Certainly with an incline, your foot has less distance to fall with each stride (and thus less impact). While conversely with a decline your foot has further to fall with more impact.
I'm sure if you compared 5% incline to 5% decline (a net difference of 10%), there would be a noticeable difference in impact. But I'm not sure there is huge difference in impact between 0 and 1.5%. But if anyone can point me towards some sound research on this, I could probably be persuaded to change my mind.
Also, runners should really be training for a wide range of different conditions - up hill and down, different surfaces, etc. Coddling the knees too much on the treadmill can make the transition to running outside even harder. That said, it may make sense to be as gentle as possible while warming up, to ensure you don't put too much stress on the knees until the muscles are properly warmed up and moving fluidly.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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