I agree that lactate accumulatoion is assosiated with fatigue. But that does not mean that lactate causes fatigue. The association comes from the fact that the higher the excercise intensity, the more lactate you produce to help keep the pace going.
If that´s the lactate that causes fatigue i don´t know.
But don´t you knoww that the lactate keeps going move up after the effort is done ? If the reason that the lactate moves up is to help keep the pace going why the lactate continues to go higer after that you finish the run for minutes after and sometimes hours after that you stop the intense activity and you go home and stand and rest ? That´s not to fight the fatigue, what fatigue and what pace going ?