Every muscle requires an impulse from the central nervous system for contraction. These stimuli determine how many muscle fibers are activated. You will be surprised to hear that the current state-of-knowledge has evidence that about 20% of the muscle is activated in the stage of high strain. The muscles need stimuli from the central nervous system to move. With the feeling of fatigue and exhaustion emerging a decline of the frequency of impulses to the muscles goes along. Exhaustion and fatigue are thus triggered by the central nervous system. Work-outs intensify and increase the flux of controlled neural muscular impulses.
A fatigued mind brings about a performance decline
It is proven that the central nervous drive decreases when the muscle grows tired. This condition seems not related to an acidic situation or empty carbohydrate stores. If the mind/brain is tired, the muscle will be tired soon too. Another strong indication for the central nervous system being involved in the development of exhaustion and fatigue is the fact that less muscle fibers are recruited by the central nervous system at high altitude compared to sea level. This so-called central fatigue kicks in before a lethal situation may occur in the organs. In all the other models it is a peripheral fatigue that determines the performance limit. But data tells us that in the situation of breaking off performance no peripheral organ, neither heart nor muscle nor liver etc. have reached their ultimate limits. Obviously, we have safety margins to protect the heart, to ensure the energy (ATP) supply and generally hold the body in balance. Staying focused during a whole training session brings about more resistance towards exhaustion as the neural muscular pathways are better trained.
Measures taken from athletes after strenuous exercise may resemble those of a disease condition
Looking at measured parameters alone can be rather misleading. There is a dreadful and in many times lethal condition called SIRS (systemic inflammatory response syndrome).Two or more of the following criteria must be fulfilled to diagnose a SIRS: body temperature over 38°C, heart rate over 90/min, breathing rat over 20/min, leukocytes over 1200/nl or under 4000, immature leukocytes over 10%. You will find the same measures in a marathon runner after a competition without suffering from this condition and then being the least bit life threatening. You can see how important the context of the measurement is and that values can be a surrogate of many components. Intact regulatory processes underneath the surface of the measurements are obviously really crucial for peak performances and not the measurement itself.
Are there central switches turning on fatigue?
There is surely a whole complex network of central and peripheral circuits that ensures that no organ or organ system is driven to its lethal boundary. Therefore the performance limit is a computational result out of many parameters, of many of which we may not even be aware of yet. It is not merely a final result but a constantly performed computational process. Training modulates these processes and increases the adaptability of the involved circuits towards safe boundaries.
Having read these chapters you will have noticed that it is very difficult to decide on one single training concept. All the facts lead us back to the beginning and the realization that there is a bit of truth in all of it. To improve endurance performance and peak at a special moment in time is a tricky job and the mission of training. More and more coaches, recommend listening to your inner voice and find out what it is like to feel good and then to train according to that feeling.
Chris McCormack once pointed that out to me. He said: »The body is a smart thing. And it is one body, everything is related to everything, people tend to forget this.« That is why there is not only one correct way of reaching a goal. To feel good is difficult. The feeling cannot be described with a heart rate monitor or stuffed in with carbohydrates. It is the art of finding out in the training which mixture your body needs to find that well-being which leads to an outstanding performance.