The Science of Yoga

system.”
    It was a fascinating idea. His activity rule, given the list of autonomic functions we had just looked at, suggested that a disciplined individual could gain leverage over dozens of the body’s most important and inconspicuous functions. His rule also suggested that different Hatha styles had very different autonomic effects. For instance, Ashtanga, with its fast, flowing movements and its emphasis on Sun Salutations, works the muscles a lot and would thus stimulate the sympathetic system. In contrast, Iyengar, with its emphasis on static poses, would seem to lend itself to parasympathetic dominance.
    Robin expanded the activity explanation a step further and showed us subtle ways in which a pose could engage the parasympathetic brake. His ideas were an elaboration on what he had told me during the inversions class.
    He focused on the heart itself. Robin noted that the right atrium—the upper chamber that gets blood from the veins—bears a sensor that gauges its fullness. Whenpressure is low, he said, the sensor signals the heart to beat faster, increasing the blood flow. When pressure is high, the heart slows down.
    Robin said inversions worked beautifully—as with carotid pressure—to fool the heart into slowing. It happened because upending the body dramatically increased the flow to the right atrium. Normally, gravity helped a little between the head and the heart. But turning the body upside down let gravity work over a much larger area, strengthening venous flow from the feet, legs, and torso.
    “It’s all downhill,” Robin noted. “So the heart overfills.”
    The rising pressure in the right atrium then signaled the heart to beat slower. That signal, Robin noted, also caused the heart to reduce the strength of its contractions. It was a one-two punch. Overall, the atrial mechanism showed yet another way in which yoga could work inconspicuously to reset the metabolism.
    Robin had us do a heart test. First, we monitored our pulses, measuring their rate against the sweep of a watch. Then he had us move to a wall, lie on our backs, and raise our legs in a relaxed state of partial inversion. Once again, we measured. He noted that the beat was probably slower (which I found to be the case).
    A good yoga practice, Robin said toward the end of class, involved poses that cycled through the accelerator and the brake so the autonomic system got a thorough workout. Robin said the resulting realization of energetic flexibility over the usual conditions of metabolic life resulted in new abilities to achieve states of inner balance and harmony.
    “A large part of the benefits,” he said, “results from going through a couple of cycles every time we practice.”
    The clues gathered over the decades about yoga’s repercussions for human emotion first began to come together in a significant way at Harvard. Herbert Benson was a physician eager to ease Western tension with regular doses of Eastern calming. He and his colleagues studied the issue at the university’s medical school, examining the effects of meditation, yoga, and other soothing practices. Benson called his insight The Relaxation Response. His book, published in 1975, sold more than four million copies and became a modern classic on undoing stress.
    Benson found that simple techniques could have dramatic repercus- sions on his subjects,cutting their heart rate, respiratory rate, oxygen consumption, and blood pressure (if elevated to begin with). Overall, he and his colleagues showed that relaxed practitioners entered a state known as hypometabolism—a wakeful cousin of sleep that exhibits low energy expenditures. He called the relaxation response “an inducible physiologic state of quietude” that healed and revitalized.
    After Benson, many scientists sought to expand his findings and zero in on particular disciplines, including yoga.
    Mayasandra S. Chaya was an Indian physiologist in Bangalore who had practiced yoga since she was ten. Chaya led a team that studied more than one hundred men and women. The scientists prescribed a diverse Hatha routine sure to press both the metabolic brake and accelerator. The dozen poses included the Triangle (Trikonasana), the Shoulder Stand (Sarvangasana), the Locust (Shalabhasana), the Cobra (Bhujangasana), the Bow (Dhanurasana), and the Thunderbolt (Vajrasana), as well as fast and slow breathing techniques. At a session’s end, the subjects assumed the Corpse (Savasana) for a period of conscious