Why are the arms so important in running

The first thing I look at as a Coach is the arms. Thou shalt not cross the sagittal plane of the body! What is the sagittal plane you ask; it is an imaginary line going down the centre of the body, that coaches use keep sprinters straight. Feet, knees and hands should not cross this plane. One of the biggest issues I have seen is the dishwasher arms, flailing around reducing efficiency, leaking energy and getting the body moving in a contradictory position to produce speed. Remember the hips will follow a knee or arm that crosses the sagittal plane, making it difficult to run in a straight line. A simple tool I have used is to get the athlete to stand in front of a mirror and practice the arm swinging back and forward (making sure the shoulder are relaxed)  rather than across the body. We are seeking to change muscle memory here, so the more times they do it the better. For the knees and feet, have them raise a broom stick (or just their hands) above their head and march in a straight line (line in concrete of lane line) making sure neither the feet or knees cross the plane. Once marching is mastered, go to skipping then high knee jogging.