A study published by the New York Times compared running outdoors on pavement to indoors on the treadmill. The conclusion was that you will burn more calories outside then in, but you will also have a greater chance of injury outdoors.

My recommendation would be to combine the two. If your legs are feeling rundown or if your in a less intense week then the treadmill is the answer. However if your fresh and looking to ramp up the calories burned, then head outside!! They both have benefits to increase your fitness level, and should be used accordingly.

“A number of studies have shown that in general, 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 other studies show that treadmill exercisers suffer fewer stress injuries in the leg. 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. This increased strain can heighten the risk of stress fractures by more than 50 percent, the study found.”