For those not familiar with Dan Lieberman, he is a Harvard professor who wrote the popular article for the scientific periodical Nature discussing the benefits of barefoot running. He was able to demonstrate reduction in force know as the transient impact that is associated with a heel strike. You can read more about Dr. Lieberman at http://barefootrunning.fas.harvard.edu.
Watch the Video below.
Source: drnicksrunningblog.com