Because they have no upper, your toes can splay as far as they need to. There's plenty of breathability, too. There's just enough thickness in the sole to allow for comfortable running even on gravel.
I think they look so great with the suede footbed that I couldn't bring myself to wear them through the mud, even for the review for Living Barefoot (which is coming soon).
You really have to have strong feet to run in these for any length of time, however. The muscles in my feet were a little sore the next day. :)
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