The "passive" core exercise as I like to call it. When sitting in a standard chair the muscles of the low back and core are not being challenged. We sit down, we slump, round the back, and literally hang on the spine. While sitting on the ball we are challenged to...well...sit on the ball. Muscles supporting the low back (in particular a group named the Multifidi) and core (in particular the Transverse Abdominus) will have to work meaning these muscles are contracting and relaxing to stabilize and keep you on the ball. Often in low back pain patients the multifidi and transverse abdominus are not functioning correctly (i.e. not responding in a coordinated manner or not responding at all) and are unable to respond to movements such as reaching over to simply pick up a piece of paper off the floor. "I was just reaching for _______ and my back went out and I had to lie down on the floor." Conditioning these muscles can be begin by simply switching out the chair at your computer for a gym ball. Try it and you may be surprised how much stronger your core over time becomes by doing this "passive" core exercise.
General Ball Sizing
Under 5'4" use a 55 cm ball.
Between 5'4" and 6'1" use a 65 cm ball.
Over 6'1" use a 75 cm ball.