Pilates has fast become the fitness craze of the Noughties.

Favoured by celebrities and sports people alike, it is widely considered the best form of exercise for improving back pain and poor posture.

But some experts are now casting doubt over its ability to relieve back problems.

Indeed, they suggest that many who take it up might actually do themselves more harm than good.

Pilates is named after its founder, Joseph Pilates, a German-born sportsman who devised the exercise method 75 years ago, initially to improve strength.

He set up a fitness studio in New York at an address he shared with the New York City Ballet.

Word spread about his workout and Pilates was soon teaching actors and athletes.

Today, it is estimated that 25million people now take regular Pilates classes in the US and more than one million do the same in the UK ¡ª its celebrity fans include Madonna, Sigourney Weaver and Liz Hurley.

Its attraction is that it promises a means of staying toned and slim with good posture.

Crucially, participants are taught how to target a muscle called the transversus abdominus, positioned deep in the abdominal area.

This they do by drawing in their navel to the spine and lifting the pelvic floor.

But according to Stuart McGill, a professor of spine biomechanics and chair of the kinesiology department at the University of Waterloo in Ontario, it is this concept of "drawing in" that is the problem.

He and his colleagues have found that strengthening this "core" muscle often worsens back pain.

It's generally assumed that when you pull in this muscle, the torso temporarily acts like a corset, protecting the lower back, and that if the muscle is used often enough, it will eventually work on its own.

But by measuring how different loads and forces affect the way the spine functions, McGill found that the transversus abdominus muscle does not play as pivotal a role in protecting the back as was once thought.

"If you hollow in, you bring the muscles closer to the spine and you reduce the stability of the back," McGill says. "Try rising from a chair with a hollowed-out stomach. Not only are you weak, it is very difficult."

More than one-third of the UK population (17.3million people) have experienced back problems for more than one day in the past 12 months, according to the charity BackCare.

More than half have endured it for between a month and a year, and for six out of ten people the pain is severe enough to disrupt everyday activities at least once in their lives.

Exercise programmes such as Pilates that promise to prevent or eliminate the dull, persistent ache seem the ideal solution. But they can be a recipe for disaster.

"I have seen many people who have done their backs serious harm through Pilates,¡¯ says Fiona Troup, a physiotherapist and qualified Pilates instructor at the Sports & Spinal Clinic, Harley Street.

"Unless it is tailored to an individual and taught correctly according to their back diagnosis, it can cause injury."

The drawing in of core muscles became a fitness mantra ten years ago when Australian researchers found that people with lower back pain used the muscle less when they did various physical tasks.

The researchers recommended that physiotherapists teach patients how to contract both the transversus abdominus muscle and another ¡ª the multifidus ¡ª in the lower back.

But the lead researcher, Professor Carolyn Richardson of the University of Queensland's department of physiotherapy, has recently admitted she was concerned that the fitness industry had made the technique as popular as conventional stretching and warming-up exercises.

"I've found that for the fitness industry, it's a poor instruction that is often misinterpreted or carried out badly," she now says.

"It is easily done incorrectly by people mistakenly holding their breath or sucking in so far that they round their back."

As a result, the correct muscles aren't targeted and, over time, the back becomes more vulnerable than before.

Claire Small, a spokesperson for the Chartered Society of Physiotherapy shares the growing concerns about Pilates and other exercise classes that claim to prevent back pain.

"In some people, attempting to engage the core muscles can trigger a reaction that makes their condition deteriorate," she says.

"In theory, all the core, abdominal and pelvic muscles should contract simultaneously so that they inflate like a balloon inside the abdominal cavity, but incorrect technique can cause the pelvic muscles to descend which is bad news as it potentially weakening for the back."

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