This was one of NZ Herald Lifestyle's most read stories in 2017

After entering through a sophisticated computerised identification gate, you descend straight down a narrow stairway into the fantabulous subterranean labyrinth of the men's changing room, where you are struck immediately by the dank smell of sweat-soured polyester mingled with the bright desperation of Lynx body spray.

You can't believe the size and provisioning of the changing room. If you brought enough protein bars and raw kale, you could happily live out the looming apocalypse down here. There are five separate changing zones and hundreds of lockers. There are several hairdryers, there are ironing stations, mirrors of every size and persuasion, endless bench seats, drinking fountains, sophisticated glass doors leading to the "wet area" with its multiple public and private showers, steam room, sauna, spa pool and plunge pool/conversation pit.

You are in awe of the nudity, absolutely flagrant and often in direct contravention of the rules - men standing conversing by the pool, for example, with their raw buttocks and penises exposed, sometimes directly in front of the sign saying "Swimsuits must be worn".

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Signs in the wet area order "No inappropriate behaviour" but tantalisingly provide no detail or photographs.

You get changed, go back up the stairs and enter the gym itself through a narrow doorway, then you follow a red line painted around the outside of the muscle floor, where surprisingly few people are using the selection of free weights and pulley-based weight stations. At multiple points, lest you be tempted to stop and try a lat pulldown or some such, signs appear with messages such as "This way to the world's best fitness classes" and "This way to the world's best group fitness".

You go up some stairs, across a glassed-in walkway, down some other stairs, around a corner, up more stairs, higher and higher, possibly endlessly, you think, until finally you emerge - as if into heaven - through the floor of the glassed-in light and glory that is Studio 1.

The feeling of the building to this point has been sweaty, cramped, intense and a bit of a hodgepodge, aesthetically speaking, but then suddenly there you are, at the epicentre of the Les Mills global juggernaut, in the beautiful, megachurch-like space that is ground zero for the group fitness classes that have been the main spur for the company's emergence as one of the world's biggest and best-known fitness empires.

It's beautiful in there, so bright and hopeful. You hear angels singing. You recognise them as the vocalists on a terrible dance track. You have arrived for your first-ever Les Mills group fitness class. This is BODYPUMP™ and, over the course of your three month membership, it will change your life. Results may vary.

During my first month's membership, although the gym is only 180 metres from my office and my regular attendance had been mandated and paid for by my company, I went only five times.

My excuses were many and not at all original: my life was too hard, I was too busy at work, had too many children at home, was morally above all that doof doof/ra ra motivational crap, wasn't worried enough about the way my body looked, was no longer trying to attract sexual partners and was already doing enough daily walking to exceed most medically mandated exercise minimums.

I had joined the gym out of professional obligation, basically in an attempt to answer in print the question of why, whenever anybody asked me where my office was, the only surefire way to get them to understand was by saying, "Opposite Les Mills on Victoria St."

How was it, I had been asked to discover, that this place, which from the outside - and frankly also from the inside - is a bit of a shitter by acceptable design standards, had become an "Auckland Institution"; a place a lot of very attractive people like to go and distribute their nudity widely across the changing rooms?

Having wasted most of my first month's membership, I also wasted most of my second month's, but by my third and final month, as my fear of failure grew and my attendance at Pump picked up correspondingly, I began noticing changes in my body that led me to start taking my shirt off in front of my wife with increasing frequency.

Initially, her response was mostly laughter but then one night, while I stroked my flexed bicep next to her on the couch, she said, "You are actually starting to look quite ripped." From that point on I was suddenly at the gym every day. Whenever I was in the vicinity of a mirror at home or elsewhere, it was all I could do to keep my clothes on.

"Shadow there, shadow there," my wife said one night, pointing to some areas of increasing definition on my torso. She began to count off the zones of my nascent six-pack: "One, two, three, four," she said, paused, looked up at me, then continued more hesitantly, "Five, six".

"You were going to stop at four," I said.

"Well, let's be honest," she said, "those two are ribs."

At Pump class, I started really loading up the weight on the Les Mills SMARTBAR™ and blasting all major muscle groups. I could feel the strength growing in my upper body and I could see the beginnings in me of the sort of physique I recognised most readily from the leading sculptures of the renaissance.

The soft downlighting over the mirrors of the main vestibule in the Les Mills changing room provided maximal emphasis of the definition on my objectively probably-not-very-changed body and there were days when I stood in front of that mirror after class finding it all-but-impossible to go back to work.

This new body obsession wasn't really related to the job I had been sent to do - but that's not to say it didn't matter.

By the time my membership period had just days to go, while I sat half-naked on the couch next to my wife one night, she asked how I was going to cope when it expired.

"I don't know," I told her, stroking the developing hump of my right triceps. "I'm actually really upset about it."

As recently as the 1990s there were two main types of idealised fitness body: the slim, toned body of people who did mostly cardio-based training, and the massive muscular body of people who did mostly weight training.

The modern fitness class, as developed and exported worldwide by Les Mills International, and particularly its industry-standard Pump class, have helped collapse those bodies into one, so that, even outside the gym you can make a good guess at the type of body that has done a lot of Pump: cleanly defined bunches of lean - but not bulging - muscle, strong shoulders and upper backs, and world-class buttocks.

Les Mills makes most of its enormous pile of money - in 2014, PwC estimated its earnings at more than $100 million a year - not through its own relatively small network of New Zealand gyms, but by creating and selling continually updated group fitness classes like Pump to other fitness clubs around the world.

Two million people worldwide take one or more Les Mills classes each week - Body Combat, Body Jam, Body Attack, Les Mills Sprint, Les Mills Grit and so on. Body Pump is the granddaddy of them all though. As of May this year, it was being taught by 58,600 instructors in 16,400 clubs in more than 100 countries.

In the press release to celebrate the 100th update of Pump earlier this year (all the company's group fitness programmes are updated quarterly), Les Mills described the class as having a "cult-like following of devoted participants".

IHRSA, the trade association serving the international health and fitness industry, credited Les Mills International in 2003 with "doing for group exercise what McDonald's did for hamburgers" - a claim the company repeats on its website. It's easy to see why. The learning curve for Pump is absurdly shallow and short. At my first class, it took me seconds to figure out how to load weight on to the specially designed Les Mills SMARTBAR™ barbell and seconds more to arrange my Les Mills SMARTSTEP™ with four supporting riser rings.

The basic set of exercises is so simple that I had mastered basically all of them halfway through my first 30 minute class. By my second class, I was comfortable with the thought of any of the 100-150 other attendees checking out my form. By my third class, while in the middle of a set of power presses, I found myself absorbed, in a classic flow state, physically and emotionally fully engaged in the movement and the dance-pop sounds of Blame by Zeds Dead & Diplo feat. Elliphant.

According to the company's CEO Phillip Mills, Les Mills is in the motivation business; its key motivational weapons being, "The teacher, the music, the moves, the studio design, the energy of the crowd, the bonds created among members and instructors."

Studio one, where by far the biggest proportion of Les Mills Victoria St branch's classes are held, is clearly designed to inspire awe. You emerge into it from narrow stairs at the back of the room, and you're immediately facing the stage at the front where, if you give yourself up to her implorations, the instructor promises deliverance from the malaise that is your current body.

It's deliberately massive, big enough for 230 people, and surrounded on three sides by floor-to-ceiling windows giving views over central Auckland's classier offices and up to the expensive sophistication of Ponsonby.

Jackie Mills, who is the company's chief creative officer and Phillip's wife, told European academic researchers in 2015: "There are two things that are crucial to getting a population moving: one is music; the other is the power of a group ... Essentially, what we are trying to do is almost to provide distractions! You have fabulous music, great lighting and a motivated instructor, so we can encourage people and help them to adhere to an exercise discipline through some type of entertainment while they exercise - we call it exertainment."

If you think this sounds a little like something the leader of a cool, modern church might say, that's crazy talk - you don't go to church to get fit.

Les Mills' standardised group fitness classes allow devoted adherents to regularly gather with a like-minded group of people in an awe-inspiring space provided by a sophisticated and powerful organisation that demands a non-trivial proportion of attendees' weekly income, for an uplifting session in the hands of a charismatic leader, for the purposes of becoming a better person.

I clearly remember standing in that first Pump class, awkwardly holding an embarrassingly light barbell, sceptical and ashamed at the thought of anyone I knew seeing me, but even more clearly I remember my last Pump class, arrogantly picking up the bar, smashing on an extra 2.5 clicks per side and blasting my chest to hell and back to the pulsating 128 beats per minute of The Black Eyed Peas' I Gotta Feeling.

I had seen the exertainment and it was good.

I had been a member at Les Mills Victoria St for 11 weeks and had just finished a very good, very uplifting class when I stopped in at reception to cancel my membership.

"Can I ask why you want to cancel?" the nice-smiled woman at reception asked me.

It was a question I had neither expected nor prepared for. I couldn't tell her that I was an undercover reporter whose company-mandated reporting period had come to an end, but neither did I have a suitable alternative explanation and I didn't want to lie.

"I've achieved everything I wanted to achieve," I told her.

Her smile didn't waver, although she took long enough to answer that I could see she was a little thrown. "Okay," she said eventually, "You feel like you've just done everything you can?"

"Yes," I said.

She messed around on her computer screen for a few seconds, then told me I was going to have to speak to somebody in accounts.

I felt nervous, like I'd done something wrong, although I suspected that I hadn't. The whole thing seemed crazy. I had given them my money for a period, and that period was up, and now I wanted to leave. Why wouldn't they let me leave? Why did I have to explain myself to them at all, let alone twice?

The receptionist picked up the phone and called accounts.

"He's achieved all his goals," she said.

There was silence for a second or two, then she said, more slowly and deliberately: "He's achieved everything he wanted to achieve."

After a while, she handed me the receiver.

"Hi," I said.

"I hear you've achieved all your goals," said the woman in accounts. I thought I detected a little laugh in her voice. It was pretty clear that everybody involved in this sordid transaction knew I was not being completely honest, and they were going to use that knowledge as leverage to ensure my exit was made maximally difficult.

"Tell me, what's the secret?" she asked brightly. "I've obviously been doing it all wrong!"

I tried to come up with a convincing explanation, or a joke, or anything really, but all I could think of was my shame.

We were engulfed by a short and painful silence, then she changed tack and tried to convince me to put my membership on hold. It seemed to go on forever, this one minute conversation, but I held my nerve and finally she and Les Mills relinquished their power over me. Or did they?

A week or so later, after I had repeatedly made clear to my wife how much I was missing Pump class, I sat down with her on the couch and discussed whether we could afford for me to take out a personal membership. She said no. I have not taken my shirt off in front of her since.