When running or walking we use the energy in our leg muscles to push backwards on the ground with one foot while the other foot is moving forward. Forces always come in pairs called action-reaction pairs. So, when we push backwards on the ground, the ground also pushes forward on us. Sir Isaac Newton first proposed this idea in the 1600s. Action-reaction pairs of forces associated with an athlete are due to gravity, friction with the ground and air resistance.
Runners move across a surface by a series of long strides.
Considerable damage can occur to the runner's ankles and knees unless the force is reduced. An elastic playing surface such as grass feels springy to run on and produces fewer injuries than more rigid surfaces such as concrete. But the time spent rebounding is higher on a springy surface, slowing the runner down. The best surface for athletics is one that is absorbent enough to limit injuries but firm enough to give athletes the best chance of achieving optimum results. The force of friction applies when you move any two surfaces against each other.