Re: why do your feet expand when you run?

Any part of your body that is being exercised vigorously will expand.  Have 
you ever ridden your bike in jeans over hilly terrain and found that when 
you got off your legs felt kind of strange and your jeans seemed tighter?  
It's the same premise: as you work your muscles, they use more oxygen - to 
get more oxygen your heart pumps your blood faster and your muscles become 
engorged.  Once you cool down your blood gets redistributed to it's normal 
resting proportions.  While running, the muscles in your feet are 
constantly at work absorbing shocks and maintaining balance thus becoming 
flushed with oxygenated blood and expanding