The Science of Yoga

they did yoga but as they walked briskly on treadmills and rested in chairs. That gave the scientists a reasonable basis for comparison. It was an experimental control meant to enhance the reliability and—not inconsequentially—the credibility of their measurements.
    Another precaution centered on skill. The scientists recruited volunteers from a university yoga class, and the subjects had practiced for at least a month. The experience factor implied that the moves and postures would be more precise and rigorous than with beginners, in theory strengthening the aerobic stimulus.It bespoke an effort to take the measure of yoga as regular exercise.
    Clay and her team also brought new precision to the measurement of oxygen intake. Unlike the before-and-after methods of the Duke and Davis studies, the Texas researchers fitted their subjects with face masks hooked up to breath analyzers, producing direct readings of respiration. The scientists judged that the gains in accuracy would outweigh any inconvenience.
    The yoga session was shorter than in the Davis study. It lasted just a half hour, compared to an hour and a half. The scientists said they designed it to resemble a routine in a health club. The Texas study, like the Davis investigation, put Sun Salutations at the heart of the session.
    The investigators cited the Davis paper in reviewing prior research. But their findings bore little resemblance. Perhaps most conspicuously, the Texas scientists explicitly addressed how their findings measured up to the official recommendations.
    The team examined a variation of VO 2 max known as maximum oxygen uptake reserve. It expresses the difference between oxygen consumption at peak levels of exercise and during rest. Since the resting metabolic rate of individuals can vary, exercise physiologists consider the reserve formula a more accurate way of making comparisons of athletic fitness. (The method is similar to how the vital index took personal factors into account.) The American College of Sports Medicine, in promoting aerobic conditioning, recommends that individuals draw on 50 to 85 percent of their maximum reserve. By contrast, the Texas scientists found that women walking briskly on the treadmill used about 45 percent.
    And yoga? The women, while doing the routine, achieved far less—only 15 percent. The results, the scientists reported, “indicate that the metabolic intensity of hatha yoga is well below that required for improving cardiovascular health.”
    The only encouraging news centered on the Sun Salutation. Clay and her team said the fluid pose turned out to represent the workout’s most aerobic aspect—a wide belief in the yoga community that had previously gone untested. The scientists found that Sun Salutations drew on 34 percent of the maximum reserve—more than twice the overall yoga session. And they suggested that the reading, though “significantly lower” than the 50 percent minimum of the American College of Sports Medicine, was nonetheless high enoughfor yoga teachers to consider putting more emphasis on the vigorous pose.
    “To increase intensity,” the researchers said, “it appears that the Sun Salutation or similar series of asanas should comprise the greatest portion of a Hatha yoga session.”
    Another downbeat finding emerged in 2005, just a month later. The study was done at the University of Wisconsin. It centered on thirty-four women with no yoga experience and no history of regular exercise. The women were divided into yoga and control groups. The yogis did fifty-five minutes of Hatha three times a week for two months while the non-yoga group did no exercise at all. Compared to the Texas study, the workout was longer and presumably more vigorous.
    The investigators in Wisconsin found gains in strength, endurance, balance, and flexibility. But not in VO 2 max. “The intensity just wasn’t there,” noted John Porcari, one of the scientists.
    The Wisconsin team did a companion study to see if Power Yoga—a demanding series of poses based on the Ashtanga system, with emphasis on flowing postures like the Sun Salutation—posed a greater aerobic challenge. The scientists recruited fifteen participants with at least intermediate experience. It turned out that the heightened vigor did make a difference, but only slightly. “You certainly sweat,” Porcari said. “But it’s not an aerobic workout.”
    He disagreed with the Texans on the idea of introducing wide customizations