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.
Friction works in a direction opposite to the direction of motion.
The size of the frictional force depends on the force pushing the two surfaces together and their roughness. Playing surfaces and the soles of running shoes must provide sufficient friction to ensure that runners do not slip. However, too much friction between surfaces and shoes can lead to ankle and knee injuries. The friction between two surfaces before they move is called static friction . Once the two surfaces are on the move it is easier to keep them moving and the value of the frictional force becomes less.