Tried decline sit ups and if aim is hip I think is more risk then reward.
Putting weight behind head rotating shoulders already puts your back at the natural curve and then lowering back even when contracting abs is not enough to counter the weight at that point. Like a see saw.
E.g let's say weight is 5kg and then the weight is multiplied by the distance to the Lower back which is already at a natural curve will put alot of pivotal force on the spine and causing major hyperextension of the back and load.
Very dangerous, not seeing any reward for hip flexor, could be better achieved through other means.
I tried 5kg, as soon as I start lowering contracting the ab, the upper back has no musculature that can support it with the lower back like the abs, my upper body continued lowering while my lower torso was maintaining angle and therefore rounding of the lower back and then I stopped. At 2.5kg I could lower myself but couldn't stop my lower back from curving even then. Even at bw back curves. It's just physiologicaly not possible to prevent lower back curving.
I need to find a way to improve elasticity of hip flexor from a quick snap action from full flexion bringing knees up and then once fully flexed bring back down in a quick motion. Just that conscious effort to do it slows it down a little, else sacrifice rom to get back and that is a fail