Force is the act of pushing or pulling an object. Force has many forms, It can be shown as pressure, friction, gravity, spring and air resistance. When you are running the main types of forces that you deal with are friction and air resistance.Two other forces that can be seen in running are spring force and gravitational force. Forces like friction and air resistance can have a positive and a negative effect on your running, it all depends on how you use them. Learning how to use these negative effects to your advantage can make you a better over all runner.
For more information go to this link: http://www.physicsclassroom.com/class/newtlaws/Lesson-2/The-Meaning-of-Force
Forces acting on you at all times
Every day, multiple forces are acting on you at all times. You are affected by friction, gravity, unbalanced forces, balanced forces and air resistance. Since your body is affected by these forces every day, your body doesn't realize that any thing is happening to it. Your body is never acted upon just on force, it is affected by every. When your body is moving this is called an unbalanced force because your feet are over coming friction to move forward. When you are just siting down or standing still your body is being affected by a balanced force, this means you aren't moving. Forces are constantly affecting your body, you just don't realize it.
In this diagram you can see that the force going backwards makes it harder to run, but the force going forward makes it easier to run faster.