"Lifting things at weights we couldn't have fathomed shows women that we are strong, and that seemingly impossible, Herculean tasks are within our reach," says my friend and CrossFit coach Alexandra Lee.
She's a devotee of the addictive exercise regime that combines gymnastics, weightlifting and conditioning in punishing hour-long classes. CrossFit, the so-called "sport of fitness", is already hugely popular in the US, where it enjoys an almost cult-like status, and celebrities from Matt Damon to Cameron Diaz are converts. This is despite it being so tough that it's attracted controversy around its possible health risks, including back injuries and kidney failure.
Now it's rocketing in popularity here, too, with more than 300 "boxes" – the minimalist, warehouse-like spaces in which CrossFitters work out – more than half of which have opened in the past year alone. On 18-19 January, hundreds of Europe's fittest athletes will descend on the UK to compete in a CrossFit event aptly named the "Battle of London".
CrossFit is not for the faint-hearted. Classes include everything from squats and lunges with heavy weights to handstand push-ups, rope-climbing and tyre-flipping. The pace is ferocious, and sessions conclude with a relentless 20-minute "Workout of the Day" (WOD), which sees participants complete a set of exercises against the clock.
Boxes change WODs daily to ensure athletes are working different parts of their bodies, learning new skills, and avoiding boredom. But a handful are repeated regularly so athletes can benchmark their progress. These are called "the girls", and include, for example, Barbara, which involves 100 pull-ups, 100 push-ups, 100 sit-ups and 100 squats, as quickly as possible. CrossFit's founder, Californian Greg Glassman, named these WODs after girls, saying: "Anything that leaves you flat on your back and incapacitated, only to lure you back for more at a later date, certainly deserves naming."
But CrossFit aims to do more than just get us fit. Rather like SoulCycle, that quasi-spiritual workout craze so beloved of New Yorkers, CrossFit is also celebrated for building a sense of community.
Many women also praise, as Alexandra does, its empowering impact. And a few months ago, her unintentionally feminist call to action inspired me to try CrossFit. I tentatively approached my local box, CrossFit Hackney, in an anonymous archway in east London's trendy Haggerston area. Despite the lack of signs, hearty yelps of "Boom! You did it!" indicate the entrance.
I'm immediately scooped up by a class of eight and thrust into a warm-up involving giant rubber bands. Next comes a skills session, in which we're taught how to squat with barbells on our backs, itself akin to a thorough workout.
Already exhausted, we prepare to begin the final 20-minute WOD. I notice those around me tense up and take deep breaths. Before fear can set in, the coach turns up the dance music to an ear-popping volume as the piercing bleeps of an electronic clock count us in to the final challenge: five seconds, four, three, two, one.
Before I know it, we're jumping on to boxes and swinging cast-iron kettlebells into mid-air. The whirlwind feels bizarre, yet infectious.
My euphoria is quashed only when the coach tells me to take the weights off my bar. I feel a pang of disappointment, assuming this is because of my gender. Yet when I notice the woman next to me add more plates, I realise the restraint is due only to my inexperience. I soon learn that CrossFit can be scaled down so that anyone – regardless of age or experience – can join in.
The drill is punishing, and although my muscles burn with fatigue, the sight of others sweating it out motivates me to push through the pain. One is Lauren Dagger, a 29-year-old business-development manager for a telecoms company who has been coming to this box since it opened six months ago.
"I was letting my work get in the way, working 75 hours per week, then I changed jobs and wanted to exercise. I didn't like the gym because it's boring and repetitive, so I thought I'd try CrossFit instead," she says.
Lauren was soon hooked and began coming almost every day. "I used to hate how I looked, I'd put on a lot of weight, so it really helped me feel better about myself. I'm so addicted now, I get fear of missing out if I don't attend class."
Lauren also takes weightlifting classes and recently competed in her first CrossFit competition. "It was the hardest thing I've ever done. I'd never lifted 35kg overhead before, but now I have so much respect for my body. I want to see what else it can do."
By becoming physically stronger, female CrossFitters say they feel psychologically stronger, too. Women are encouraged to lift as heavily as possible, and although it flies in the face of size-zero convention, Alexandra and I continue to espouse the mantra, "Strong, not skinny".
The regime is fiendishly addictive: my Instagram account becomes a homage to my new-found hobby, as I delight in posting distinctly unfeminine visual badges of honour, such as red-raw palms covered in chalk and bruises, a testament to weightlifting. My best friend's partner chides us for debating the appearance of new muscles over dinner, so we wait for him to leave before continuing unabated.
But during one early-morning class, I work so hard that by 9am I can barely lift my arms to type or answer the phone. I refer to the multitude of CrossFit blogs and chatrooms, where hundreds of thousands of CrossFitters discuss – almost boast about – health problems ranging from women peeing during workouts, to spinal injuries, to something much more alarming: rhabdomyolysis.
When skeletal-muscle tissue breaks down, products of damaged muscle cells are released into the bloodstream, some of which, when deposited in the kidneys, can cause renal failure. Caught in time, it can be treated, but if not, rhabdomyolysis can kill.
So could CrossFit be dangerous? Dr Charles Tomson, a consultant renal physician at Southmead Hospital in Bristol, says: "Rhabdomyolysis can definitely be linked to extreme exercise such as CrossFit. People do intense exercise and if not treated, muscles fail and the kidney shuts down. We see two or three rhabdomyolysis cases a month. It's most common in heroin addicts, but occasionally those who over-exercise. For example, a colleague recently had a soldier forced to run in full kit who got rhabdomyolysis."
You can develop the illness to a degree and then get better, according to Dr Tomson, who says the medical establishment is still unsure why some people get it and others do not. "If you tested the blood of everyone as they finish a long race such as the Great North Run, then it's likely many people would have some level of broken cells in their kidneys."
The CrossFit community is sitting up and taking notice of the risks. Some boxes make beginners attend "foundation" courses, to ensure they can move safely. This includes CrossFit Hackney, where my coach, 32-year-old Andrew "Stretch" Rayner, cautions against inexperienced instructors jumping on the bandwagon. "Anyone can go on a weekend course and qualify as a CrossFit Level 1 coach, pay for affiliation and open a box, so people who give classes can vary widely," he says.
Stretch himself came to personal training four years ago, after a career in IT. "The fitness industry is full of people who don't care about individuals – they just want to make money. But for me, coaching CrossFit is about doing something I love. Here, we tell people how to move safely, eat nutritiously and sleep well. When someone deadlifts their own body weight or does their first competition, like Lauren, those are the moments that make you proud as a coach."
He justifies CrossFit's high fees – which can be two to three times those of a conventional gym – because, he says, the programme is more akin to personal training.
And CrossFit works. It will make you ache in places you didn't know muscles existed, while the sense of camaraderie will keep you – somewhat masochistically – coming back for more. If you're serious about getting fit in 2014, this is one new year's resolution you won't give up in a hurry.