Sydney Sweeney, the actress last seen at the centre of a controversial American Eagle jeans advert, has undergone a transformation. In preparation for her latest role — the lead in Christy, a biopic devoted to the female boxer and 1980s sports icon Christy Martin — Sweeney gained two, deeply, impressively muscular stone, boxing training for several hours a day, every day.

“It was incredible being able to completely embody such a powerful woman,” Sweeney said when promoting Christy this week. “I felt even stronger. It was truly inspiring.”

She is not the first woman to discover how profoundly boxing changes you, both physically and emotionally. Nor, to be fair, am I … though that is precisely what it’s done for me in the last nearly nine years I’ve been training in my local club, Islington Boxing Club.

My 50 minutes with Sydney Sweeney, the pin-up dividing America

Boxing is a rigorous workout. You burn between 350 and 600 calories in an hour, which is roughly the same as running. Unlike running, however, boxing works your body evenly and completely: upper and lower, your abs, arms, glutes and thighs. It’s good for reflexes and hand-eye coordination. It’s also fun, so you rarely even notice how hard you’re working until you stop (and collapse).

Boxing has entirely altered my shape. Once I was skinny, waify; now I have a little bulk. Definition on my thighs from all the squatting. Abs. Side abs! Then there are the emotional benefits of training. Boxing is an epic stress reliever, a great diminisher of anxiety. All the petty injustices I felt so acutely when I first started ebbed away, transforming themselves, magically, into better shoulder definition.

Above and beyond even this, boxing has bestowed on me a greater confidence than I knew I was capable of. This is something that happens to you once you understand how to throw a punch. There’s a sense of being useful. Could I hit an attacker, say, if required? My guess is: I’d have a go. There’s stuff that has now been trained into me, stuff that wouldn’t give in easily; stuff that means these days I walk with more composure and swagger.

Sydney Sweeney at the Echo Valley European premiere.

Sweeney trained in jujitsu as a teenager

KARWAI TANG/WIREIMAGE

It was back in 2017 that I first joined a Saturday women’s class alongside 20 others. Our ages, bodies and fitness levels varied so dramatically there was no possibility of feeling out of place. No one looked anyone else up and down, as women are inclined to do in smart gyms. No one judged.

There were moments in that first class when I thought I’d either die or puke, or both. I hadn’t done any cardio exercise since school; what sort of an idiot attempts her first burpee at 40-odd? But for all that, for the times my vision clouded over and blood buzzed in my ears, there were also moments when I found myself utterly enchanted. The first time I raised my gloved fist, swung it into a punchbag and marvelled at how good it felt. The first time I swung my hip behind a punch, twisting on the ball of my foot as if squidging out a fag. The first time I watched the trainer, Sunni, spar with one of the group regulars capable of matching her. There’s something about seeing women unleash the aggression we’re otherwise trained to hide, suppress, deny.

When the first hour ended, I crawled out, knowing I’d be back the next week and the one after that; that I wouldn’t even have to force it. I was right. I have still never dreaded a boxing class, never told myself, “I should go, I suppose …”

Christy Martin: ‘My husband almost killed me — now Sydney Sweeney is playing me in a film’

Given that, it’s wonderful to see how many other women have also fallen in love with boxing in the last few years. Sweeney, who apparently trained in jujitsu as a teenager and did a little bit of kickboxing before landing her breakout role in Euphoria, is just the latest in a list of female celebs who have “found” boxing, and who now rave about it as I rave about it. There’s Gigi Hadid, Karlie Kloss, the comedian Amy Schumer, the actor Elsa Pataky, the singer Ellie Golding, Kim Kardashian (obviously). Woman in a Boxing Gym, sweaty in Lycra and lounging against the ropes, or poised menacingly by a dangling punchbag, gloves raised to her cheekbones, weight precisely balanced between her feet, her expression menacing, her intention obvious … it’s become an Instagram classic. I should know. I’ve done it, many times.

The UK’s boxing gyms and classes are increasingly filled with women squatting, drilling, sparring, pummelling bags, loving boxing. Sweeney’s transformation might have been for a role, but if she gives up boxing now having discovered all it gives women, I’ll be surprised.