People often tell me that “humans are not designed to run on hard modern surfaces, like concrete and asphalt.”
Actually we are not designed to run on ANY surface without being able to sense directly how our bare soles are interacting with that surface. That is, we’re more likely to run with greater impact, skidding, etc., if we cannot feel those interactions directly on the soles of our feet.
It is the sensations in our feet that tell us to land more gently, with less abrasion, than we might do while wearing shoes to protect our soles from those sensations.
Hard smooth modern terrain are actually some of the most easy terrains for barefoot running – that is besides being hard, they don’t provide much of a challenge … and hardness of terrain is more challenging for running in shoes, because the shoes inhibit our natural ability to sense when we are landing too hard, or too abrasively.