The bottom line - The man who gave J-Lo her perfect bum
He keeps A-list bodies trim, toned and in tip-top condition -- but don't expect trainer-to-the-stars Gunnar Peterson to brag about it
You often hear 'Gunnar' cited on American chat shows, and name-checked in supermarket tabloids: here, as the man responsible for Sly's biceps, with their angry-haemorrhoid veins; there, as the craftsman behind J-Lo's posterior, and the svelte waist Halle Berry achieved just a few months after giving birth.
Gunnar Peterson is also, famously, the trainer who honed the bottom belonging to reality TV star Kim Kardashian.
Not long ago, LA's starriest personal trainers was quoted by US Weekly, ascribing his lofty achievements to "dumb bells, medicine bells, and satiability balls".
Heaven knows what a 'satiability ball' is (it sounds like an Oriental sex toy, but could also be a US Weekly misspelling of 'stability ball'). Either way, in Hollywood terms, this feat of engineering is priceless.
Peterson is shortish, buff, and has light hair clipped into a sort of flat-top.
I peruse the collection of memorabilia which adorns every spare inch of wall space in his Beverly Hills gymnasium. The studio, which is perhaps the length of a swimming pool, is like a museum of American sport.
There's an important point to this display, besides giving visitors something to gawp at. It demonstrates that Peterson's client list includes as many professional sportsmen as it does Hollywood stars.
They come because, unlike many so-called 'celebrity trainers', he actually knows his onions.
Whether you're a film star looking to discreetly achieve the perfect six-pack, or a football player needing to shave a few milliseconds off your 60-yard dash, he's built a reputation as a man who gets results.
So, while tabloids write endlessly about his sessions with Hugh Jackman, Bruce Willis, or Angelina Jolie, Peterson is most proud of, say, helping to prolong the careers of 34-year-old NFL star Tom Brady, or tennis's Pete Sampras.
Given the enormous amount sports stars earn (last year's US Open winner trousered $1.7 million), these kind of endorsements makes his services staggeringly valuable.
He won't tell me his hourly rate, but I am given to understand that it is not unadjacent to $400.
Peterson, who is 49, first got into personal training by accident.
Born in Texas, but educated in Switzerland -- a result of his father being posted to Saudi Arabia -- he was an overweight child who was taken to WeightWatchers by his mother at the age of 10.
"Scarred" by the experience, he started working out at university.
By the time he arrived in Los Angeles in 1987, where he had secured an assistant's job at a talent agency, he was what you might call a fitness fanatic. Pretty soon, he boasted memberships at five different Hollywood gyms.
One day, a fellow gym enthusiast offered to pay him to help him work out. Within a few months, he had picked up several clients via word of mouth.
"I pretty soon realised that I made more money training people part-time than I did from my job." He went pro in 1989.
It was good timing. The modern fitness industry, not to mention the concept of personal training, was in its infancy, putting Peterson in the centre of a fast-growing market. Within a few years, he'd acquired enough clients to open his own gym.
Peterson's career has seen tectonic shifts in the way action movie stars are supposed to look.
"Jimmy Stewart said a few years ago that it was much harder now to be a leading man than when he came up. Back then you needed to be a good actor and have a certain look.
"Nowadays you've got to be a good actor, have a certain look, and also have this amazing physique."
The fact that leading men who manage to look "amazing" can command as much as $20 million a film has only added to demand for his services among the A-list.
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His assistant Caralyn is also his romantic partner (Peterson is divorced, with two children from his marriage). Part of her job (in the former, rather than latter role) is to weed out would-be clients who want to train there purely because they might bump into a celebrity.
"Gunnar is one of the few people in his clients' lives who isn't going to want something out of them, like wanting them to do something, or read his scripts. We try to make this a place they come and be an extension of their home."
He leaves me with an eternal truth in his industry: that anyone in the fitness industry who promises results without hard graft is a charlatan.
"The word 'work' is in 'workout'. It's not called a playout or a hangout. It's a workout. There are no short cuts. Never believe anyone who says otherwise."