Going weak at the knees is usually associated with spotting the love of your life for the first time. In this case, my knees are not just weak, they are shaking uncontrollably, along with the rest of my legs. And there’s not a handsome suitor in sight. It’s just that I’m halfway through a barre class and my muscles are hitting the limit.

The good news is that everyone’s legs are shaking, even the instructor’s – that’s the point. Barre uses dozens of repetitive movements to strengthen and tone. A squat starts out big for 30 counts – then the instructor reduces the size of the movement so that my legs are only bending a few centimetres, in what are called isometric movements.

This sounds easy, but controlling the bend against gravity gets increasingly tricky – and then, at the midpoint, the instructor asks us to hold the bend… and hold it…and hold it some more.

I try to focus on the pumping beat of the music and the fact that the lady in front has 20 years on me and isn’t even wobbling.

But that’s when the shakes start. Muscles rebel, the quivering sets in, and we’ve only completed the opening warm-up. If I thought that it wasn’t going to be enough of a workout, I’m rapidly set straight – shaking indicates muscle fatigue and I’ve got it bad.

The ethos of barre is simple. It combines the toning and controlled movements of pilates with the flexibility of yoga and the discipline of ballet and dance. Some of the exercises require support from a barre as in ballet practice – however, a high backed chair will do just as well while additional equipment is usually simple.

Controlling the bend against gravity gets increasingly tricky

It’s easy to be put off by the ballet aspect, but grace and co-ordination are not required – we’re not doing any pirouettes. This is a good workout for beginners or people who need a low impact option – the exercises strengthen muscles without putting as much strain on the associated tendons and ligaments. And it might actually help to avoid injury by working muscle groups that protect the back, offsetting the effects of sitting at a desk all day.

The idea of an exercise class that uses a ballet barre is not exactly new. Lotte Beck, a German ballerina, developed a routine in London the late 1950s after she hurt her back, and her clients reportedly included Joan Collins and later, Yasmin le Bon.

But a new wave of barre classes has sprung up in the last decade and its now one of the hottest fitness trends around.

Barre is usually about strength and toning, but the one I attend incorporates a cardio aspect. That has me doing speed push-ups with a hop incorporated, and powering through the pain of star abs, where, from a prone position on your back, you lift up to touch the opposite hand to a raised ankle.

The addition of exercises that raise the heartbeat means that we burn fat as well as improving muscle tone.

By the end of the class, it feels like every part of my body has been pummelled. We’ve squashed a squishy ball between our calf and thigh 100 times while doing a donkey kick; we’ve made tiny circles in the air with a raised foot until the burn in the glutes became a full blown inferno; we’ve held a plank for a minute (or tried to). I’m knackered but invigorated.

Until the next day. I have to literally roll out of bed. My muscles, challenged by the unfamiliar exercises, are leaden. They simply don’t want to move. When I walk, I look like I’ve just dismounted from day-long bareback horse ride. I make coffee at the speed of an octogenarian. And it’s not just the day after – the stiffness goes on for the best part of the week.

But here’s the good part. The next class doesn’t hurt so much, both during and after. And two classes a week for a month start yielding visible results. The muscles in my arms look leaner and longer, my thighs are definitely more toned, and when I go swimming, I can actually feel the improvement in strength – 40 lengths of crawl feel much more doable. There’s some science to support that notion – barre works on the slow-twitch muscles which are what you need for endurance, as opposed to cardio exercises which tend to work on fast-twitch muscles for speed.