Muscle development = itchiness?

[guest@endurance.net]

Suzy Fraser suzy@vicinity.com
Sorry, I'm not an endurance rider but a dressage rider, but this seemed
like a more logical place to look for an answer to my question. My horse
is starting to develop more muscle all over. While I'm grooming and
tacking up, he begs to be scratched, or so I thought since he'd twitch his
upper lip as though I had hit an itchy spot. What I have discovered,
though, is that he really likes to have his large muscle groups massaged
and with a fair amount of pressure. I'm curious as to what's happening
with his muscles that makes him twitch his lip as though he itches. Any
ideas? :-)