My brother does a lot of competitive endurance type sports (triathlons, adventure racing). He sent me this interesting article about muscle fatigue and how it can be an “emotion” rather than a specific physical thing.
Isn’t the body amazing? It sense when we are at the limit of some part of the system and creates the signals needs to that the whole organism will slow down. Of course, people can (and often do) override those signals which just gets us into trouble.