Why does running on a treadmill feel easier than running outside at the same speed?
Show answer & explanation
Answer: There's no air resistance to push through
There's no air resistance to push through ✓ — Outdoors you push air aside with every stride; on a treadmill you stay in place, so there's no headwind. That's why coaches suggest a 1% incline to match the effort of running outside at the same pace.
The moving belt does part of the work for you — The belt doesn't push you forward; relative to the belt, your legs work just as they would on the ground. If the belt did the work, you could stand still and still be carried along, and you can't.
Indoor air holds more oxygen than outdoor air — Indoor air has the same share of oxygen as outdoor air, about 21%. Your lungs get no bonus indoors; the difference in effort comes from air resistance.