On a blustery Saturday morning, I am about to climb Alpe d’Huez near Grenoble. I have waited years for the chance. For followers of the Tour de France, this is hallowed asphalt – a mountainous ribbon of road where mortals have pedalled into legend. The race I have entered finishes at the summit. My thighs already crackle with heat when the going gets steep.

After 21 hairpin bends, eight miles and a final sprint against a Dane called Arne, I cross the line in 44th place. As I slump over my handlebars, my beard dispensing sweat like a full sponge, I receive a text message: “Can you come and do this nappy?” I get off my bike, turn off my iPad and hobble downstairs. Perhaps Arne is doing the same.

The next morning, I am transported to New York, where I take part in a spinning class alongside hundreds of other masochists. Robin Arzón, the instructor, leads a brutal interval class of sprints separated by “active recovery”. Each pedal stroke is matched to the beat of the music. As Britney Spears tells me, “You better work, bitch,” Arzón spits one of her catchphrases: “I don’t babysit on this bike… you gotta own this hustle!”

I have cycled seriously for almost 15 years, clocking up thousands of miles in and around south-east London, where I live, and on famous climbs in the Alps and beyond. My bike brings me freedom, fitness, friendship – and that wholesome cheek-glow that only follows outdoor exertion. But for the past few weeks I have been riding alone, holed up in a steamy spare room.

The temporary cycling gym I have created is my route into a fitness boom led by two startups that are trying to change the way we work out. Peloton sells smart, connected exercise bikes, to which it streams spinning classes from its Manhattan studio; Arzón is one of its most popular instructors. The bikes launched in the US in 2014 and the company arrived in Britain last autumn with a seemingly bottomless marketing budget and endorsements from celebrities including the Obamas and David Beckham.

Peloton’s bikes, which have big touchscreens, cost £1,990, on top of which riders pay £39 a month to access the classes. (An app-based subscription that isn’t compatible with the bike is £19.49 a month.) Calling itself the “Netflix of fitness”, it has doubled its membership in the past year to more than 1 million people. Subscribers can join live classes or choose from a growing on-demand library. A London studio is due to open in Covent Garden next year, and the company is poised to float on the New York stock exchange. Its most recent round of fundraising valued it at $4bn (£3.2bn).

To climb Alpe d’Huez, I used Zwift, which also has more than 1 million subscribers. If Peloton brings the spin class home, Zwift brings the road. Also launched in 2014, it is essentially a computer game that turns a bike into the controller. I bolt my road bike to a compatible “turbo trainer”. These heavy machines, which start at about £250 and can cost more than £1,100, pair wirelessly with a laptop or a tablet and apply resistance to a bike’s back wheel or its chain. There is no steering, but the resistance varies automatically to mimic what I am doing in the game. When I ride uphill, it gets harder; when I enter the slipstream of the cyclist ahead, it gets a bit easier.

In the game, which costs £12.99 a month, I can choose to enter a live race, tackle a structured training session or ride around various city-based race circuits recreated for the game. The Alpe d’Huez replica is part of Watopia, an otherwise fictional island paradise of roads set among jungles and volcanoes that dispenses with such inconveniences as potholes, cars or rain.

The brands are benefiting from a reversal in the supposed cycling boom. In April, Sport England revealed a significant drop in the number of people cycling for sport, leisure and commuting – and a spike in indoor riding. British Cycling, the sport’s governing body, blamed potholes, pollution and death by lorry for putting off even some of the most dedicated riders.

Both apps use elements of social media. Peloton has a live leaderboard for each class and the instructors can see who is in each ride. Sometimes they congratulate members for a 100th class, say, or a birthday. When I first tune in and the instructor Ally Love asks: “Are you ready?!” I have a moment’s panic, thinking that she can see me in my dodgy Lycra.

Zwift allows riders’ avatars to wave at each other and supports instant messaging. As I mingle with hundreds of other virtual cyclists, it is hard not to imagine each real person, grimacing and perspiring like me, in a basement or garage somewhere. Both services sync with fitness-sharing apps such as Strava, allowing members to post their rides more widely. Strava lays Zwift’s virtual routes over real maps; zoom out of a Watopia ride and you discover the fantasy land has been built on Teanu, in the Solomon Islands. In reality, it has no roads.

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Historically, indoor cycling has been a dismal experience. The abandoned exercise bike turned clothes horse is a symbol of fitness at its least motivating. Mechanical turbo trainers, through which resistance is controlled by a lever attached to the handlebars, have long been an alternative. While training for events, I have resorted to using one, motivated by nothing more than a stopwatch. It is irredeemably boring, but with a busy job and a toddler, it is hard to find the time to keep my bike maintained, never mind to ride it.

Eric Min had the same problem. The Korea-born entrepreneur grew up in New York, where he rode laps of Central Park, just 10 minutes from his apartment. But then he moved to London to start a software company. The city’s popular cycling parks – Regent’s and Richmond – were a good 30 minutes away. Then came kids. “That’s when I started riding indoors,” he says.

Min used a basic turbo trainer and tried cycling fitness DVDs, but they required a giant leap of imagination to be distracting. Each session was threatening his love of the sport. “And none of them had the social interaction that I craved,” he says. This led him to create Zwift.

Min, who believes gaming can get kids more active, offers free subscriptions to under-16s (parents have to sign a consent form). “Let’s use the very thing that’s keeping them inactive to become active,” he says. Among his biggest young fans is Ben Wiggins, 15, the son of Bradley. “He comes home from school and, rather than go and play Fortnite until 3am, he goes on Zwift,” Wiggins said earlier this year. “We’re happy, because he doesn’t have to go out in the dark,” Wiggins added.

Min has big ambitions for Zwift beyond its safety, convenience and appeal to gamers. As the virtual world has grown, riders have arranged to meet online and race. There is now a busy schedule of races organised by the community. One Saturday, between child-rearing duties, I enter the DIRT DADurday Chase Race for fathers. “My two-year-old only wakes up early when I’m waaaay down in the basement Zwifting,” one dad says on the chat thread during the warmup. The race, on Watopia, starts at a blistering pace; I hang on to the lead group for only 10 minutes.

Now, Zwift is taking virtual racing professional. In December, the company launched the Kiss Super League, its first competition for riders from pro teams, who race alongside amateur teams drawn from the strongest racers. At the end of March, British Cycling hosted its first e-racing championships, in London, which were broadcast live by BT Sport. The riders lined up on turbo trainers and the winner in each event took home a champion’s jersey, as well as a virtual version to wear on Zwift (all riders in the game can earn rewards to customise their avatar, bike and kit).

Zwift then staged a race at the Giro d’Italia in May, after recreating one of the event’s time-trial stages in Bologna; four pro riders raced the course at a special event in the city. Min is now in talks with the UCI, the world governing body, about a world championships event. He also hopes to take indoor racing to the Olympics. “I think my answer to that idea is: ‘Why not?’” says Jonathan Rigby, British Cycling’s commercial director.

Ian Bibby, a pro rider with the Madison Genesis team, was among the finalists at the British Cycling championships, but missed out on the podium. As a schoolboy racer from Preston, he used to hate indoor training. “I’d try and watch TV, but then I’d end up just sitting on the bike not pedalling,” he says. “Nothing could take your mind off the boredom.” He started using Zwift in 2016 and welcomed the escape from winter training. In 2017, he did all his training for a big race in Australia inside his house. “Then I went out and won it,” he says. “No way would I have done that a few years ago.”

Bibby also values the safety of riding inside. He recalls a message his mother sent him after she saw a live stream of one of his Zwift races. “She said it was the most fun she’d ever had watching me, because she didn’t have to worry about me crashing,” he says.

Rachael Elliott, a British amateur racer, is one of the strongest women on Zwift. She has ridden more than 30,000 miles in her house. But the social aspect of indoor riding appeals as much as her many victories. Last February, she suffered a stroke while training on Zwift. Other users noticed she had stopped and helped raise the alarm. “I started getting dizzy during my warmup,” she says. “I remember getting off and lying on the floor thinking I was fainting, then reaching for my phone.”

Elliott spent a month in hospital, where doctors said anything could have triggered the stroke. Zwifters sent her an iPad and under-desk pedals so that she could start riding again on the ward. She still has reduced vision; she can ride outside only on the back of a tandem. Many Zwift riders have become friends in real life. “It really brought home how useful Zwift is as a community tool,” she says. She also says safety comes into her thinking; a stroke on her bike outdoors could have been much worse.

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John Foley, CEO of Peloton, is a big road cyclist, but when he was head of digital at Barnes and Noble, the US bookstore giant, life was busy. To stay fit, Foley, who is 48, would attend spinning classes, which have come to dominate gym schedules all over the world and spawned a network of dedicated studios.

The motivation was there – good spin instructors are equal parts DJ, life coach and drill sergeant – but Foley, who was involved in launching Nook, a rival to Amazon’s Kindle e-reader, saw a way to bring it home. “I was helping disrupt the book world,” he says. “We had stores to sell the product, a software team and a hardware team in Asia, and relationships with content producers. I was in a spin class and thought: I could do the same here.”

I had always looked down my road cyclist’s nose at spin classes, but while I have been amazed at how immersive Zwift is – I really was on that mountain – I have surprised myself by using my Peloton a lot more. Partly, it is comfort. I have a new dad’s bad back and I can adjust the Peloton to a more sedate position than my roadbike allows with Zwift. But I have also sweated out my scepticism, because spin classes are insanely efficient. Within two minutes of leaving my desk, I can be on the bike choosing from more than 4,000 classes (I have done only a couple of live rides, which are roughly every hour; Foley says only 20% of rides are undertaken live).

I sort the rides by duration first, tending to go for 30- or 45-minute options. You can also filter by music genre or instructor. In every ride, the instructor tells you roughly what resistance to apply to the pedals, via a red knob below the handlebars, and how fast to pedal. Both measurements show on my screen, along with the power I am producing, in watts, and my heart rate, via a chest-strap monitor. In an interval class, I have to spin fast and hard between breaks. “Climbs” involve longer, slower sessions of growing resistance. In Tabata rides, the recovery time is half that of the sprints. I have rarely felt closer to death than when doing 20-second all-out sprints with 10-second recoveries.

I know the instructors can’t see me, but the illusion is striking. “Stay with me,” Arzón might say, or: “Put it back on!” It feels like she knows that I am wavering or have taken off some resistance. Meanwhile, the leaderboard ranks users elsewhere according to their power output, adding a sense of competition, but also encouraging the exchange of virtual high-fives.

Some instructors have emerged as minor celebrities. Love and Arzón, for example, have 385,000 Instagram followers between them. Leanne Hainsby is one of two British instructors who joined the platform last November. She records Peloton rides at a temporary studio in east London. Next year she will join a bigger team at the Covent Garden site. “Before, I was teaching 50 people in a studio; now, I have the opportunity to inspire thousands,” says Hainsby, a former backing dancer for Taylor Swift and One Direction. She had started leading spin classes as a sideline before Peloton took her on full-time. “I get some lovely messages on Instagram about how my classes have helped people lose weight or get through something personal,” she says.

In its marketing, Peloton has take a premium route, with images of its bikes being used in improbably swish apartments by buff models. But the official Facebook users’ group paints a different picture. In between complaints about buffering, there are daily accounts of dramatic weight loss and recovery from illness, typically from modest suburban homes. For reluctant, shy or snooty spinners, Peloton’s privacy is appealing.

The bikes are also easy to parody. In January, a Twitter user wrote captions under some of Peloton’s photos of bikes in fancy homes. “I had my carpenter build a $9,000 finished wood riser for my Peloton bike in my glass-enclosed zen garden/home gym,” one read. Elsewhere, Clare O’Connor, a journalist and prospective Peloton buyer in New York, shared an email from her father in which he slammed the bike’s high price; it went viral. “I have heard of the Peloton, and have concluded that, like the use of cocaine, it’s another way of God saying people have too much money,” he wrote. In its defence, Peloton points out that it is competing with gym fees. (If you were to spread the cost of the bike and the classes over five years, it would work out at £72 a month. A UK gym membership costs an average of £43 a month.)

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The rapid rise of Zwift and Peloton has startled the fitness industry. Established players are racing to catch up. Technogym, the Italian giant of gym equipment, has just announced a rival bike. Rowing studios are popping up as alternatives to spin studios, with the same potential for home streaming. Gyms are worried and will have to up their game, says David Minton, a fitness industry analyst at DB Leisure.

Peloton has launched a treadmill in the US with a similar screen; it also streams yoga, stretch and weights classes. When we speak by phone, Foley admits that getting the rights to music has been tricky during the company’s breakneck growth period. A few weeks later, several music publishers slap Peloton with a $150m lawsuit, prompting it to remove thousands of classes. In a statement, the company called the suit “unfortunate and disappointing” and said it was in talks with the publishers concerned.

Demand will fuel further technological advances. I suspect we will see Zwift-compatible simulators that allow steering, leaning, and simulate, say, the rumble of cobbles. Virtual reality may take the immersion further. But isn’t it a bit depressing that a wholesome pursuit of such mechanical simplicity should be brought inside as a digital product?

“It just means I don’t go outside for the sake of it now,” Min says while looking at a giant photo of the renowned Stelvio pass in Italy on the wall. “I’m going to go outside because I’ve got the chance to ride something like this.” His father, who is 85, rides almost every day on Zwift using an old, non-smart Schwinn exercise bike. “He still doesn’t understand what my last business did, but he gets this one,” Min says, smiling.

I have done more than 50 activities on Peloton and Zwift. I have ridden more than 600 miles and burned tens of thousands of calories – losing about half a stone of dad bod – and sweated enough to flood a small village. I would never have exercised so much without the strange machines upstairs. Moreover, I have avoided losing the feeling in my toes to cold, or negotiating potholes or roundabouts or speeding cars. I have stopped inhaling bus exhaust fumes at traffic lights.

But after two months of cycling in my spare room, my horizons are closing in. Soon, perhaps this weekend, I will have cleaned my bike, filled my bottles and picked out my favourite jersey. A morning chill will hold the promise of a sun-warmed back and I will wheel my bike outside and head for the hills with a friend. But if it is raining, or life allows me only 45 minutes... well, I will see you upstairs.

Posed by models on location. Styling: Kara Kyne. Hair and makeup: Desmond Grundy at Terri Manduca.

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