Any sufferers? Pretty curious phenomenon that I've been looking into that seems to be a bit of a blind spot in the fitness community. Combat sportspeople, runners, dancers, yogis, all sorts of people with active lifestyles you'd expect to have strong cores, and who in many cases have BF% of 12 or less, with defined 6-packs - report being unable to lift their upper halves off the floor without somebody holding their feet. Basically, what is thought of as a standard situp 'from scratch' - just lay down and do it, anywhere, as you would pushups or planks or squats or any other bodyweight/equipment-free calisthenics. The reason behind that isn't any great mystery - it's because this movement is controlled less by the abs than by your hip flexors, particularly the psoas. The point of bafflement is how some people's psoas get so disruptively weak & tight, even if they stretch regularly and have strong cores, and generally know what they're doing and perform the right exercises with proper form to avoid injury. It seems to be quite a widespread problem, with many anecdotal reports on bodybuilding websites, Reddit and Quora, etc. - but there's a surprising dearth of authoritative medical or sports science literature on the issue, at least that I can find published online. That is, science-based advice on preventative and curative measures for people inclined to be severely afflicted. I can sit in plank, correctly, for an entire Iron Maiden song, and then rip off a few dozen pushups (again, correctly ) and as many crunchs. Couldn't do a single sit-up, though. Not without my feet either being pinned or jerking wildly off the floor, and even then it feels like my weight distribution is such (top-heavy with my chest and shoulders having gained a deal of muscle with my recent strength training) that my hip flexors are impossibly over-taxed, and I can barely make much headway past a full crunch. Can now do a couple after doing some psoas wall holds and other flexion based exercises and stretches but, man ...uphill battle.