One of my favorite exercises on the resist-a-ball is the back bend over the ball. The longer I sit, the more my lower back stiffens up. The tightening is usually my SI joint (or tailbone) locking up (or at least the soft tissue, muscles, ligaments tightening up). Some of you might wait it out to go see your Chiropractor, but I find rolling on a ball is a quicker and more passive way to resolve my pain.
There are many stretches I can do that also help with this issue, but the rolling seems to provide immediate relief in the SI area.
When you begin to slowly roll over the ball, you want to make sure that you breathe naturally. If you are having difficulty, try breathing only through your nose. When you roll yourself over the top of the ball, make sure your feet are securely grounded on the floor. You can begin by using your arms to reach over the ball, or keep them on your chest until you feel comfortable. As you bend over the ball think of a few things:
1. The ball will support you, just move slowly, so melt into the ball and relax.
2. If you are reaching your arms over the ball as well, imagine someone puling your arms and legs away from each other so you are lengthening your body.
3. Your butt and hips are your anchor side, so allow them to weight you down. This will prevent you from going over the top of the ball. It also helps your lower back feel a sense of release when you achieve that lengthening feeling.
I suggest back bends before doing deep stretching of your back. When you are coming out of a long period of sitting, it is important to move passively in all stretches.