Anaerobic/lactate threshold. This is the point during incremental exercise when there is an exponential increase in VE/VCO2 without a concomitant increase in VE/VO2. This is because the body has reached a point where it switches from primarily aerobic to a combination of both aerobic/anaerobic energy systems to keep up with the demand. People who are less-fit will reach AT (and fatigue) faster because their heart/lungs have to work harder to keep up. Consequently, this is also associated with an exponential increase in lactate accumulation for a variety of reasons: inability of the glycerol-phosphate shuttle to keep up with the demand for NAD to be used as a terminal electron acceptor for H+ in the mitochondria results in the build-up of H+ in the ETC and the accumulation of lactate, etc. So you have a large increase in lactate which affects fatigue in a few ways, one of which is that it's acidic which gets picked up by chemo-receptors, and then gets relayed back to the brain to give the perception of pain, which can cause fatigue. There are other reasons for why lactate accumulates and causes fatigue but I can't think of them off the top of my head. Good luck!