking to them. That’s his brand: the celebrity trainer who does not care about celebrities,” a 2011 Bloomberg piece argued, citing Peterson’s ease with the 1 percent as his foothold in the industry.

I’m sure it is easier for celebrities to shoot the shit with someone from their financial bracket, but I doubt Peterson’s family’s history with the Forbes 400 would matter much if he didn’t coax results from his scrutinized clients. Peterson’s star has risen in a trajectory reminiscent of the reality television family that works out ardently with him. Like the Kardashians, Peterson has endorsed and participated in a wide and eclectic array of partnerships and media plays in search of exposure and extra dough, even appearing on The Nanny. He has 18 fitness DVDs, which lead people through a “Core Secrets” workout, and he has had partnerships with Under Armour, LG, Gatorade’s Propel, Clif Bars, and Adidas. His most, uh, unusual gig: working as a spokesperson for the “PetFit Challenge” promoting workouts for dogs and cats. I am extremely sad to report that the videos of Peterson’s workouts for dogs appear to have been wiped from the internet, although descriptions from media reports still, mercifully, linger online: “The videos are slightly reminiscent of those goofy high school health education films, complete with a stiff introduction by Dr. Chad Dodd, synthesized beats and a few unintentional laugh-out-loud moments,” SFGate wrote at the time. Peterson also contributed pet fitness tips to Prevention magazine, where he encouraged creating “dogstacle courses.”

Peterson groomed himself into a media mainstay, writing a column for Muscle & Fitness magazine and serving as a contributing editor for Glamour, as well as writing for Clean Eating. He has become a staple guest whenever a nightly news program or talk show needs an exercise expert, an unflappable, good-natured sound-bite machine in a polo. He’ll even talk to TMZ, as long as he stays on message. “It’s all about getting the work done,” he promised the online tabloid about the Lakers.

As he has gained a reputation as an “enter-trainer,” Peterson has doubled down on his actual training hours and has mostly abandoned his cheesier partnerships. With increased visibility, he has pulled back on the salesman aspect of the job in favor of the job aspect of the job. In this key way, he differs from earlier celebrity trainers like Jillian Michaels, who has become a television personality first and a trainer second.

“Gunnar is an OG. He’s not necessarily a trendsetter, but that can be a good thing,” Self special projects director Amy Eisinger told me by email. Eisinger is a personal trainer herself, and sees Peterson’s practicality and avoidance of fads to be one of the secrets to his longevity. “He’s been at this since before ‘wellness’ was ever a buzzword. His focus on functional fitness and foundational training has allowed him to be an innovator without falling victim to fitness fads. He’s not out there pedaling strange gear on infomercials, he’s using weights, resistance bands, and people’s own bodyweight in original and inventive ways because that’s what works.”

While Peterson is certainly invested in the concept of health, he tends to shy away from the pitfalls of the “wellness” industry that have emerged around him.

According to the Global Wellness Institute, the “global wellness industry” pulled in more than $3.7 trillion in revenue in 2015 (its most recent tally). This market includes a glut of celebrity moguls, including Gwyneth Paltrow’s GOOP, Alex Jones’s Infowars (a supplements business disguised as a conspiracy scare factory), and Jessica Alba’s Honest Company. It also includes fitness pursuits like the Instagram-centric empire of Australian “bikini body” entrepreneur Kayla Itsines. Upstarts like Leanne Ratcliffe, a.k.a. “Freelee the Banana Girl,” have gained notoriety and surprisingly wide reach, often by espousing extreme ideas. Ratcliffe’s YouTube videos have been viewed over 265,703,500 times altogether (at the time of this writing), and she advocates a “fruitarian” diet, focused on eating bananas. Big names are also still jumping into the field, for example Arianna Huffington, who launched a wellness startup called Thrive Global. While Huffington has no forceful banana opinions, her foray is also on the quirky side; Thrive Global sells products like a $100 13-inch-long solid-wood bed for smartphones, complete with miniature satin sheets.

For a “Hollywood fitness guru,” Peterson’s training advice is, in 2017, refreshingly reasonable. He doesn’t recommend any specific gym memberships or restrictive diets. He doesn’t promise quick fixes or unrealistic results. Reading his book, The Workout, is like talking to your most level-headed friend who goes to the gym. Sample advice: “You need to give every workout your best effort, but realize that not every workout is going to be your personal best.” He preaches consistent exercise above all else, focusing on old-school movements like squats and push-ups.

Peterson is also much more equivocal about the supplements industry than some of his peers. Although he had sold his own supplement, the Gunnar Peterson System “14 to Lean” kit, Peterson was openly skeptical about supplements when I asked him about the industry. “I would go to a nutritionist,” he said, “so you’re not wasting your time and money buying stuff you don’t need or maybe your body doesn’t process properly, or maybe it wasn’t made from a quality level that even works for you.”

While he closely follows new workout tech, Peterson’s primary interest in introducing new machines and gadgets is to keep people engaged and having fun in their workouts, rather than trying to inculcate them into a fad. “You own a restaurant, you’ve gotta buy new plates and glasses. That’s just how it works,” he told me. He is equally matter-of-fact about his shift into fixing the Lakers. “They’re not at their best right now, but they’re building something,” he said, with genuine enthusiasm.

His gimmick, as far as I can tell, is that he works incessantly and is good at his job(s) and develops a goofily bro-y rapport with his clients. “Gunnar is a riot,” Khloé told me. The guru stuff is just window dressing, a way to entice people to sweat with him. This man wakes up at the crack of dawn and appears earnestly overjoyed about hurling medicine balls at walls. He’s forthright about expecting clients to show up and put in effort, and while he’s as effusive as any of his contemporaries about the benefits of a healthy body, his focus remains on exercise and sensible eating rather than complete lifestyle overhauls involving herbal dusts or charcoal juices.

The most excited Peterson became when I talked to him was when I asked him to show me Hoop Hands, a basketball training device he invented years ago. It’s essentially a basketball with an eye-hook attachment to turn it into a resistance ball for shooting and passing drills. I’d read about Peterson’s foray into fitness technology invention, and wondered if it was part of his earlier media-and-endorsement blitzes or whether it was something he actually thought people would use.

“I didn’t want to be the guy who goes, oh, well, I came up with a cool idea but never really did anything,” he said, noting that even though he’s never sold any Hoop Hands, he’s given them out to college coaches and used them with his kids over the years. But just because he hadn’t manufactured it didn’t mean he’d given up on it—he had already brought one down to the Lakers’ gym.

Like everything else about Peterson, I suspected Hoop Hands would be mostly glossy hokum, and then it turned out to be endearingly practical and focused on performance. He was, as it turned out, entirely fit for the job.