I'm not using fatigue to describe many things, that is what fatigue 'is'. Fatigue is any accumulative effects that lower function or performance. The specific reasons for that fatigue can be different and different combinations of various things. Example, force loss with very heavy loads and low reps is partially due to the Pcr supply running low. a loss of 'fuel' is a form of fatigue. The only reason our muscles even recruit more fibers and increase rate coding during a set is due to fatigue. The only reason you can't do your 1Rm more than once is due to fatigue Usually I see people using fatigue to decribe metabolic issues rather than neural though, and I really wasn't point to failure as the key. Just that 'fatigue' in general is what moves a muscle to a point where the lacking performance signals a need for adaptations. It's that way in all systems, a callus is formed from mechanical fatigue damaging skin cells and stimulating it, etc. Fatigue is a general 'thing' : Muscle fatigue is a decreased capacity to perform a physical action. Fatigue causes a decline in performance and a decrease in your ability to exert force.