I have an exercise routine wherein I lie face-down on an exercise ball and hold my arms out in front of me for a few seconds, then drop them and pick them up again. I do this 10 times. Following that exercise, I hold my arms out to my sides for a few seconds, then drop them. I also do this exercise 10 times.
Perhaps as an unfortunate result of my muscular restrictions, I sometimes feel my butt tightening up to the point where it's kind of annoying. After all, I'm trying to exercise my back, not my buttocks.