When William Broad held his first yoga pose in 1970, his intention was to attain better health, both in mind and in body. Now a practitioner of more than four decades, he’s looking at yoga from another perspective, trying to mete out the benefits of yoga from widely held myths, and along the way, revealing a downside of yoga that’s placed him firmly at the center of a decidedly un-zen firestorm of controversy. Broad is a senior writer with the New York Times, and author of the new book, “The Science of Yoga.” In January, the Times excerpted a chapter of his book called “How Yoga Can Wreck your Body,” spurring a frenzy of online responsefrom yoga defenders and detractors alike. Of course, that chapter is only part of a much broader book, in which Broad relies on research, interviews and peer-reviewed science to examine the myths and miracles of a practice that’s taken hold throughout our culture.