Steve,
The fatigue mentioned comes from the constant work the brain does to keep us upright and walking balanced. With a healthy patient that has two hearing ears, two vestibular nerves, two good eyes, strong neck/feet they don't even THINK about staying upright and balanced when walking.
As a surgical patient with the vestibular nerve cut (this is where the tumor grows from) and SSD my balance is precarious at all times, but if you saw me walking around, running around you wouldn't even know it. If you were in my brain, boy would you run as it's very noisy in there working CONSTANTLY through the help of my eyes, one so-so ear, one so-so left over vestibular nerve, and proprioceptors in my neck/feet area trying to work together. Now with all that going on let's add everyday noise, conversations, and our jobs. We are truly fatigued and will remain fatigued.
Hope this helps you and by the way, WELCOME!