As I suspected my core and back are sore now from Kenpo yesterday. It's in a good way though. When you really pushed and moved your punched by driving your hips/core, it's a good core exercise as well...can't agree more. Besides like Tony said, core strength is essential to hold your posture and balance.
Yesterday, my core was still a bit sore from Core syn and to top it off, I pushed harder in Yoga Belly 7.
Anyway, I enjoyed my extra sleep this morning for sure.