Thousands of other clients join her sought-after training programme in person and online. Her Instagram account, @theannakaiser, has more than 288,000 followers.

The mother of two is bouncing with positive energy when I arrive at the Manhattan studio for a chat ahead of her class.

She rolls a yoga ball across for me to sit on in the wooden-floored studio, which is full of sunlight and flecked with light reflected from disco balls hanging from the ceiling. Those balls are a feature here; and mirror Kaiser’s infectious sense of fun.

“It was always my goal to deliver everything you need in one place, for mind, body and spirit, while fusing the fun and celebratory spirit of dance with the efficiency of training,” says Kaiser, who was a professional dancer and choreographer for 20 years before setting up Anna Kaiser Studio.

Kaiser’s approach is “taking the guesswork out of weekly workouts” by offering a one-stop shop, everything one needs for a well-rounded workout over the course of the week.

It’s not just for celebrities or those who can attend in person. All classes are now available online, anywhere, for US$39 a month.

Her group classes are in five formats: Step uses step boxes to elevate heart rate; Sculpt is weights-based, to tone; HIIT stands for high-intensity interval training; Dance is self-explanatory; Mashup is a mix of the first four.

All classes feature a dance component, designed to simultaneously tone and give high-intensity cardio bursts, with weights and resistance, the music set to a fast beat.

The dance moves are the ones Shakira’s backing dancers use in their actual choreography.

“The dance stays the same for two weeks, so you have a chance to learn it, improve and push harder through each of the movements, improving your endurance and stamina,” Kaiser says.

For those wanting to get in shape quickly, the classes may provide a good solution. Kaiser’s philosophy is emblazoned on a board by the studio door: “In 2 weeks you’ll feel it. In 4 weeks you’ll see it. In 8 weeks you’ll hear it.”

Alongside her classes, Kaiser offers a nutrition programme, which, for Shakira, was 1,200 calories a day – 40 per cent protein, 25 per cent carbohydrates and 35 per cent healthy fats, supplemented by an electrolyte-rich water.

The combination of the workouts and nutrition plan helped Shakira turn things around quickly.

She notes that she and Shakira are both 157cm (5 feet 2 inches) tall; people of different heights and weights need different calorific intakes to sustain them, and everyone metabolises differently.

We wrap up our conversation, as Kaiser must prepare for her class; I grab a pair of weights and head into the room where around 15 women are already gathered with box steps and resistance bands.

The atmosphere is friendly and welcoming, a sisterhood. Today I am doing the Mashup.

It’s a fantastic workout – not only do I sweat copiously, but I don’t once check my watch. It’s fun and invigorating and the soundtrack is brilliant.

The next day I take a Dance class – huge fun – and later try her Sculpt class online through an iPad at home.

Kaiser’s philosophy centres on the brain as much as the body – for her, cognitive function is as important as physical strength.

In her workout sessions, she always includes “mirror” routines, so you are doing the same moves on both sides of the body.

“Being fit and healthy is extraordinarily different than being thin. You may lose weight, but if you are fit that is due to the change in your body composition,” she says, noting that more muscle equates to less fat.

“If we cut down our eating dramatically, that will cause a drop in weight due to the loss of fat and muscle and slow your metabolism down significantly (the less muscle you have, the slower your metabolism).”

After a week spent with Kaiser and her disco balls, heeding her advice seems not only more of a priority, but also so much more achievable.