Compared to the pumping intensity of spin or Zumba, a tai chi class looks like it’s being performed in slow motion. Watching the gentle, graceful movements of this ancient Chinese practice, it’s hard to imagine that tai chi can burn off a single calorie or strengthen muscles. But this exercise program is far more dynamic than it looks. “The slowness that you see from the outside can be deceptive,” says Dr. Peter Wayne, research director of the Osher Center for Integrative Med