True strength is an insane concept used in martial art storylines: such as movies, video games, and comic books. It really has no basis in reality.
A perfect example of this is Ryu from street fighter. He wanders the world over searching for true strength. Nothing else in life matters.
Mas Oyama searched for it in the mountain tops in a Korean movie called "Fighter in the Wind."
Daizaemon Kaze in the manga Gantz also searches for true strength.
The closest thing we have to a concept of true strength in reality is "he got heart," or "a true soldier."
Whitebelt: What is true strength?
Sempai: Beating Sub Zero in mortal kombat
Whitebelt: Hey I was serious
Sempai: Stupid questions get stupid answers