Grunt and groan and sweat and strain. This really can't be yoga - can it? Peter Guttridge on an ancient art's latest gung-ho guise
Perspiring heavily, Sarah balances on her hands, her legs threaded back under her armpits, her ankles crossed behind her neck. Anne, a marketing executive for a London-based bank, slides into the splits on a drenched exercise mat. In one movement, Rob, an Oxford management consultant, drops from a standing position into a kind of press-up, makes his back concave, then swings his butt up to hold himself in an inverted V. Phew.

It's just another morning of astanga vinyasa yoga at the Practice Place in Crete. The popular perception of yoga is gurus, meditation and gentle stretching exercises. Not astanga vinyasa yoga. No dogmas, no meditation - just a lot of sweat. Regarded by its proponents as the truer yoga of which other hatha yogas retain mere fragments, it uses standard yoga postures (asanas) but puts them together into 90-minute flowing sequences that are very vigorous and very chic - celebrity practitioners include Sting, Kris Kristofferson, Koo Stark and the US basketball star Kareem Abdul-Jabbar.

This aerobic yoga, which comes on like a gymnastic dance, was rediscovered by the yoga master Pattabhi Jois in the Thirties. It has been growing in popularity in America and Australia since the Seventies as a "work- out" yoga. It's the Nineties now, and the new, muscular moniker is power yoga. A very catchy tag - adherents claim that in combining the raw energy of the modern aerobic fitness craze with a New Age interest in ancient, alternative approaches to well-being, power yoga's time hascome. For the past 10 years Derek Ireland, 46, a former professional footballer, and his long-time partner, Radha Warrell, 47, once a Roedean sports coach, have run astanga vinyasa sessions throughout Europe. "We began in New York in the mid-Eighties," Mr Ireland says. "We'd been teaching traditional yoga, but I took to this immediately because of its dynamism."

Living testimony to their calling, Mr Ireland and Ms Warrell have taught power yoga for the past six years at their centre in southern Crete. Sarah, Rob and Anne have all done the yoga before. They are "self-practising" under Mr Ireland's watchful eye. "Most people we get are professional," he says. "They spent 15 years getting their careers together but forgot about fitness.

"We used to teach two classes a day. By the third day, nobody could move. We go easier now. "

During a 90-minute practice, the postures are arranged in a sequence that allows the skeletal system to go back into correct alignment and the muscles to open out. Performing the basic moves, known as the primary series, exercises the whole body - detoxifying, stretching and strengthening. It's tough going. Everything depends on matching breath to movement. The breath (the vinyasa bit) releases the energy to complete the series and the heat generated allows the student to do advanced stretching exercises safely.

In another room, a dozen beginners are finding everything a bit of a shock. They include a personal fitness trainer and a group of yoga practitioners who study a more traditional form - iyenga.

Yoga students always find astanga difficult at first; it seems contrary to good yoga practice. "I was expecting something ... gentler," gasps one red-faced woman. But the personal fitness trainer thinks it's "brilliant. There's been nothing new in aerobics for years. I can see this taking over because you get the suppleness as well as the aerobic movement."

Those who like it love it. Anne, for instance, is going to India to study with Pattabhi Jois. Jois, now aged 80, is a hands-on teacher. His approach includes, on occasion, lying on top of people to get them into a posture. In a article in the US Yoga Joumal, some criticised him for his "risky or even violent" methods. Mr Ireland and Ms Warrell have a much gentler approach. Even so, there are one or two drop-outs on every course at the Practice Place who aren't fit enough to persevere. People who aren't particularly supple (most of us) can suffer niggling knee injuries and strains.Recently, in a UK yoga magazine, one yoga teacher told of a serious injury she had done to herself at the Practice Place.

Mr Ireland says: "It's important to be focusing on what you're doing all the time. But we've developed a way of teaching so that most people have made remarkable progress by the end of two weeks. If they take it home and carry on with self-practice, it will keep them fitter, more energised. You won't need any other exercise."

Still, he knows it's not for everyone, certainly not those who want to do less than the recommended six times a week for a full 90 minutes. He laughs and shrugs: "It's never easy. I've been doing the primary series for 15 years... and it still kills me."

Details of the Practice Place from 12 Beatty Avenue, Coldean, Brighton BNI 9ED, 01273 687071. 'Power Yoga', by Beryl Bender Birch (Prion Books, pounds 8.99).