Weighted situps of any kind, but especially on a decline, work the psoas (hip flexor) muscles through much of the range of motion. There are certainly more ideal methods for training the abs through flexion.
Secondly, flexion is not the only movement that should be used to train your abdominals. Your abs also control lateral movement and rotation. But the main function of the abdominals (and I’m talking internal obliques, external obliques, rectius abdominus and transverse abdominus) is most likely stability of the spine rather than any type of mobility. All the above mentioned muscle groups have muscle fibers that run in different directions in order to provide stability. Which is why heavily loaded exercises that force stability of the abs (ie. heavy squats) can be so fatiguing. Certainly the abs do contribute to mobility as well, so that form of training should not be overlooked, but it is not the only function, either.
Hope that all makes sense.
Well then tell us the most effective way to train your abs without working those damn hip flexors. You don’t have to say squat, press, or deadlift because we all already do those.[/quote]
I would say good abdominal exercises that minimize hip flexor action would be kneeling cable crunches, planks and other isometric holds, russian chair twists and woodchoppers to name a few. They do not eliminate hip flexor action, but they certainly minimize it more than sit-up variations.
And I hope you didn’t interpret my post as cutting on you, because that was not my intention.