How we can measure fatigue using EMG signals? in what domain (time/frequency) do you take measurement; and why?

1)How does muscle fatigue happens from physiological stand point ?

2. How we can measure fatigue using EMG signals? in what domain (time/frequency) do you take measurement; and why?

3. What are the major criteria used in the literature to measure fatigue? Which one(s) are more reliable? which one(s) are more common?