Aside from causing lordosis, research has proven that leg raises and sit-ups are not effective as abdominal trimmers or developers.
I have always maintained that arching the back causes the work to be done by the back muscles (Psoas Major) rather than the abdominal muscles (Rectus Abdominus). Because an exercise is traditional does not mean that it is good. Many exercises were developed as far back as the 18th century. With no scientific knowledge behind them! They just seemed good for the particular muscle.
Toe raises, for instance, performed where you raise out on the toes instead of up on the big toe and ball of the foot with the heels coming together produce not only a bow-legged look, but stretches the structures of the feet and weakens the arches.
(IronMan Magazine Nov 1973 Vol. 33 No. 1)