For the last year and a half I have been ditching direct core work and instead only did deadlifts, squats, weighted dips and barbell rows. Now I tried doing some crunches and sie crunches out of boredom and noticed an instant incredible burn in my abdomen and spinal erectors as I was doing them. Not onset muscle soreness, but instant soreness. I was barely able to crank 10 crunches to each side before the burn became stinging and cramping. I learned a few things today, primarily that after years of deadlifts my hip strength is fine but my core strength... Well I'm weaker than most children on core strength. And on second point, that somehow the spinal erectors are involved in side crunches.
But HOW? Isn't the spine supposed to do the opposing movements of the abdomen?