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

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Answer this questions from the files
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?