Q. Is there any difference, fitness-wise, between running on a treadmill and running outdoors?

The answer depends on how fast you're running. For the average person, running 6-9 mph on a health-club-quality treadmill, the difference is slight, perhaps nonexistent. Some studies show no difference at all between treadmill and outdoor running; other research shows outdoor running burns 3-5 percent more calories. "The treadmill belt is doing a little bit of the work by helping pull your feet back underneath your body," says John Porcari, Ph.D., a professor in the department of exercise and sports science at the University of Wisconsin, LaCrosse. (A cheap treadmill, with a belt that doesn't move smoothly, won't assist you as much as a high-quality machine, so you'll probably burn the same number of calories as when you run outside.)

When you run on a treadmill, you don't have to overcome wind resistance, so that may also explain the small difference in calorie burn. If you're running faster than about 10 mph -- a very speedy six-minute-mile pace -- outdoor running may burn up to 10 percent more calories than running on a treadmill does because you're working harder against wind resistance.