One must galvanize both his body and soul in intense concentration while praying.
Physical exertion is not necessarily expressed through physical movement.
It is told that the prayers of the Rebbe of Ruzhin were not particularly lengthy, and that there was no noticeable change in his demeanor while praying.
Yet upon concluding his prayers, he would need to change all his clothes, as they had become soaked in sweat.
The onlooker observed no extra effort or unusual movements on the part of the Rebbe of Ruzhin as he prayed.
Despite this, it was clear that his body expended tremendous energy.
It is quite possible that specifically the externally undetectable “movement” is far more difficult and grueling than any external action.
Rabbi Adin Steinsaltz