Miyamoto Musashi and "Internal" Swordsmanship
There are some direct (and some subtle) references that seem to point to some of the factors necessary for internal structure and power, in a description of proper posture and demeanor by legendary 17th-century swordsman Miyamoto Musashi,:
“The face is calm, neither turned upwards, downwards or to the side; the eyes slightly closed without movement of the eyeballs; the brow unwrinkled; the eyebrows slightly gathered, the bridge of the nose straight; the chin neither stuck out or drawn in too much; the nape of the neck equally straight and the spinal column full of energy. Below the dropped shoulders the body is perfectly relaxed; the spinal column is in place; the buttocks drawn in; the legs, from the knees to the ankles are firmly set on the ground; the hips are not twisted; the stomach is firmly shaped.”
Interesting to consider, at least.