But they also are co-opting an ancient spiritual philosophy, many yoga experts contend. A sacred practice, they complain, is increasingly being debased and commercialized.
Yoga is a lucrative and growing business. About 16.5 million Americans now spend nearly $3 billion annually on classes and products, a February poll by Harris Interactive and Yoga Journal magazine revealed.
Compare that with two basic tenets of yoga--that it is unethical to charge money to teach it, and that you need nothing but your body to learn it.
The sun salutation, perhaps the best-known series of asanas, or postures, of hatha yoga--the type most commonly practiced in America--is literally a Hindu ritual.
"Sun salutation was never a hatha yoga tradition," said Subhas Rampersaud Tiwari, professor of yoga philosophy and meditation at Hindu University of America in Orlando, Fla. "It is a whole series of ritual appreciations to the sun, being thankful for that source of energy."
To think of it as a mere physical movement is tantamount to "saying that baptism is just an underwater exercise," said Swami Param of the Classical Yoga Hindu Academy and Dharma Yoga Ashram in Manahawkin, N.J.
What Americans are doing--practicing everything from hip-hop yoga to yoga with pets, using Hindu deities as knickknacks--is "hurtful and insulting" to the 5,000-year-old tradition, Param said.
The debate has intensified among yoga scholars and teachers as yoga practice has grown in popularity.
Between 1998 and 2005 alone, the circulation of the 30-year-old Yoga Journal tripled. Now there are yoga cruises, yoga book clubs, yoga dating services, yoga snacks ("created specifically for yoga"), yoga music ... the list goes on.
Todd Jones, senior editor of Yoga Journal, explained the evolution. Yoga "did start primarily as a meditative-spiritual practice. But it's gone in so many different directions." There are so many styles practiced in America, he said, it's nearly impossible to describe a "typical" yoga class.
"We live in a market-driven culture," Jones said. "If you're a yoga teacher, there's pressure to separate yourself in some way from the hundreds of others." Instructors often do this by "emphasizing whatever feels most compelling and authentic to them, and that differs from person to person."
But when Swami Param, now 56, was curious about yoga as a 16-year-old in New Jersey, it was by no means ubiquitous. So he turned to a dictionary.
"I still keep that Webster's with me," he said. "I looked up yoga and it said, `Sanskrit, Hinduism.' That's what it is. Just look at the facts."
Sanskrit is the language of sacred Hindu writings. "Every Sanskrit word these teachers are saying in yoga classes, they are using a religious language," he said.
Imagine the outcry if Christian, Jewish or Islamic prayers were commonly and casually used in nonreligious contexts, Param said.
The word yoga is most often defined as a yoking, or union. Its practice strives to unite the individual soul with the "greater soul" of the universe, traditionally through four main paths: karma (action), bhakti (devotion), jnana (wisdom) and raja or ashtanga (mental and physical control).
The physical postures of hatha yoga are practiced by Hindu yogis to enable them to more comfortably meditate for hours, freeing the mind from the distracting pains of the body. "A yoga master in India is a highly evolved spiritual being, not a gymnast," said David Frawley, director of the American Institute of Vedic Studies in Santa Fe, N.M., who writes and lectures on the controversy.
But Americans tend to focus on fitness alone, perhaps because "as a culture we are extremely physically oriented," as Hindu University's Tiwari put it. "We are enamored by the physical aspect of who we are. Some of us even worship our bodies."
Everyone agrees that yoga is physically beneficial.
"It's a very nice exercise activity," said Cedric Bryant, chief exercise physiologist for the American Council on Exercise in San Diego, Calif. "It improves muscle strength and endurance levels, joint range of motion and flexibility, and balance."
The Yoga Journal's Jones believes these physical benefits can ultimately draw participants into a deeper, more spiritual understanding of the practice.
"I'm more peaceful, I have more energy and more patience--but I certainly didn't go into it looking for that," he said.
Even that is unacceptable to Swami Param. "Why be covert?" he asked. Participants should be invited upfront to "come study Hinduism," which is what they're doing when learning hatha yoga, he said.
His New Jersey ashram does offer one nonspiritual class called "Stretch and Relaxation Based on the Hatha Yoga of Hinduism." He urges other hatha yoga teachers to explain to participants that they're taking a fitness class based on a religious practice.
"Then, they could even charge money," he said.