http://www.exrx.net/WeightExercises/Rec ... itUpX.html
http://www.exrx.net/WeightExercises/Hip ... Situp.html
Could someone writing articles for exrx help to clarify the difference in terminology here?
So far as I can tell, incline/decline is judged more upon the angle of the upper leg, and not the actual angle of the table your body rests on? That things termed 'incline' begin with flexed hips whereas things termed 'decline' begin with extended hips?
Both seem to be okay with hooking the feet under an anchor. Both seem to be okay having flexed knees... personally I sort of like straight-legged situps too. I feel like, sure, one might be more strenuous, but that once you're doing flexed-knee with high weight maybe you could start learning extended-knee ones too!
All in all though, it seems like there might be some naming ambiguity going on here, and that there's really only one name to give to the exercise, and that it needs resolution. A disambiguization page to explain the naming problems on the exercise would be very useful to add for confused persons.
So far as I know, so long as a segment of the spine anterior to the one below it is 'behind' it, the spinal flexors will be engaged to bring it on 'top' of it, and if you go further then that to bring it in front, then you'll be eccentrically lengthening your back muscles (erector spinae) to support that descent, and using them concentrically to get back to neutral posture.
As for hip flexors it's basically the same thing, so long as the spine is behind your ass, you'll work hip flexors to get it on top, and if goes past your ass, you'll be working your hip extensors (glutes, hams) to get it back up there.
So really, there should be one criteria for the neutral exercise, and then incline/decline discussing alteration of these angles. One would make it more strenuous, the other would make it easier. Difficulty meaning for the hip and spine flexors. An exercise can still be quite strenuous, but for other muscles (hip extensors, erector spinae/spine flexors) which the exercise is not designed for. Their contraction actually helping the targetted muscles relax and thus incredibly 'easy'. Essentially the alteration from a situp to a 'glute-ham raise' or a back extension. Very similar motion and simply regarding how gravity is attacking our moving through them due to our positioning.
At the culmination of maximal hip flexion (thighs against chest), the most difficult exercise being if the upper thigh is facing the ground, the easiest being when the upper thigh is facing the air.
At maximal hip extension (muscles in a lengthened position) the most difficult being if the upper thigh faces the air and most easiest being if the upper thigh faces the ground.
For partial-range, if the upper thigh faces a line perpendicular to the ground is harder, though attaining maximal hip flexion is still quite hard due to tightness in the hip extensors and over-active insufficiency in the muscles, though I still like the idea of attempting that over-active status, like maybe it won't be as insufficient if you do it a lot.
Spine flexion is much more complex so basically using similar thought process given from the example of hip flexion you can sort of draw images of the force curve too.