"According to several studies," the Times wrote, "the answer is not so simple. Researchers have found in general that while outdoor running tends to promote a more intense exercise, running on a treadmill helps reduce the likelihood of injury, and thus may allow some people to run longer and farther."

The Times article cites studies that have found "outdoor running burns about 5 percent more calories than treadmills do, in part because there is greater wind resistance and no assistance from the treadmill belt. Some studies show, for example, that when adults are allowed to set their own paces on treadmills and on tracks, they move more slowly and with shorter strides when they train on treadmills."

But the other side of the debate is this: Treadmills offer a softer landing surface, and the Times says other studies show that treadmill runners suffer few stress injuries in the legs because of this.

"One study published in 2003 in the British journal of sports medicine, for example, analyzed a group of runners and found significantly higher rates of bone strain and tension during pavement running than during treadmill running, particularly in the tibia, or shinbone," the Times wrote. "This increased strain can heighten the risk of stress fractures by more than 50 percent, the study found."

The bottom line: "Studies suggest that running on pavement generally burns slightly more calories, but also raises the risk of stress fractures."