Shin Splints (rider)

["guest@endurance.net" <guest@endurance.net>]

Alison Dunn alCritters@hotmail.com
Boy, after this weekend SASO IV, my horse seems ready for a fifty,
but I'm not certain about me.  I used to ride in the 'flexi'
plastic stirrups with a thick foam pad, and was fine for 25 to 30 miles.
I was concerned that the stirrup was a little too small for my 
generously sized feet, and got the aluminum stirrups with the
thick rubber pad.  Twice now on 25/30s I've come up with aching
shins on one or both legs - very sore to the touch. What causes 
this?  Is it because I might be pushing off from my toes when
posting rather than dropping my heels?  (As I fatigue, I might be
shifting to posting from my toes.)
Any advice or explantation would be appreciated!