Shakey mare

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

sherrie hall mhrarabs@gwtc.net
I only trailride and not at a overly fast speed, walk and trot occasionally. My mare is 7 years old and is ridden 2 times a week for 3-4 hours. She is otherwise out in a 20 acre pasture. She is a purebred Arab. The question I have is this year I have noticed she will start quivering after about 1 hour into the ride. I have noticed this in cool weather as well as warm weather. We live in the Black Hills of South Dakota so its not that hot here. It doesen't seem to matter if she is sweated up or not. Is this something I should give electrolytes for? I have had this mare from birth and this is her second year under saddle. I would sure appreciate any advise in this from you all. I am very fond of this mare and will be talking to the vet soon,but in the mean time......
Thanks, Sherrie