I overstretched my brachial plexus from (a) having a droopy shoulder and (b) swinging the arm vigorously for 45 mins a day while walking. And look, for $300 - $600, I can get a fancy illustration of it:

http://www.medivisuals.com/nerve-com...99048-04a.aspx

No more arm swinging for me, especially on that side. Hand goes in the pocket or on the hip.