Full or half sit-up: Which is more effective?

According to Martina Sprague, author of Strength and Power Training for Martial Arts, full sit-ups don't work the abs any more than partal sit-ups. "The abs are used primarily during the first 30 degrees of the situp. Then the hip flexors take over, which are strong muscles with the function of bringing the legs toward the upper body, or the upper body toward the legs. So full situps don’t work the abs more than partial situps. However, there is no harm in doing them." Martina also notes that a stability ball is preferable to the hard floor: "The rectus abdominis muscle also works through about 15 degrees of spinal extension, which means that in order to get full benefit from your ab workout, you should avoid lying on a hard floor that prevents you from extending your spine. This is one reason the stability ball is a good ab training device."