I am not so sure that
people like Bruce lee, never explored its internal aspects
I have read a lot of what Bruce Lee wrote and it seems to me that the internal work that Bruce Lee did with martial art was more aimed towards the cultivation of realness in mind and body. While not an art of developing internal qi strength, this goes to the foundation of what we are all building on in tai chi, qi gong and other internal arts.
I would say that devotion to 'self actualisation' is just as internal as anything I can think of.
I think that this sums it up:
A musician must make music, an artist must paint, a poet must write,
if he is to be at peace with himself. What a man can be, he must be.
This is the need we may call self-actualization ... It refers to man's
desire for fulfillment, namely to the tendency for him to become
actually in what he is potentially: to become everything that one
is capable of becoming ...
[ Dr Abraham Maslow ]
If this is not internal work, then what is?
Anyways....maybe I am a bit off track with the thread here....but I am a huge fan of Bruce Lee....
It is curious to note the old sea-margins of human thought! Each subsiding century reveals some new mystery; we build where monsters used to hide themselves.
[Henry Wordsworth Longfellow]