Arm movement is the body's way of counterbalancing leg turnover. Watch any race, and you'll see some runners pumping their arms like mad, and others who keep their arms at around chest height with very little movement. As momentum and gravity drive you forward, your arms are keeping you from tilting off balance. A natural instinct for runners is to try to use their arms to drive themselves forward, but this an unnecessary waste of energy. Just like with your stride, your arm movement should be comfortable and as fluid as possible. In "Jeff Galloway's Book on Running," expert Jeff Galloway recommends less arm swing. "The arm motion won't help you run farther or faster," he says, "but too much arm motion can slow you down through upper body fatigue."
So, while our arms play an important role in running form and can help us run more efficiently, it's important not to overemphasize their use and risk burning valuable energy that could be redirected down toward the legs, where the real work is happening.