To understand the basic physics of running, you can think of your limbs as pendulums. A pendulum's velocity depends on
the length of the pendulum, not the mass at the bottom. If a pendulum is shorter, the speed of the mass at the bottom is faster.
Think of your feet as the mass at the end of the pendulum. Then assume that your legs are the pendulums. In order to shorten
the pendulum, you bend your knees. This is very simple, and obviously you have realized that it is very difficult to run with
your knees locked. Therefore, you will run faster if your knees are bent.
You can also think of your arms as pendulums.
They act in the same way as your legs do. Now try this:
-Swing your arms by your side like you do when your running, but don't bend them.
-Then bend your arms at 90 degree angles and then swing them.
You probably found that swinging with your arms bent is much easier. That is why, when running, you bend your arms. It
helps to move them faster.
For more information on the basic physics of running and how improve your own running, you can check out the website that
gave us the above tips as well as a lot of others.
The Physics of Running