I don’t know much about the standards of E-IQ other than the global definition of recognizing awareness of self and others, the ability to control and make use of emotions and apply them to solving problems, and the ability to manage and regulate self and apply to others to encourage or calm.
That said, I wonder if there can be a static (single) definition of strength more than a vision of what is attainable in the given moment… you never step into the same river twice.
For me, strength is found in the simplicity of the daily Eightfold Path:
Now, within that strength is an incredible value of self, because bad things can happen to good people and when it does, the foundation of calm so that those things strengthen you with the stamina being mindful brings. It defines you with the good baggage leaving the bad behind, or destroys that particular portion of you weakening the infrastructure to allow doubt of one's endurance to persevere.
spoke or managing your own impulses while recognizing others attempting, with attempting navigation for a healthy outcome... an everybody wins result and the most favourable by far, but there will be times when one simply cannot maneuver these tasks, conditions, and standards into a win-win or even a quid pro quo where both humbly agree to disagree, and that is where the seed of strength grows or withers.
Desire of any kind is an interesting exercise, often resulting in a destabilization of some form or another. That destabilization can be positive, like the breaking down tissue to build muscle in an emotional sense, or it can be negative and lead you in an emotional game of "crack the whip". It's all in the warm-up and stretching in the beginning that insures an injury-free exchange.
So, still speaking of desire, eventually one can attain such mindfulness that when such emotions come at you, the congruence of the mind and heart are so in tune that it's not that you have to dodge them or take the hit and withstand it... the ability will be that you simply part the waves of them where you can observe as they move past but few, if any, touch.
That is one avenue of how I define strength, or at least the first that comes to mind.