Stretching the Truth Yoga and Pilates enthusiasts tout their workouts with a religious zeal, boasting about the life-changing physical and mental benefits they confer. In the past several years, scientists have been studying what these activities really can—and can’t—do for the body:  Heart Health Yoga: Hatha yoga provides little cardio benefit; power yoga does a bit better, giving the heart the same aerobic workout as a brisk stroll. Pilates: You may feel the burn with moves like “the hundred,” but your heart won’t. A beginner routine offers only a mild aerobic challenge, at the level of a slow walk. An advanced class is better, kicking the heart rate up to that of speed walking. Weight Loss Yoga: A 50-minute hatha class will burn about 145 calories; a power yoga class, about 250. If your goal is dropping pounds, experts recommend you do a high-intensity activity, like jogging, as...

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