A group of privates will snap into "attention" at the word of command with splendid muscular control; the same number of officers would find great difficulty in doing this.
Now as the man loses muscular control he loses poise and carriage.

His head rolls about in a slack way on his neck, and has a tendency to drop forward; the muscles of the neck and the upper part of the back grow soft from lack of use and control and he begins to become round-shouldered; his chest falls in as the shoulders come forward and the chest cavity is reduced.

This means a gradual cramping of lungs, heart, and stomach.
By way of compensation he lets out a hole or two in his belt and starts in to carry more weight there.

In other words, he exchanges muscle for fat, and as the fat increases he has less and less muscular strength to carry it.

It is as though in a motor-car one added hundreds of pounds of weight to the body and reduced the horse-power of the engine.