Russ - Along the line of what Diane Nelson said, the rider needs some variety 
in use of muscles during a long ride.  Personally, I post a lot, ride 
two-point some - 'floating' just above the saddle, and if terrain is 
appropriate (like steep downhills) I get off and jog with my horse.  At a 
canter I more or less concentrate on 'riding lightly' and steadily to be easy 
on the horse's back.  You see a lot of riders flopping around all over.  It 
makes me feel for their horses! 
 
The same thing about variety holds true for the horse.  That's another thing 
in favor of setting the speed and then allowing him to choose his gait.  If he 
is forced to trot the entire ride, for example, using the same muscles in the 
same way he is bound to get sore.  Changing to a canter for awhile or walking 
will give those muscles some relief.  Also, switching diagonals frequently 
gives him a break and helps avoid fatigue from using the muscles in the same 
way all the time. 
 
In the past I have gotten so sore that I went backwards down stairs to ease 
the pain to the muscles on the front of my thighs (this was after posting for 
60 miles).  The next ride that did not happen, though I did get generally sore 
for a day or two.... 
 
Hang in there  -  It does get better. 
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Dave Bennett      		 
e-mail:  idj3q@mhs-tva.attmail.com      
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