The tragedy of life is what dies inside a man while he lives. (Albert Schweitzer)
If you use the thesaurus and try to find another word for 'middle', you will come across 'dead centre'. Dead centre, according to the Oxford Dictionary, either refers to: exactly in the centre, or to the position of a crank when it is in line with the connecting rod and not exerting torque.
When there is no rotation, no movement, no motion, there is nothing, zip nada. When there is nothing going on, only emptiness, that is Death. It is the opposite of Life.
This is all still a bit vague but fortunately there is an old song that explains it better than a thousand words could ever do. Just click under the jukebox below and listen.