I often see sit ups beeing suggest in online discussions, as a good exercise to workout and tone your abdominal muscles. This is wrong advice.
Sit up are ineffective for training abs
Traditional sit ups emphasize sitting up rather than merely pulling your sternum down to meet your pelvis.
When you sit up you are using mainly your psoas muscles. 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, sit ups are inefficient at exercising your abs.
They're inefficient because the psoas work best when the legs are close to straight (as they are when doing sit ups), so for most of the situp the psoas are doing most of the work and the abs are just stabilising. Putting the thighs at a right angle to the torso to begin with, like when doing crunches, means that the psoas can't pull it any further, so all of the stress is placed on the abs.
Sit ups are bad for your back
Another even more important disadvantage to doing sit ups is that they grind the vertebrae in your lower back.
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.
Consider crunches as a more effective and healthier alternative to sit ups, or any of the dozens of stomach exercises you can find on FlatStomachTips.com