Found this an interesting read on the models of fatigue in prolonged endurance cycling.
https://www.researchgate.net/profile/Paul-Laursen/publication/7583082_Models_to_Explain_Fatigue_during_Prolonged_Endurance_Cycling/links/0c960529d44685177c000000/Models-to-Explain-Fatigue-during-Prolonged-Endurance-Cycling.pdf?origin=publication_detail