Most people do not give much thought on the motion of their arms during a run. Proper use of the arms can increase ones pace any where from 5 to 20 seconds per mile. This is one topic where even some advance runners may not be taking full advantage.
If used properly your arms can help your generate more speed. Their proper use will help you get to the top of the hill faster. On the downhills they help with the balance and increase tempo.
To see how important how use of the arms can be on your next run after you are fully warmed up and a running at a decent clip just drop your arms and do not swing them at all. If you do this for a half mile or so you should feel that your legs are doing more work than normal. When you go back to swing your arms you should be able to discern a slight lift of the knees on each stride. Using this knowledge we can increase the motion of the legs even further by perfecting your form.
To use your arms correctly we must start with proper postioning of the hands. As an experiment, in the standing postion, clench your hands into a fist. Now closely examine the effect that has on your arms. Feel how the forearms tighten up. This tightneing goest all the way up to your upper arms and shoulders. If you clench and relax your fist you can even feel some tightness in your back.
So this is one of the big keys to running succesfully. Keep your hands relaxed. It is pretty difficult to carry tension in your arms if your hands are completly relaxed.
The arms are the key to good form. Too gain an understanding on what good form is lets examine what we want to avoid. Many people not only swing the arms back and forth but they also swing them side to side. This common mistake leads one to turn the back back and forth. It really doesn't have much benifit in driving the legs any faster. But it does help consume alot of energy.
The rule then is to make sure your arms do not cross the center line of your body.