Do you also have your arm extended up in the air, or bent and on your hip? (Obviously the arm that is not supporting you.) Just wait until you get to isometrics where you hold that leg up, ha ha.
It always amazes me on side planks how the dominant side is such a big factor - I'm much stronger on side planks with the raised leg when I'm on my dominant side. I suppose there is still a lot of leg strength involved, and not just core.
Those Good Mornings are tough for those of us with super-tight hamstrings. It's just a bow, sure, but if your hamstrings don't like to stretch, then it turns into a bit of a curved back instead of the hip hinge.
Glad you think the core routine is helping out!
