After 10 minutes, my muscles are screaming and I swear I can taste blood. At 12 minutes, I start to make peace with my maker. By 14 minutes I can no longer stand and a paramedic is holding an oxygen mask to my face.

Welcome to the "most dangerous gym class in the world", aptly named Flatline, which comes with emergency medical assistance and a legal disclaimer. If it sounds like torture, well, it is - but there are plenty queueing to put themselves through it. Mainly, young male City workers, pumped up with testosterone, adrenaline and stupidity. The steel buckets placed around the course in the techno-gym-cum-nightclub suggest several will vomit along the way.

Before I attempt the taster session, I am asked to sign a waiver which points out risks including "permanent disability, stroke, single or multiple organ failure, physical damage to organs, spinal injuries, paralysis, heart attack, heart failure, brain swelling and death". It would almost be funny if not for medical evidence which suggests that, for certain people, extreme sports and fitness may be dangerous.

And yet extreme fitness is booming. Regimes such as CrossFit and F45 are based on the principle of High Intensity Interval Training, or HIIT- extremely intense bursts of activity followed by brief periods of recovery, strung together in a class and put to music. CrossFit, which began in California, appears to inspire particular obsession, with devotees proudly posting videos online of themselves vomiting with exhaustion during and after sessions.


At Gymbox in Farringdon, London, the home of Flatline, paramedic Keith Young checks out my heart rate and determines that I am fit to proceed: "It's an extreme class so its vitally important that we have a medical professional here," he explains.

I am fitted with a heart monitor which transmits my heart rate to a tile on a display screen: the session is designed to get participants' hearts to function at their maximum capacity or "redline".

Gymbox Master Trainer and sculpted Adonis, Firas Iskandarani devised the class and explains it involves four circuits of seven exercise stations at which a certain number of repetitions must be completed within 45 seconds. Participants then have 15 seconds to move on to the next station. Tasks include climbing a rope to the ceiling four times, hoisting a 60kg dead weight over the shoulder six times and running several yards carrying two heavily weighted bars.

It doesn't sound too daunting and two stations in, I am quietly confident; I spend hours each week in a gym and am blessed with above-average fitness for my 47 years. Halfway through the third station however, I realise that I am in trouble and begin gasping for breath. From then on, my heart has no chance to recover and races from a resting 65bpm up to 177bpm. Before the first circuit is up I am damned. Each subsequent station is a descent into a circle of hell, stoking the inferno burning in my chest.

After two circuits and 14 minutes I can barely breathe, think or move. I am close to tears. Firas barks at me to keep going but I flop to my knees, gulping for air. Oxygen arrives. Keith looks concerned. It is only afterwards, when I recover enough to speak, that Firas tells me that even a British Obstacle Course Racing champion only managed three circuits. That makes me feel slightly better but begs the question, why?

"I wanted to design the toughest and most difficult class on earth," says Firas. "And that's what I've done. The only way anyone will ever be able to finish it is if they do it again and again. As you do it more, you work out that there are certain ways to do each exercise which makes them easier. We are not trying to be nice. People will come, they will try it, they will collapse and throw up."

But isn't that dangerous?

"We would never let someone hurt themselves," he concedes. "There is always a health professional on site when the session is on. Your body is actually smarter than you are and if it can't do something, it won't."

But Flatline poses questions about HIIT. The NHS advises at least 150 minutes a week of moderate-intensity activity; while most of us risk doing too little, for some over-exertion could cause physiological changes that overlap with existing problems. In 2012, broadcaster Andrew Marr suffered a stroke after an intense session on a rowing machine caused a carotid artery to tear.

Dr Roby Rakhit, clinical lead for cardiology at the Hospital of John and St Elizabeth, says: "Too much exercise can accelerate cardiovascular disease and promote ageing. [But while] doing a severe amount of exercise is potentially harmful, that should be put into the context of other risk factors. It can't be good if you have underlying, undiagnosed coronary disease and undergo rapid heart rate response."

Still, last year the Heart, Lung and Circulation journal published a review of several studies of people with heart disease who tried HIIT under medical supervision, and concluded that it was associated with a pronounced improvement in heart function.

Flatline, however, is designed to be stressful, and Dr Rakhit points to research published in the American Heart Association's journal Circulation in October, linking anger or emotional upset while engaging in heavy physical exertion with tripling the risk of cardiac arrest.

So, while the adage "everything in moderation" may hold true for extreme exercise, this class is still not for the faint-hearted.

Is your body up to it?

If you can answer yes to any of the following, consult your doctor before starting any exercise programme...

• Do you feel pain in your chest when you do physical activity?

• Have you had any chest pain in the past month when not doing physical activity?

• Do you lose balance through dizziness or do you ever lose consciousness or collapse?

• Do you have a bone or joint problem that could be made worse by exercise?

• Is your doctor prescribing medication for your blood pressure or a heart condition?

• Are you pregnant or have you given birth in the last two months?

• Have you been told by your doctor that you should only do physical activity recommended by a doctor because you have a heart condition?

• Do you know of any other reason why you should not do physical activity?