Thanks, that was clear. That's how I run, and with your description in mind, I'll try to make sure my nephew runs that way too. I do notice that a lot of runners kind of curl up the arms inside their torso, but they also tend to be slouched, and often have a pained expression on their faces.
Here's a nice summary from Steve Magness (
http://www.scienceofrunning.com/2011/02/180-isnt-magic-number-stride-rate-and.html) on the whole rate/length issue:
"What I’ve found in my years working on running mechanics and in being taught by some of the best minds on the subject, is that I nor them have ever focused on stride rate or length. Those are outcomes of what you do. They are feedback. They are not things you directly change. If you take care of the mechanics, whether it’s arm swing, body position, force application, footstrike or whatever, the rate and length will optimize. Is it useful to measure length and rate? Sure, but remember that they are data. If one of them seems “off” you’ve got to figure out how to fix it."
Basically, he's saying stride rate and length are effects not causes, and that's what has occurred to me, and why it seems like posture and then foot landing are most important. Mostly I'm glad this thread has convinced me that I can go back to my world of three days ago and not think about cadence anymore!