Another thing to consider re: the overuse injuries: weight. I remember from reading McDougall's book that he's a fairly big guy; don't know about Lieberman. I would guess that heavier (and I don't just mean obese--I'm talking about the very solid, muscular builds as well) people are more likely to get the stress fractures and other issues from running barefoot on asphalt and concrete just because there's more force being exerted on their foot structures with each step, no matter how good their form is, than for a lighter person.