When he performs his swordplay, to the eyes of others there is no flaw in his movement. But to make sure that there is no flaw, Li Xian had to use his internal energy to compensate the other parts.
Internal energy does strengthen the body but it’s effect is during the duration of the internal energy applied on the body
And that is the only time it strengthens the body
For example, someone hit his back with a piece of wood.
At the time he senses it, Li Xian would activate his inner energy.
And because he is proficient in the use of inner energy, Li Xian could direct his inner energy at his back, minimizing the usage of his inner energy.
When the piece of wood hit, Li Xian would feel at most a slight discomfort and then he could attack.
It is different however if he practices his body
