I thought about writing a post to dispute the ever-persistent "Concrete is harder on your legs than asphalt" debate. Then when doing research I saw an extremely well-thought-out, scientific assertion (which encapsulated many similar findings) showing they are essentially equal. Following it were comments such as:
"Very interesting stats, but I know my brain tells me that asphalt is softer."
"I'm not buying this. Physics aside I once read concrete is as much harder on the legs than asphalt as asphalt is harder than dirt or grass, and I believe it."
I then realized writing a post trying to convince people one way or the other is probably a fruitless endeavor. If people are not going to listen ("physics aside"?!) they are not going to listen. It is like when someone denies climate change, adds "Look, I am no scientist" and then goes on to refute the scientists who universally agree it is happening. If you believe concrete is worse for you than asphalt, I am not going to change your mind. Like a political debate, no one has ever changed from Republican to Democrat (or vice versa) when showed the other's stance.
So, rather than waste my time on a blog, let me sum this up thusly. While there are some things which make asphalt better (less dog crap, less uprooting by neighboring trees), if you actually are not so set in you ways that you will allow physics and facts to overcome "Well, I just know my knees hurt more" suffice it to say there is no difference between the two when it comes to hardness and impact on your knees, legs, muscles, etc.
The same goes for trail and dirt which, actually, given their often uneven nature, roots, rocks, etc are more prone to injure you in other ways like ankle turning. I know this goes against all those who think trail running is the one true Lord, but don't argue with me. I am not physics. I am just a guy who has run countless miles in the past decade on concrete with not one running injury to show for it. Undoubtedly, you have your own opinion what works for you and science and facts don't get in the way. Except for the fact that facts don't care what you think.