The Science of Yoga

risk factor for stroke, heart attack, and kidney failure. Because of such dangers, the human body over the ages has evolved a striking array of sensors and defense mechanisms that constantly take pressure readings of the blood vessels and make suitable adjustments.
    Robin said the Shoulder Stand tweaked one particular kind of sensor. It lay in the carotids—the major arteries that run through the front of the neck carrying blood to the brain. The carotid sensors make sure the brain gets the right amount of blood and, given the brain’s importance, get serious attention. Sensors embedded in the arterial walls monitor bulging or contracting that indicate changes in blood pressure.

    Shoulder Stand, Sarvangasana
    But in the ShoulderStand, Robin said, the chin presses deeply into the neck and upper chest, clamping down on the carotids and making the local pressure very high. That rings alarm bells and the parasympathetic brake flies into action. It assumes that the delicate tissues of the brain are reeling from too much blood and orders the heart and the circulatory system to compensate with pressure cuts. The main response signals go through the vagus—the large nerve that starts in the brain stem and wanders among the lungs, heart, stomach, and other abdominal organs.
    Robin clapped his hands to illustrate the urgent nature of the parasympathetic commands.
    “Don’t pump so often. Don’t pump so hard. Open the diameters. Vasodilate”—the term for vessel relaxation that allows blood to flow at a more leisurely pace.
    I thanked him. Gune may have recommended the Shoulder Stand to Gandhi for its calming effects, but he had nothing on Robin in explaining how it worked.
    Robin volunteered that the scientific approach to understanding the poses made some yogis uncomfortable.
    “There are people who say, ‘You’ve crossed the line. That’s not yoga. Look at Patanjali. There’s nothing about the workings of the sympathetic nervous system. ’
    “They are very, very traditional people,” he continued. “My own feeling is—I agree. It’s not yoga. It’s about yoga and understanding it, and that lets you do better yoga.”
    And with that, Robin turned his attention to other students.
    Better yoga. The phrase echoed in my head. A few days later, I called Yoga Loft and signed up for a series of four courses that Robin planned to teach on the science of yoga. The last focused entirely on the autonomic nervous system.
    Right off, he cast the topic in a new light. Most portrayals stress psychological factors—such as existential threats that prompt the fight-or-flight response, or peaceful interludes that bring about rest-and-digest states of contentment. But Robin said the systems could be stimulated not only by environmental factors but from the bottom up by conscious actions. An example, he said, centered on the muscles.
    “If you’re frightened, your muscles get tense,” he said. “But if you do muscle work, that also excites the sympathetic nervous system.” It was a fascinating observation thathad all kinds of implications for life and explaining the influence of yoga postures.
    Robin pointed to a place in his book that listed which parts of the body came under autonomic control. The table, spread over four pages, described more than one hundred functions— everything from sleep and gastric secretions to vasoconstriction and shivering. Each entry was followed by a reference number, or several reference numbers, that pointed to the book’s end section of scientific reports. The table seemed to represent a labor of love that summarized decades of research.
    Robin had us turn to another page that listed nine unusually potent effects of sympathetic stimulation. They include a quicker heartbeat (to prepare for action), dilated pupils (to admit more light for better attention to potential threats), and changed blood chemistry (to stimulate clotting in case of bleeding). Most people have no conscious control over such autonomic responses. But two items on the list stood out as relatively easy to influence—muscle tone and respiration rate. Robin called them keys to the hidden world of autonomic control.
    We practiced poses that worked the muscles, seeking to excite the sympathetic nerves. “Any kind of exercise, any kind of muscular work” will do it, Robin told us. He added that the same held true for respiration. “Anything you do to speed up your breath will speed up most parts of the sympathetic