Model of the central regulator Why is it that at some point the brain gives the muscles a command to

Model of the central regulator Why is it that at some point the brain gives the muscles a command to stop or slow down the movement? Does the brain itself become tired or do some factors lead to fatigue developing in it? How does he know that you need to get more fiber into your work, put pressure on and make the final breakthrough? How does the brain determine that the number of activated muscle fibers is enough? How does our brain pick the right tempo when running "on sensations"? Many questions are not answered, but the meaning of the brain reaction, which leads to the formation of "fatigue", is understandable. There is evidence that the brain gives the signal to stop or slow down to protect the body ... from ourselves and from those irreversible damage that (as the brain thinks) we can inflict on the body. Every second and in any situation, the brain conducts a check of all organs and systems of the body. 

Obviously, if the percentage of muscles involved in the work increases as fatigue accumulates, then the "classical" theory is correct. This simple method was used by scientists to find out whether the muscle is a source of fatigue. What happened? According to the results of the conducted studies, it was not found involvement of a large volume of muscle mass during training, as well as 100% involvement of muscle fibers in the work with high-intensity training.