LA's Mr Muscle: Meet the man who shapes celebrity

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.

A pearl of wisdom greets visitors to Gunnar Peterson's Beverly Hills gymnasium. It reads: "Never trust anything that can shit and run at the same time!". This maxim, with which it seems foolish to argue, was dreamt up by the evergreen action hero Sylvester Stallone, who scrawled it in black marker pen on a framed photograph which shows him riding a galloping horse. Plenty of running takes place in the room where Stallone's message now hangs. The Rocky star is one of many local celebrities who employ Peterson, who is one of LA's starriest personal trainers. 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. He is also, famously, the trainer who honed the bottom belonging to reality TV star Kim Kardashian (a woman whose celebrity status, as it happens, is due to be sealed today in a lavish California wedding).

Not long ago, Peterson 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. You might say that Peterson's studio – on the upstairs floor of an address that I am sworn to keep secret, so as to ward off paparazzi – represents hallowed ground.

The man himself is busy when I arrive, administering a massage to a very tall man who I do not immediately recognise. "This is Kevin," he says, manipulating the client's nether regions. Later, I realise that he's talking about Kevin Love, a star basketball player for the NBA's Minnesota Timberwolves and one of the most valuable young men in American sport. "We'll be done in a few minutes," adds Peterson, who is shortish, buff, and has light hair clipped into a sort of flat-top. "Make yourself at home."

I peruse the collection of memorabilia which adorns every spare inch of wall space. Peterson's studio, which is perhaps the length of a swimming pool, is like a museum of American sport. On one wall, you see boxing gloves belonging to Mike Tyson and Sugar Ray Leonard, who he once trained. On another hangs a pair of framed Y-fronts, complete with sweat stains, belonging to an athlete he (sadly, but wisely) declines to name. In one section are autographed basketball vests; in another, ice-hockey sticks. On the ceiling are dozens of balls, bats, and other signed items of used sports equipment.

The overall aesthetic recalls one of those Italian restaurants where the walls are filled with thank you messages from famous diners. Many of the autographed items in Peterson's collection carry chummy little in-jokes which betray the close relationship he has managed to forge with the owners of the world's buffest physiques. "Looking is for free," reads a suggestive note written by NFL star Reggie Bush. "But touching's gonna cost you!"

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, who publicly thanked him after the 2001 US Open, which he won at the age of 31. 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.

'One of my guys won the MVP award in the NFL this year, and I just love that," he says, when I ask what he enjoys about coaching pro athletes. "I love watching on television and hearing the commentator talk about a client and say 'Every year this guy brings his A-game'. I know it's because he calls me in the off-season and works hard." Sampras was probably the "hardest worker" he ever met. "I like being around people like that, because gyms are one of the few places where the harder you work, the better you always get."

We decide to pump some iron. First, like every Peterson client, I get to choose both a soundtrack ("What's your music? Pop? Hip-hop? Classics?") and a flavour of Gatorade to refresh myself. Then I must surrender to Peterson's commands, barked out enthusiastically, with the occasional "Yes sir!" when he reckons it might inspire me to break more sweat.

The next hour gives lie to the notion – often circulated by keep-fit refuseniks – that gyms equal boredom. We flit from state-of-the-art machine to state-of-the-art machine, exchanging small talk about anything from politics, to the summer movie market, to the blockbuster ski season California recently enjoyed. From time to time, he dispenses economical but effective nuggets of information and advice on our workout activity. If I'm using a device incorrectly, which I occasionally am, he gently tells me. Sixty minutes fly past.

We pummel muscle groups I never knew existed. He escorts me from familiar pieces of kit, such as exercise bikes, to completely alien ones, including a moving staircase which speeds up the faster I run, and a rotating platform designed to improve balance. In between drills, I push a weighted wheelbarrow from one end of the room to the other, or swing along a set of monkey bars. The session, he says, is called Peripheral Heart Action [PHA] training, in which you exercise different extremities of the body consecutively to increase cardiovascular activity. It certainly works. At one point he proffers a pink bucket, which properly exhausted clients are invited to throw up in.

As Peterson sees it, personal trainers have two roles. The first is to motivate, inspiring the lazy to push harder or go faster than they would in their own company. The second, and far more important, is to educate. "How much fun is it, if you've never played tennis, to pick up two rackets and a can of balls, and just hit? Not much, right? But you're with a pro, you'll hit more balls in 10 minutes than you would with an hour and a half without. The same applies to working out. But you would be amazed how many people never bother to see a trainer to make sure they're doing it right. It's a waste."

There are, he says, common gender errors in the way most people use the gym. "Women, as a rule, will only work their weak spots. Not their whole body." Men always try to lift weights which are too heavy. Both sexes are guilty of failing to vary their workout sessions. "I update my workout for every person, the night before I see them," he says. "I want them to always do something different, because each day, your body's different, and your mindset's different."

You might not call this rocket science, but devotion to the basics is almost certainly the key to Peterson's success. He's constantly reading up on developments in the fitness industry, trying new machines, or workout techniques. He likes to make every session a journey, rising at 3.45am on a normal workday to fine-tune his plans (and fit in his own daily workout). It's been this way ever since Peterson, who is 49, first got into personal training, a development which turns out to have happened 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 (with a view to making a career in entertainment), he was what you might call a fitness fanatic. Pretty soon, he boasted memberships at five different Hollywood gyms.

One day, when he was leaving an early morning workout session, a man approached him. "He said, 'Hey, I see you're always here in the mornings. I'd love to work out with you some time'. I said sure." Peterson thought the man wanted a casual gym partner. "But then he said, 'What do you charge?'. Suddenly I found myself making money from it." 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 left the talent agency job and 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. And he turned out to be pretty good at what he did. "I had been reading everything I could lay my hands on about fitness for eight years. I was obsessed by it. But I remember looking around at the competition and wondering how they got away with it. They were making basic mistakes, telling people things that were factually wrong." 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.

"I won't lie, there are days when you look at the people who come in here and think, 'Wow... did that really just happen?'," confides his assistant Caralyn, who 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."

Other trainers do things differently, and Peterson has a mixed view of rival fitness gurus. "If you are taking people who were not doing anything, and making them do stuff and feel better about themselves, then you are doing good," is how he puts it, as we wind down afterwards. "But if you are filling them with empty promises and trying to fill your bank account, then shame on you." 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." Eyeing a pink vomit bucket as I finish an hour in his remarkably smart gym, I am inclined to agree.

Gunnar's top tips for a better workout

* Have a plan and write it down, so that when you walk into the gym, you'll know exactly where you're going. You don't have to write down every weight you will lift, or the exact number of reps. But you do want a general map. The five minutes it'll take you to write a plan will save you double that amount of time during the workout.

* Make good choices outside the gym. Don't look at your training session as the fix-it for everything that's wrong with your lifestyle. This does not give you carte blanche at the dining table. It does not give you a hall pass at lunch or a get-out-of-jail card at happy hour.

* Mix it up. Don't just be Captain Eclectic, but always try to vary your workouts. If you're a woman and your butt is your target zone, learn to also work the quads, hips, abs, and arms. Firstly, it will stop the gym becoming boring; secondly, it will improve the whole body.

* Sleep. If all you do is train, your body won't have time to recover. Lack of sleep and lack of recovery time puts you under stress, and that's how you get injured. Plus, you can't eat junk food when you're sleeping.