Strength isnt always measured by how big your biceps are, or by how much guts, or balls, you have, fact of the matter is, the things that seperate the men from the boys are just something that you can sense, that you can feel, that you know are there, theres just a barrier, something which guys break from, that they push through, there is no word or expression for it.
If I was to put my finger on an answer, Id say integrity, branch of courage.
I say courage this time simply because the question calls for a different answer. This isn't quite the same question, in my opinion. Anyway, I believe courage/determination is the core attribute, but you need muscles to back it up or it's just a show.