Tuck in your elbows when you run and walk

I've been noticing that some people swing their arms across their body while they run. As your arms move across your body, your whole upper body tends to twist. People swing their arms this way, because they keep their arms folded across their body. This type of arm-movement isn't considered good running form, because it takes extra muscle-movement and extra energy to move your arms across your body. The solution is to tuck your elbows in toward your waist such that your arms move in and out in a motion parallel to the direction you're going. By doing this, you can keep your upper body stationary and only move your hips and legs.