Core strength from heavy barbell movements like front squats and deadlifts without isolation.

All right, that's really what I think it is. I really think it's just bracing the core for those big lifts. I know there's a lot of debate people say you have to train your core, weighted. I'm in the camp where if you're doing heavy, lower body, barbell movements, there's no way your core is not going to be trained if you're bracing properly, honestly. There's no way. There's no way. You can't do front squats, heavy front squats for reps without a strong core. I mean, you just can't. I've done front squats and had sore abs the next day.