Different surfaces allow for variations in running techniques based on hardness, degree of unevenness and hills. Flat surfaces such as roads, running tracks or treadmills force us into repetitive movements, which may over time cause an overuse injury.
Additionally, running on natural irregular surfaces brings about a variety of tissue stresses which can reduce the risk of overuse injuries. The best running surface is the cross-country-type surface, ie. firm and irregular. Since more body parts are being stressed and engaged, you can optimize adaptation and reduce the risk of overuse injuries.