of all the jobs I've had unloading was the best, you get to be alone with your thoughts, as long as you are moving packages out at a good rate, most sups will leave you alone. The work becomes automatic.
Sure at first there is that sore muscle thing and usually they will pair you up with some crazy person, just to show you the speed they are looking for. Do not worry, its part of the mind games that they play.
Wear light , loose fitting clothes, have a couple of rags to get the sweat, bring a frozen bottle ( or two ) with you { those feeders,
ups slang for trailers, have been sitting in the hot sun allday...those frozen bottles will melt quickly } , have a quick drink at every slow down you come to.