My core strength seems...odd. Please help I seem to have plenty of core strength but my lower abs are sorely lacking.  I can't do L-sits, I can't do full sit-ups without having something hold my feet down (just crunches), but doing roman chair situps with 70lb on my chest or squatting 3-plates is not difficult for me.  From what I've read this seems to indicate that my lower abs are weak compared to my upper abs.  Is this accurate?