As a martial term of the art, "內勁"has a pretty specific meaning of internal strength. The term internal strength when applied to martial arts in general has a fairly specific context in which it is used and which has existed as a term of the art for several centuries (at least according to Wikipedia).
As pointed out earlier, Phillip is referring to a different concept of which we have other more appropriate words in the english language to describe. If Phillip had selected a different word then perhaps this discussion would be in a different sub forum.
My Pop use to refer to it as "mental toughness" and "instestinal fortitude."