Why during a prolonged effort are we forced to slow down after a certain period of time? 
Why after 3, 4 or 5 hours on the bike are we not able to repeat the same intensities o the first 2 hours, notwithstanding the regular and proper CHO intakes to keep a constant glycemia? 

It’s been demonstrated that muscular glycogen exhaustion compels a drastic reduction in the intensity of the effort, but it is also true that the administration of caffeine (J.Appl.Physiol. 1991;71:2292-2298) or other stimulants of the central nervous system allows a sensible delay in the rising of fatigue, and an extension for the intensity of the effort. 

It seems that caffeine has a direct effect on the brain, reducing the perception of fatigue, while it seems that it does not accelerate fats metabolism, with a consequent saving in glycogen, as recently supposed. 

Therefore the brain acts as central governor for the sensation of fatigue, before the complete exhaustion of glycogen stores, with a preventive/protective action in order to avoid any excess and possible damage. 

In a similar fashion the brain reacts to hyperthermia to prevent heat strokes, or to hypoglycemia to avoid any damage to its own functionality, activating the signal of FATIGUE. 

The action of the brain as central governor reduces the amount of recruited muscle fibers, with a consequent drop in strength and effort intensity. 

It is probable that great athletes, besides being gifted with great peripheral capabilities (muscular, cardio-vascular, pulmonary, efficiency of gesture etc.), have a “reduced” inhibitory central function, being capable to hold up the coming of the perception of fatigue, moving further up their own limits.