I’m on all fours, scuttling across the carpet. It’s a shade of sickly mauve that makes me think of an office corridor. Strapped to my face is a black mask that makes me think of war. “Right arm forward. Left leg forward. Then right arm. Keep pushing! Now reverse!” I have forced my mind into neutral, but I have just enough awareness to detect a quiver of amusement in the voice of my trainer, Dean.

My gym clothes are coated in a sleek membrane of sweat. With the mask on, my breathing sounds thick and mechanical. The effect is something pitched between Darth Vader and one of the Venezuelan killer spiders in the Arachnophobia movie. The stuff of budget horror films — or 1980s children’s TV. The three other people in the gym — all male — are lifting weights, determinedly acting as if nothing is amiss; every so often, clandestine glances in my direction betray their bewilderment.

The Conrad London St. James hotel has just launched the new extreme altitude mask fitness class that I am currently wheezing my way through. Train Dirty London, a personal training company, started running the sessions at the luxury establishment in September, making Conrad London St. James the only hotel in Europe to offer such a regime. A flurry of eager women have signed up. “Some have already committed to follow-up sessions,” says Dean. I nod with polite incredulity between ineffective gasps of breath.

In the mask, the sensation of breathing is something close to trying to fill a bucket of water when it has a walnut-sized hole in the bottom.

Exercise involving altitude masks (also known as hypoxic training) has traditionally been the preserve of professional athletes. The England football team, India’s cricket squad, and Samoa’s national rugby side have captured headlines for using them. Unexpectedly, Conrad London St. James’s offering is aimed at women looking to burn calories rather than men seeking to bolster their tough-guy image. “Females who do yoga, with its emphasis on controlled breathing, are particularly excelling in the workouts,” says Dean, as I swivel and strain my way through a series of Russian twists — a particularly joy-sapping exercise that involves sitting on the floor, lifting the feet, leaning the torso back, and then turning the body to the right and left.

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However, Train Dirty London’s new fitness regime is also called Skinny Rebel. At best, this evokes images of a dodgy slimline cocktail; at worst, a pro-anorexia blog. Maladroit marketing aside, Skinny Rebel promises results; one session can burn up to 1,000 calories in 45 minutes.

The next 10 minutes are a smothering haze of mountain climbers, squat jumps and kettlebell swings. In the mask, the sensation of breathing is something close to trying to fill a bucket of water when it has a walnut-sized hole in the bottom.

Conrad London St. James, a luxury property in Westminster, is the only hotel in Europe to offer the workout.

When it comes to the burpees, I cave. I rip the mask off and do the next few exercises without it. Things feel a bit more manageable now — but all the while I can see the discarded mask dangling from one of the bench presses on the edge of my peripheral vision. Taunting me to reclaim it. In some devious twist, the source of the last 30 minutes of physical turmoil is now seducing me from the corner of my eye — like a packet of Percy Pig sweets at the supermarket checkout. I put the mask back on.

I’m a runner, so the final task — interval training on the treadmill — delivers a genuine, crackling rush of adrenaline. In the closing moments, I can’t resist the triumphalist temptation to mentally airbrush the last 45 minutes of my life: “well that wasn’t so bad”; “tough but fun”; “I could do it again”. And I think I will.

Skinny Rebel workouts run weekly at Conrad London St. James for both in-house guests and non-hotel guests. The workout is priced at £40 for one 45 minute session or £35 each for six sessions. For more information and to make a reservation, email: info@traindirtylondon.com.