Im not sure if this qualifies as "internal"but there is the concept of "spirit training", as can be found in many different external martial arts. Will power, determination, grit, whatever you want to call it.
two different things.
internal power = mind control of body internal structures to do some sort of physical work -> in contrast with athleticism. for example, Chen Taichi internal, koryu internal, and so on.
internal power = will power, toughness, strength of spirit, don't quit, etc and etc. for example, Ghandhi, Mother Teresa, and so on.
you will find folks argue the two points above against each others. pretty much talking pass each others.