Sit ups have been listed in one of the more dangerous exercises to perform, yet is included in the National Fitness Tests for high school students, along with push ups, sprints, and the mile run. The exercise in question causes the vertebra to squeeze the discs and place pressure on the fluid inside of them, which can lead to serious herniated disc injury. In addtion, there is no functional fitness to sit ups; no average person is going to need that movement strength. A recommended replacement for sit ups could be the plank pose, timing how long it can be held.
http://www.thedailybeast.com/newsweek/blogs/the-human-condition/2009/06/...