Thirty years ago, virtually none of these businesses existed. A gym was a place where a man might go to learn the art of self-defence, and a woman to work at the front desk, or clean the floors. Male bodybuilders trained to grow frightening muscles and avenge the insult "that made a man out of Mac". The idea that a normal person might go to a gym to lose weight would have been as absurd as the concept of grown-ups paying to do a PE class in the park before work.

Nobody had ever heard of the "box gap" and there was no real fitness industry, just a minority of the adult population who jogged, swam, surfed or played sport and, apparently, people were fitter and thinner than they are today.

According to Fitness Australia, the fitness industry nationally is worth more than $1.2 billion a year. There are 5522 registered fitness professionals (group instructors, gym instructors and personal trainers) in Sydney and the Blue Mountains, and 212 gyms. But, according to measures used by the Australian Bureau of Statistics, in 2012, 63.4 per cent of Australians aged 18 years and over were overweight or obese, up from 56 per cent in 1995.

And yet, Sydney is a city of beautiful bodies - of ripped abs, triangular torsos and, er, box gaps. It's health-obsessed, stoned on vitamins, dietary supplements and wonder fads (remember shots of wheatgrass, anyone?) and constantly perving behind its sunnies.

On the surface, it would seem that if we have both an explosion of gyms and a growing population of overweight people, the fitness industry is actually making us fatter. But the suburbs where personal trainers work alongside the football ovals are not the homes of the newly obese.

"Obesity's been called a 'postcode disease'," says Sydney epidemiologist Garry Egger, founder of the men's weight-loss program Gutbusters, "because it's related to where you live - and where you live, of course, is associated with how much money you earn. In developed countries, people in poorer areas always suffer more from obesity-related problems, but if you look at China and India now, the fat people are the rich ones."

In the Third World, excess weight is proof you can afford more food than you need to survive. As a nation becomes richer, says Egger, "It becomes more of a symbol of wealth to be thin, because it means you are able to pay for the right resources to enable you to get thin in an obesogenic environment."

Egger caused a storm in a twee cup recently when he said personal training in public areas was "middle class", intrusive and embarrassing. He says he wasn't criticising the fitness industry or the middle class, just stating the fact. "Without the fitness industry, and without an emphasis on fitness, we would be much, much fatter than even what we are now," say Egger. "Obesity is a problem throughout society, and dealing with it depends on the individual.

"If you're middle class and you choose a middle-class way of dealing with it, then that's fine, as long as you don't interfere with other people's rights to do what they want to do."

In the cafe at aboutlife naturalmarketplace, Rozelle, where you can drink organic soy chai latte or Fairtrade organic coffee, while shoppers fill their baskets with antioxidant juices and gluten-free breads, I meet celebrity trainer Michelle Bridges and her manager/business partner Bill Moore.

Twenty years ago, there was no such thing as a celebrity personal trainer. Just as a celebrity chef used to be a chef who cooked for a pop star (generally Elton John), a celebrity personal trainer was someone who trained an actor or a singer (almost always Madonna). But Bridges became a TV star for her role in the Network Ten show, The Biggest Loser, which turned fat-people-trying-to-get-thin into a compelling spectator sport. "The human body is a machine," says Bridges, who then tells me, "Your body is perfect. Your body is perfect."

Although Bridges' body looks perfect, I'm getting old and stiff and overweight, and that's why I'm interested in Bridges. Two female friends of mine have each lost 20 kilos following the Michelle Bridges 12-Week Body Transformation Challenge, a web-based exercise, nutrition and motivation program. Bridges obviously knows what she's doing, but she continues to insist my body is perfect.

"It responds perfectly," she says. "If you eat well, [with] high-nutrition foods, get out and get moving, it will respond, there's no question about it. If you sit on the couch, eat shit, it will respond perfectly. It's like the most amazing piece of machinery on the planet." Like celebrity chefs, celebrity personal trainers tend to swear a lot in public, offering a welcome break from the corporate management-speak common to the fitness industry.

It's quite difficult to pin anyone down on what the 12-Week Challenge actually involves - apart from eating less and better and exercising more and harder - perhaps because there are so many workout choices in the program. "You should do the program," Bridges tells me. I'm thinking about it. Actually, I'm thinking about excuses not to do it.

Bridges leans towards me across the table. "I do ask a specific question right at the start of the program," she says, "and that is, 'Are you a man of your word?'" Me? Hell no! "That's really what it's about," she says. "It's about being your word." Does she have a program that's about not being your word? "No," she says. "As I said to one of my contestants, You're standing on the corner of Man-Up Road and Pussy Street. Which way are you going?'" I'm going to walk down Pussy Street, I'm afraid.

Bridges says she's "bringing back" fitness DVDs and has three titles in the pipeline featuring home-based workouts composed of multifunctional exercises which train large muscle groups simultaneously. Instead of asking a client to perform a simple biceps curl, for example, Bridges - and many other coaches - now demand they do a biceps curl and lunges at the same time.

"And a lot of exercises that used to be contraindicated are now the latest fad," she says, "like burpees. Once upon a time in the fitness world, people were like, 'Don't give anyone burpees: they're so bad for you.' Now, I prescribe them in the newspaper."

Luke Istomin of Rollex Health in Paddington is a celebrity trainer in both senses of the word. He is well-known in the local fitness industry and had counted among his clients Hugh Jackman, Nicole Kidman, Liev Schreiber, Leonardo DiCaprio and Tobey Maguire. He's currently working with Joel Madden from The Voice. "We're going to make a big push and get him pretty fit," says Istomin. "He's under pressure, being on a show on TV and wearing a singlet. He wants to make sure he looks good for that."

At Rollex, too, says Istomin, burpees are big, and even rolling the ab wheel is back. "That was ruled out a couple of years ago because people thought it was too bad for your lower back," he says, "but if your core strength is fine, you can do that."

Bridges and Moore believe the fitness industry let people down because it became overly prescriptive, contradictory and complicated. "There are wankers out there, personal trainers who've got their watch timed so it goes off and tells them to eat half a banana at a certain time of day," says Bridges. "How can people relate to that, really?"

But, worst of all, fitness went soft. "It's like we got so wrapped up in everything having to be safe that fitness got boring," says Bridges. "We probably underestimated what we can do," says Moore. "We got a bit scared. Fitness professionals were getting educated, and you have to educate them with something so it got very technical and scientific. Contraindication was the catchphrase of the day."

It used to be felt that crunches were bad for the neck, press-ups could damage the elbows, and certain yoga stretches might be bad for the spine. Now, everything seems to be back on the table. Whereas once it was thought that low-intensity aerobic work, performed without interruption over a long period - such as a 60-minute jog - was the most efficient way to lose weight, the new buzz acronym is HIT, or High-Intensity Interval Training, which is based on a jog-sprint-jog-sprint (or even sprint-rest-sprint-rest) model, but can be applied to any workout regime from lifting weights to dance.

Another of the Biggest Loser coaches, Shannan Ponton, is an engaging, perceptive trainer, with an interesting overview of the industry. He runs his own Shannan Ponton Fast Track 8 Week Challenge, and I've never done that, either. He says people always ask him, "What's the easiest way to lose weight?"

"There is no easy way," says Ponton, "and the more fads and gadgets people come up with - the Abdomenizer 1000, 2000, 3000 or 4000, where you can get abs in three minutes a day - it makes business better for us.

"Because people know that once they've come home at three o'clock in the morning, drunk, and bought an Ab Sculptor off infomercials and it doesn't work, that they've really got to come in and do some hard work.

"It takes commitment, it takes consistency and it takes discipline, which, at the moment, rules out about 85 per cent of our population because they're just soft."

"I think the big-muscle-bulk type training of the '80s - that sort of massive bodybuilder -type thing with veins popping out everywhere - seems to have died a slow death," says Ponton, "which is great. The new trend is really healthy-looking athletic bodies that are functional, that can climb ropes, that can climb walls, that can surf, that can run, that can play with the kids, so you enjoy a better ease of life from your training."

Until recently, hardly anybody ever had any cause to climb or carry a rope, but now gyms and parks are often rigged up like sailboats. Much of the impetus for this comes from CrossFit, a style of workout that takes many of its movements from Olympic weightlifting and gymnastics, and emphasises scrambling up ropes, army-recruit style.

CrossFit Sydney has been around since 2005, but there has been an explosion in the number of CrossFit gyms - or "boxes" - in the past five years. Andrew Cattermole, the head coach of CrossFit Sydney, estimates there are "over 80" CrossFit gyms in Sydney.

They're called boxes because they tend to be much simpler than conventional gyms, with no treadmills, cross-trainers or pulley machines.

CrossFitters practice what they call "mixed modality workouts" - often running, then lifting weights, then performing a gymnastic bodyweight movement.

In a class, there is usually one coach to every six participants. The coaches watch each individual and, says Cattermole, "They're going to load that person to make sure they're on edge and a bit pushed. And that's the very debatable issue in CrossFit at the moment: how much intensity should one person have?"

In Sydney, even the trainers have trainers, and they have trainers of their own. Ponton, who fights in white-collar boxing matches, has great respect for Richie Cranny, a mixed martial arts trainer in St Leonards, of whom more later. Cranny, in turn, credits some of his success to his strength coach, former physiotherapist Rachel Guy.

It's Guy who first tells me about the box gap. "I get a lot of girls coming up to me and asking how do they grow their booty," she says. "Booty is becoming really, really popular now, and I'm renowned for having a big one. It seems that skinny is so, like, two years ago, and now the girls are all wanting curvy bodies." But how does one curve one's booty?

"All building a booty is," says Guy, "is making sure your glute max muscles get working. First of all, you have to wake up your butt muscles, because so many people spend all their time sitting down." She recommends exercises such as glute bridges, lunges, squats and deadlifts.

Beyond the booty, women always want a flatter tummy, she says, but, "One of the big things I've been asked this year is how to get a box gap." A what?

"When you see a girl standing in a bikini," she says, "it's the gap between her thighs. I kid you not, I've been asked to give lectures and seminars on how to get a box gap. I presented on the topic in Melbourne last year. Essentially it's a lecture on how to lean out your legs, and the most important thing is, if you want to have nice-shaped legs and a box gap, you have to be doing a good amount of weight training, cardio and dietary modification as well. That's one thing that's become very fashionable."

Dietary modification for the box gap involves portion control. "I don't like to take things out of people's diet," says Guy. "I like to replace it with something else. So instead of wholewheat pasta, maybe they'd like to try zucchini pasta." How is zucchini pasta like pasta? "It's basically zucchini cut into shapes like pasta," says Guy.

Although few people actually stand up and say it any more, practically every diet I hear about is a variation on cutting out bread, potatoes, pasta and rice, particularly after lunch. When fitness professionals protest, "Carbs are great! I eat carbs!" they're usually talking about sweet potato at best.

"You can't out-train a bad diet," says Istomin. He laughs when I ask if he eats bread. "I keep myself as gluten-free as possible," he says. "I've eliminated all processed foods from my diet." A lot of CrossFitters subscribe to the paleolithic diet, says Cattermole, "eradicating all the processed food and leaning towards quality protein sources, quality fats, unrefined fruit and vegetables and very little sugar."

CrossFit is a practice that tends to be male-dominated, although Cattermole says about 40 per cent of his students are women. But one of the most visible changes in the fitness industry has been the rise of women-only gyms. It began with Fernwood Fitness, then came Curves and Contours.

In mixed gyms in the 1980s, says Ponton, "It was almost like gym apartheid. There were the girls who turned up in their G-strings, leotards and fluffy pulled-down socks who went to the left, into the aerobics room, and then there were the boys in their tight singlets who rolled in to the right and started throwing heavy weights around."

Fernwood Fitness was founded in Bendigo, Victoria in 1989, by Diana Williams, a woman who loved weight training. "I thought how nice it would be to have a serious gym, with serious weights," she says, "but all the special little touches women like, like flowers, oil burners and that kind of thing."

She brought in yoga and tai chi classes, which were hardly heard of in regular gyms, and the business grew from one gym in Bendigo to 72 across Australia, including "about 12" in Sydney. They now offer "food coaching" (which "used to be called weight-loss programs", says Williams) and personal training. Inevitably, there is also a three-month challenge, in this case the "Foxy Challenge, where you have 12 weeks to get foxy".

Yoga has become fantastically popular, with variants on the theme ranging from Bikram Yoga - which, notoriously, is practised in a hot, humid room - to Warrior Yoga, which seems to take place in only one gym in Sydney, Richie Cranny's Platinum Extreme in St Leonards. Yoga alone will increase flexibility and, to a degree, strength, but it won't actually build fitness. Almost everyone in the fitness industry agrees it is good thing, and everyone should do more of it but, as Moore says, "In Australia we have an obesity crisis. We don't really have a flexibility crisis."

At Platinum Extreme, yoga is one of many tools used to condition the body to fight. Cranny's well-appointed gym is home to the fitness craze that would have been the hardest to pick 20 years ago: white-collar mixed martial arts. Cranny teaches office workers the training regimen and techniques used in the Ultimate Fighting Championship, and has attracted a great deal of attention and praise within the industry for his work.

He offers me a session of sport-specific MMA drills, beginning with a warm-up using ropes. "They're really good," he says, "You'll enjoy it. Well, you won't really enjoy it."

He's right. I don't really enjoy shaking heavy ropes into wave-shapes, but I can feel it working my muscles like nothing I've done before. We run through a pummel drill, which is what it sounds like, and then he teaches me how to "sprawl", using a wrestling dummy, a punching-bag-style figure shaped like a man. This seems uniquely appropriate, since the only opponent I would be able to beat in Ultimate Fighting would be a stuffed toy. It's invigorating, challenging and as fashionable as pink shorts and it's probably the workout I would pursue by choice, if I had the energy.

Happily, however, it's pretty much the only discipline with no gym in Balmain.

FEEL THE BURN …
… with these glute-toning, ab-shaping fitness gurus

• Shannan Ponton still occasionally works as a personal trainer, as well as running boot camps and workshops. Contact him at shannanponton.com.au

• Rachel Guy is available for strength coaching. Email her at rachel@rachelguy.com, or phone 0416 955 720; rachelguy.com.au

• Contact Richie Cranny and his team at Platinum Extreme, 657 Pacific Highway, St Leonards on (02) 8905 0772; platinumextreme.com.au

• Luke Istomin runs small-group classes six days a week at Rollex Health, 30 Alma Street, Paddington. Monday and Wednesdays are for cardio work, Tuesdays and Thursday are resistance-based sessions and Friday is for stretching. There's a bonus super-circuit on Saturday mornings. All classes are the same throughout the day, so if you miss a time, you can always slot in to the next. Contact him on 0402 307 654; rollexhealthpt.com.au

• For patience, good humour, and determination in the face of terrible adversity (ie: me) it's hard to go past my martial arts trainer, Jesse Reid, who teaches private clients at Sweatbox Gym, 57 Victoria Road, Rozelle (in the Yellow Express Building). Phone her on 0438 282 502; sweatbox.com.au

• Michelle Bridges doesn't take on clients in the flesh, but will cyber-tutor you through her extremely successful 12-Week Body Transformation program at michellebridges.com.au