You can think of your arms as pendulums and your feet as the weight of the pendulum. When you want to get a pendulum to swing faster, you need to shorten it. We have mastered the concept of shortening the pendulum by bending our knees as we run. We would look like narps if we ran with them locked. Our body works like gears on a bicycle. We warm-up to loosen the muscles and that's when we feel the burn in our quads and that is how gears work. After a warm-up you have reached a constant stride. Physics helps us with running because it allows us to calculate the amount of time it will take us to run a certain distance or to calculate what speed we should maintain to have a consistent split time.