Jonathon Hewitt brought up a good question- what about sports where you can't use your arms to assist your running, like carrying a football or a lacrosse stick?
Jonathon, you have stumbled upon one of the greatest core exercises- running with your arms crossed across your chest! Never forget, when we run force is not only generated from the ground up, but also from the top down. Arm action:
- blocks over rotation of the torso & pelvis in the long axis so the rear leg can swing forward.
-helps lift the body off the ground.
-increases horizontal thrust in the sagittal plane.
-helps maintain balance.
-Mechanically "dampens" abnormal body postures created by distal postural asymmetries.
I look pretty silly when I run with my arms crossed across my chest. The osteochondritis dissecans in my left ankle from an old sprain is transmitted up through the torso causing me to weeble-wobble down the track. Yet, it is an important conditioning activity for me.
Without using the arms, the torso's lever arm advantage is removed. The core of the body must compensate for extraneous body movement in all planes- reactively, subconsciously. Watching someone run without the arms can be a useful evaluation tool.