I was prepared to give the world of ballet one more shot, more for its sake than mine. This pointy-toe stuff will do nothing for my core strength, I thought, but I have been unfair about its core, a discipline before mindfulness, what we once called concentration. Plus, Define is a ballet class with a difference: it promises you’ll add two inches to yourself, though I didn’t think that through, just assumed “add” was a misprint of “lose”.

I should have realised something was up when we started the class with jumping jacks. Inelegant, boisterous and extremely hard, these feature nowhere in my understanding of the genre, and nor did anything else. What’s happened here, people, is that an ex-dancer, who is also a New York badass – Ashley Verma – has identified everything related to a barre that might slightly hurt, and exaggerated it wildly to make it hurt more. Pliés have become squats that last six minutes; arabesques have mutated into glute torture. In regular ballet, the barre offers just the suggestion of support, a thing upon which gracefully to rest your hand, like touching the back of a chaise longue as you make a witty remark while smoking. In this kind of ballet, you grip it with both hands and hang on for dear life, as ridiculous demands are made of your flexibility and muscle tone. Whatever circle my active leg was turning, my supporting leg was usually shaking like a leaf.

“How tall are you, 5ft 9ins, 5ft 10?” Verma asked. I gasped a strangled “Whatever”. My height? I didn’t have the headspace for my own name. I was trying to keep a soft, football-sized ball between my thighs, as they burned. “You’ll be 6ft before you walk out of here,” she said, through her 80s aerobics-instructor headset, with her dazzling, 80s aerobics-instructor smile. Oh. That’s what she means by the inches. I don’t even want to be taller. I’m already too tall. “The thing about dancers is, they’re athletes,” she said, afterwards. “People think it’s so fluffy, but they’re incredibly tough.”

It’s true, they are hardbodies, but I always thought they were born like that and the profession was self-selecting. But, of course, they don’t get that way, lean as beans, strong as oxen, by dancing; they just have to be that way in order to dance.

So if you want to get muscle-fit, you really need the single-minded nous of someone who needs it, or once needed it, for their job. They divide their body into its raw power units, its triceps and its hamstrings; they flog them like pit ponies, smile while they’re doing it and don’t stop until everything hurts. “It never gets easier,” she says, “you just get better.” It recalls the lost world of the workout video, that fun, brassy straightforwardness, where nobody was pretending to exercise for spiritual reasons. We all just wanted to look like Jane Fonda.

I miss the 20th century.

• For classes, go to define.london

What I learned

There is no more effective abs exercise than the plank. Lie straight with only your forearms and the balls of your feet touching the ground. Stay like that. There.