Explanation:
look at the free body diagram
As we can see from the free body diagram
when a runner run two forces act on him.
gravity force which is acting downward and a reaction force by the ground which can be resolved into two components Fx and Fy.
Fx act in the forward direction to push runner to move in forward direction while Fy provide a upward force to the runner.
These two forces help a runner to run.
