Yoga is as ancient as Sanskrit and as American as Walden Pond — Thoreau proclaimed “even I am a yogi” in 1849. Modern yoga is as rooted in the nation’s capital as Dumbarton Oaks in Georgetown, where the marvelously named philanthropist Mildred Bliss performed yoga postures in her famous garden, guided by a personal trainer imported from Austria in the 1930s. Today images of yoga bodies are as mainstream, and as annoying, as the parking tickets illustrated with “calming” poses issued last year in Cambridge, Mass., as part of a public art project.

But what is yoga? Almost all definitions mention the practice of a combination of physical poses, breathing, and mental exercises of South Asian origins, intended to achieve liberation from suffering. William J. Broad begins his new book, “The Science of Yoga,” with the assumption that readers know and agree that yoga is primarily a form of exercise. “Our own” yoga, as opposed to “old yoga,” involves “bending, stretching, and deep breathing” learned in classes at gyms, health clubs, studios and spas. No more nuanced definition appears at any point in this entertaining but haphazard tour through select scenes from the past century-and-a-half of investigations into yoga’s alleged powers — healing, sexual, magical and supernatural.

Within a loose weave of chronology and theme, the book traces a contemporary odyssey, a journey East and into the past. It’s a wisdom quest with a twist. Instead of conducting a classic search for spiritual enlightenment, this seeker sets out on a fact-finding, myth-busting mission “to track down the best science” and answer questions accumulated over 40 years as a “knowledgeable amateur” yogi. Traveling to archives, ashrams, libraries and laboratories in the United States and India, he meets with dozens of researchers, doctors, scholars and practitioners, and surveys texts ranging from crumbling 19th-century treatises to online catalogues of 21st-century clinical trials.

An award-winning science journalist at the New York Times, Broad is at his best when he provides straightforward summaries of contemporary biomedical research and sketches of researchers in action. Noting that the National Institutes of Health have funded yoga research since 1998, he frequently cites their PubMed electronic database of 1,000 papers, with 100 new reports published each year.

Some of the studies show that yoga “works remarkably well” to lower cardiovascular risk factors; it “can reduce stress, the heart rate, and blood pressure, helping to boost immunity and prevent diseases” and, according to one study, has implications for “increases in life span.” A section of the book on Dr. Loren Fishman shows integrated medicine in action: The former student of B.K.S. Iyengar leads classes for patients, conducts research and writes on yoga’s role in treating back pain, multiple sclerosis, arthritis and osteoporosis.

’The Science of Yoga: The Risks and the Rewards’ by William J. Broad (Simon & Schuster)

There’s a great deal of science woven into Broad’s narrative, and the anecdotes he strings together are fascinating. They range from descriptions of Indian and Hungarian investigations of yogis who claimed they could survive being buried alive to the recent use of MRIs to map what “Sex and the City” called “yogasms.” But there’s a tendency here to pile on details indiscriminately: In the chapter titled “Moods,” readers need less information about what Harvard Medical School researcher Sat Bir Khalsa ate for dessert, and more on his work on how yoga can promote sleep and ease performance anxiety.

The more serious problem also is a matter of proportion. At the big-picture level, Broad loses his footing. His de facto definitions of both yoga and science are too narrow for the complexity of his subject. He announces early on that he will not explore “meditation and mindfulness, liberation and enlightenment,” leaves the brain science of meditation for others, ignores Ayurvedic medicine, and does not analyze the complex historic relationship between science and yoga in colonial and postcolonial India. Instead of looking at yoga as a mass-culture phenomenon, he takes easy shots at its commercialization. For an introduction to the other contemporary yoga boom, readers must look elsewhere — to interdisciplinary research in the social sciences, religious studies, and social history of the embodied practice of modern yoga, such as Elizabeth de Michelis’s work on yoga as a secular healing ritual and the illuminating insights of Joseph Alter, Mark Singleton and other emerging scholar-practitioners.

Other omissions and errors detract from a book that in places is as compelling as a good mystery, even if it’s not, as the author claims, the first book to offer an “impartial evaluation” of yoga. Broad’s chapter-length warning of the potential risks of yoga-induced injuries is valuable, but flawed by its reliance on only a handful of case studies over the past 40 years and its lack of comparative risk data.

Like yoga, the fable of the blind man and the elephant originated in India. Science may be the central nervous system of modern yoga — or one way to attempt to locate, analyze and describe it — but it’s not the whole beast. This rambling, time-traveling book is not the comprehensive, balanced account of the science of yoga that we need. But it’s a start.

Cathryn Keller is writing a book on Selvarajan Yesudian, author of the first international bestseller on yoga and health.


The Risks and the Rewards

By William J. Broad

Simon & Schuster. 298 pp. $26