Media playback is unsupported on your device Media caption The BBC's Jane O'Brien attended the opening gala of Yoga: The Art of Transformation at the Sackler Gallery of Asian art

Early yogis struck lovely poses - and also fought fierce battles. An exhibit in Washington shows the complex dimensions of yogi life.

Do you like your yoga hot or powerful? Maybe you practise in the nude or spend a fortune on the latest Lycra. Are you a follower of BKS Iyengar, or do you prefer Vinyasa or Ashtanga yoga?

Whatever your taste, yoga is a $5bn (£3.1bn) industry in the US practised by millions of people seeking physical fitness, improved health, or spiritual enlightenment. Almost every gym offers a class and the experience can be enhanced by drinking specially blended teas or listening to suitably soothing music.

The Indian government has become so concerned about the commercialisation of yoga in recent years that it started a campaign to patent hundreds of postures to stop them being appropriated by Western companies.

Yogis were seen as quite sinister and dubious figures Debra Diamond, Smithsonian

But the world's first exhibition exploring the visual art of yoga has revealed aspects of the ancient tradition that many purists will find troubling: in the 2,500 years of its known existence, there has never been one single type of yoga.

"Five years ago I did think I would find that single yoga tradition," says Debra Diamond, curator of the exhibition Yoga: The Art of Transformation at the Smithsonian's Sackler Gallery of Asian Art.

"But yoga constantly transformed and developed over time. Although there are a couple of main goals, there's nothing that shows up in every single yoga path.

"For some traditions it was heightened consciousness and an end to suffering, a way to get out of the cycle of birth, death and re-birth that is so painful. But for other yoga traditions some of the goals were things like supernatural powers and the ability to control other people."

Image caption The Kedar Ragini depicts an ascetic listening intently to a musician

The exhibition showcases 130 objects from 25 museums and private collections around the world. Many have never been seen in public while others are known masterpieces. Together they mark the start of a new field of study - how yoga's visual culture can shed light on its profound mysteries and hidden meanings.

A group of fearsome looking yoginis, more than 1,000 years old, greet visitors at the start of the exhibition. These are flying goddesses who attained divinity by practising tantric yoga. They sit in audacious poses, baring their sharpened teeth and voluptuous breasts, their loose hair marking them as wild women.

Some are adorned with snakes while another has her fingers in her mouth, ready to pierce the night skies with a war whistle.

"They're the tough chicks of the 11th Century," says Diamond.

The theme of the warrior yogi continues through the exhibition, culminating in a dazzling and intricate 16th Century watercolour illustrating the Battle at Thaneshwar, at a holy site in northern India.

The blood flows freely as bands of armed yogis decapitate their rivals and battle over bathing rights at a festival.

"Compared to what happens later on, it's a relatively small skirmish," says Sir James Mallinson, a Sanskrit expert and one of the exhibition advisers.

Image caption Garbhasana, a watercolour from 1600, depicts a yogi in practice

"By the 18th Century the yogi sects are so powerful and so big that we have reports of huge pitched battles at these festivals where thousands and thousands of these yogis get killed."

The painting is the earliest known depiction of a fight between yogi sects, which were hired as mercenaries by the Mughal emperors until the British disbanded them.

Sir James is a yogi himself and has spent many years living and studying yoga in India. He is the first Westerner in his sect of master yogis to be ordained as a Mahant, or commander, with the authority to raise his own troops.

He sees no conflict between yoga's peaceful pursuit of spiritual enlightenment and its more violent manifestations.

"For these guys, even to this day, they see no contradiction," he says. "They understand yoga and their other practices are generating a kind of internal power which they can then use in various different ways, giving blessings or curses or indeed fighting."

Given yoga's bloody history, it may seem odd that in the US it is practised largely by women seeking to relieve stress. That trend has its roots in the 19th Century when women's exercise emphasised stretching rather than the more masculine activity of weight lifting.

"For guys, exercise is sports and competition, which seems to fly in the face of what yoga is about - but doesn't actually. Men just don't know what yoga is," says John Schumacher, director of Unity Woods Yoga Center in Washington - and one of a handful of American yoga teachers to qualify under Iyengar.

"The very qualities that make a top athlete are the qualities necessary for excellence in yoga - not just physical flexibility, strength and stamina, but the whole quality of meditation which involves focus and clarity."

The exhibition explores how such perceptions of yoga have changed as the practice crossed cultures and continents.

Image caption Yogis used to wage war, as this 1590 painting shows

"This is a very complicated and tricky section because what we have are very often images of European desire, what Europeans believed about the exotic in India and what they believed about yogis," says Diamond.

On display are a number of early photographs of yogis presented as "native views" with elaborate backgrounds and props that fuelled the imagination of Western viewers.

"Yogis were seen as quite sinister and dubious figures," she says. "They claimed to have worldly or supernatural powers, they were scantily dressed or naked, which was very upsetting to 19th Century sensibilities. They wandered around and often smoked dope. They became reviled as the stereotype of everything that was decadent about India."

But as this ground-breaking exhibition shows, yoga has retained its power to transform, not just the bodies and minds of its practitioners, but yoga itself. The yoginis of the 11th Century wouldn't recognise their sisters in the 21st. Or would they?