The answer to Abs is not sit ups.
My view on working and building abdominal muscle should come from a variety of movement patterns that create a need for stabilisation.
The sit up, for example, does have abdominal activation but can be taken over by momentum, also hip flexer muscles and lower back can take over creating a lower back issue.
By doing exercises that create an imbalance and the abdominal muscles having to regain balance, will create a more optimal result, for example - weighted Russian Twists, weighted cable crunch (good form essential), lying bicycle crunch and a bench 'V' sit.
If anyone else can think of any further good exercises in this category please post them for further discussion.