horse behavior

[shari <snaylor@pacific.net>]

I noticed that my 11 year old Arab mare often trembles in her legs before
going on a ride and sometimes during a ride after we stop for a rest. She
is never hesitant to load for the ride and doesn't appear to have any other
percular behaviors but this one. She has a lot of stamina, and does very
well pacing herself on long rides, 20 miles and more. She seems to tremble
more when she is around other horses. She seems to like other horses and
always wants to greet them when I ride with others. Could this trembling be
because she is anxious or excited, or could there be another problem that I
need to be aware of. I ride by myself most of the time and I don't usually
notice this behavior, except when she is cold in the early mornings.
I'd appreciate knowing if this is normal or not for some horses.
Thanks, Shari