I agree with tblver its a respect issue, but it also could be a fear thing. My mare has free run of her stall, with a door opening up to the pasture and she can come and go as she pleases, she will stand in it all day during the summer, to escape the flies, and hot sun. But if I close the door and lock her in, she starts acting like yours does, paces non stop, and circles the stall, (and what a mess to clean that up in the morning!) She always has a buddy in the stall next to her so she is never alone, just doesn't like that fact that her escape route has been closed off. This past winter she was locked in the stall for 3 days because of a blizzard that we had, and let me tell you when I finally opened the door she bolted out so fast if the snow wouldn't have been almost up to her stomach she would have fallen! Bucked and kicked and ran around for a good twenty minutes, and then voluntarily walked quietly back in the stall cause it was still so **** cold and windy outside. I have noticed that if I play the radio in the barn the horses are quieter. But some folk say not to do this cause it mask noises and the horses can easily be spooked. I figure, Mask the noise, that way they don't hear as much going on outside, and therefore they don't worry about it. To each his own I guess.