Question

Explain why a runner presses the ground with his feet before he starts his run .

Answer 1

According To Newton's Third law of Motion:
" For every action , there is an equal and opposite reaction ".
Action=-Reaction

-->So when the runner pushes the ground[action force] he experiences an equal and opposite reaction force from the ground which he utilizes to run faster.

--->The harder  he pushes the ground, more reaction force he gets which helps him to move forward.

Answer 2

Using the  Newtons's third law of motion - "For every action there in an equal and opposite reaction". The runner presses the ground with his feet before he starts his run in turn the ground deploy a reaction force on the runner in the forward direction to help him run.