Core activation refers to how the nervous system coordinates your posture, breathing and muscles to produce an efficient movement. Core activation ≠ core strength. Now you may be able to hold a plank for 2 minutes or more - well done. But you may be using a less than optimal strategy, such as posteriorly tilting your pelvis to brace with your surface back muscles. Or you might be holding your breath. Generally, the trunk, spine, and pelvis should remain relatively neutral during many core exercises. Core strength can be developed when you have optimal core activation- and the most effective core exercise is the one that you can perform with good alignment and breathing.