Here are some alternative exercises to the traditional situp.
The first alternative exercise trains eccentric deceleration of the spine into extension. There is a progression that you must follow as you can see below.
This exercise is to train the movement and to establish core stabilization especially at the transition at the top of the movement. It is important to completely stand upright and initiate the movement bracing the core to prevent movement from the spine, it should be all hip.
The second exercise demonstrates the progression into the more dynamic version of the exercise using a band where you must brace on the rebound to prevent extension. The trick is to brace at the very upright position bracing at the last second.
So now you have an alternative you have no more excuses for doing situps or crunches. Next time I will cover another alternative exercise that trains more of the isometric function mentioned earlier.