Because there are so many factors that affect outdoor running and terrain is so varied across regions, there’s no concrete recommendation as to what incline level on a treadmill most closely mimics outdoor running.  However, Penner recommends bumping the incline up to level one or two in order to undo the propulsion of the running belt. “The motor and belt naturally propel you forward a bit during your stride, so slightly increasing the incline better mimics the resistance you’ll feel when running outside,” he says.