If it is an experienced horse with lots of miles - so his soft tissue
structures are strong - and he is well balanced then trotting down hill
should not be a problem. The disclaimer is very steep hills where you
should be walking - leading the horse.
There was an interesting comment in "Endurance News" by C. Hart in his
"pacing" article. He comments that most downhill injuries occur at the
bottom of the hill where it levels off becuase of the addition force
applied by the centripetal force involved. After all when you come to the
bottom of a hill you are on a circle and the centripetal force places
additional stress on the horse.
So the conclusion is to slow down before you come the the bottom of the
hill to avoid a larger force component on the horses legs because of the
centripetal force.
A good application of basic physics to riding!
Truman
Truman Prevatt
Sarasota, FL