https://www.acefitness.org/getfit/studi ... rcises.pdf
This study appears to compare several different Ab Exercises to the lowly crunch in terms of % of muscle activation measured via EMG.
I've heard Rob recommend the hanging leg raise several times. This correlates to the findings in this study (similar to the Captain's Chair.) One huge advantage the hanging leg raise/Captain's Chair has over the Bicycle Manoeuvre is the ability to easily add progressive overload in the form of increasing weight held between the feet."Topping the list of most effective exercises was the bicycle manoeuvre, followed closely by the second ranked, Captain's Chair."