Classes are by invite only and feature squats, weights and many burpees

Siobhan Norton tries out the fitness craze that operates by invite-only

The first rule of the SB Collective is that you don't talk about the SB Collective. Well, not quite, but the exclusivity surrounding this fitness craze certainly feels more than a little Fight Club. The website is beguiling while giving very little away.

I am neither skinny nor a... well, I reckoned 7am classes would up my bitchiness score no end. I arrived to find a room full of beautiful people, who were not so much skinny as lithe models of perfection. In fact, many were models. Clad mainly in the model uniform of black. I lurked at the back, feeling like a Day-glo cartoon character who has wandered into a Calvin Klein ad.

This was no sweet little aerobics class. Squats, weights and oh so many burpees. It moves between abs work and cardio bursts, with the occasional order to grab your weight, hold it over your head, and RUN! The exercises are animalistic – crawling, jumping, pushing and pulling.

These were no bitches, though. There were no sneers as I wobbled through the gazillionth split squat. Afterwards, everyone was chatty and friendly and a glance at SBC's Twitter and Instagram feeds shows just how much dialogue there is. Founder Russell Bateman says this is key. "It is invite-only – if someone has the wrong attitude it affects the dynamic of the group. We want it to be supportive and positive."

It is, he says, more than just classes. It's a changed attitude to fitness, how you eat, and your mindset. The whole thing is culty in the best sense of the word, and I'm well on my way to being brainwashed. Now I just have to pray I get in.

Cheapskates' version: Lunge and squat like your life depends on it. Crawl around your kitchen floor. Always wear black.

Price: £40 per session. SBC classes take place at Fitness First, 55 Baker Street, London W1;