Why is it harder to run on ground as opposed to a treadmill?

Question

Why is it harder to run on ground as opposed to a treadmill?

Answer 1

i wonder the same thing. when i run on the treadmill indoors, it is very easy for me to run for long periods of time without getting out of breath. but when i run outside at the same speed, i get tired quicker. I always thought it was the wind/temperature or something, but i don't know....