Tim Noakes, a researcher and ultra-marathon runner, has authored many papers on a theory called, the Central Governor Model of fatigue. In this theory, fatigue is similar to thirst or hunger in that there is a monitor in the central nervous system that regulates the sensation (2). For example, hypoglycemia during exercise may elicit a fatigue feeling from the nervous system. By exposing the nervous system to hypoglycemia repeatedly, as in training for a marathon, the nervous system may lower its threshold for the fatigue alarm of hypoglycemia. (Perhaps this is also the explanation for the "Keto Flu" that occurs for first few days of ketogenic diet as the body gets used to a new set point for serum glucose.) This theory is in opposition to the Energy Depletion Model that was first proposed by cardiologist AV Hill (1):
"In 1923, Nobel Laureate Archibald V Hill developed the currently popular model of exercise fatigue. According to his understanding, fatigue develops in the exercising skeletal muscles when the heart is no longer able to produce a cardiac output which is sufficient to cover the exercising muscles' increased demands for oxygen. This causes skeletal muscle anaerobiosis (lack of oxygen) leading to lactic acidosis. The lactic acid so produced then ‘poisons’ the muscles, impairing their function and causing all the symptoms we recognise as ‘fatigue’."
Summarizing Noakes theory, there are 2 types of fatigue - one is peripheral, so that when you are lifting weights at some point you are fatigued and muscle can't go further. Then there is second type which is central and relates to the exhaustion or hitting the wall when doing endurance exercise. In the article that you can link to below it talks about "pacing" as a factor in the experience of fatigue (1):
"...complex brain mechanisms are able to determine the physical state of the body on a moment to-moment basis and to adjust the work output specifically to insure that exercise can be conducted safely without the development of a catastrophic biological failure."
(Figures from source 2.)
My interpretation is that it seems that the purpose of the exercise and the confidence in the exerciser is an important element leading to success or exhaustion. There is interesting evidence for both the Energetic Depletion Model and the Central Governor Theory of fatigue.
Source (1):
Noakes, T.D., 2012. The Central Governor Model in 2012: eight new papers deepen our understanding of the regulation of human exercise performance. British Journal of Sports Medicine 46, 1–3. https://doi.org/10.1136/bjsports-2011-090811
https://bjsm.bmj.com/content/bjsports/46/1/1.full.pdf
(2) Noakes, T.D., 2012. Fatigue is a Brain-Derived Emotion that Regulates the Exercise Behavior to Ensure the Protection of Whole Body Homeostasis. Front Physiol 3, 82. https://doi.org/10.3389/fphys.2012.00082
https://www-ncbi-nlm-nih-gov.revproxy.brown.edu/pmc/articles/PMC3323922/?tool=pubmed
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