;)
One of the challenges I often get in relation to barefoot (or minimal footwear) running is that our bodies were not meant to cope with modern hard surfaces such as concrete.
There is a website I have looked at recently that covers this in it's FAQ section, answering this and other related questions with a scientific bent.
It is by Daniel Lieberman (a Harvard Professor researching and practicing Barefoot Running)
Biomechanics of Foot Strikes & Applications to Running Barefoot or in Minimal Footwear
According to the website:
"it has been developed to provide an evidence-based resource for those interested in the biomechanics of different foot strikes in endurance running and the applications to human endurance running prior to the modern running shoe."