Running indoors on a treadmill is slightly easier than running outside because you aren't moving through the air and there is no drag force to oppose your motion. A 65 kg man is running at 4.5 m/s on an indoor treadmill. To experience the same intensity workout as he'd get outdoors, he tilts the treadmill at a slight angle, choosing the angle so that the component of his weight force down the ramp is the same as the missing drag force.
What is the necessary angle? The cross-section area is 0.72 m2 and the density of air is 1.20 kg/m3.