Re: why does my back hurt when i do sit ups on the ball?
If you're truly doing 'sit ups' and not crunches stop doing them and change to crunches. Sit ups (even those done on a swiss ball) can damage your vertebrae in your lower back, this could be the pain you are feeling. This is because to work the abs effectively you are trying to make the lower back round, but tension in the psoas encourages the lower back move into an exaggerated arch. The result is the infamous "disc pepper grinder" effect that helps give you chronic lower back pain in later life.
Traditional situps emphasize sitting up rather than merely pulling your sternum down to meet your pelvis. The action of the psoas muscles, which run from the lower back around to the front of the thighs, is to pull the thighs closer to the torso. This action is the major component in sitting up. Because of this, situps primarily engage the psoas making them inefficient at exercising your abs.
Sit ups are inefficient because the psoas work best when the legs are close to straight (as they are when doing situps), so for most of the situp the psoas are doing most of the work and the abs are just stabilising.
Also, many people excessively train their hip flexors thinking that they're training the abdominals. Doing sit-ups (as discussed ad nauseum above), leg raises, "flutter kicks," and hanging leg raises are all primarily hip flexor exercises. That doesn't necessarily make them bad, but most people tend to have chronically short hip flexors, which can compromise the structural dynamics of the lumbar spine. Short hip flexors are also associated with low back pain. Just something to keep in mind.
Are you also exercising your back muscles? This helps immensely; working only your abs will leave you with stronger abs and weaker back muscles, which can cause back pain as well. You want to keep the core muscles in front and in back equalized to avoid compensating and ending up with a back ache.
To do a crunch on the ball:
Sit on the ball with your feet flat on the floor. Let the ball roll back slowly. Now lie back on the ball until your thighs and torso are parallel with the floor. Cross your arms over your chest and slightly tuck your chin in toward your chest. Contract your abdominals raising your torso to no more than 45 degrees. For better balance, spread your feet wider apart. To work the oblique muscles, make the exercise less stable by moving your feet closer together. Exhale as you contract; inhale as you return to the starting position.
A foolish consistency is the hobgoblin of little minds.