In sports for the longest time we studied the peripheral components of fatigue, from pure muscular fatigue to metabolic factors involved in fatigue. Although we long understood that the brain has a role in fatigue, only recently we started to look into the role of the brain and the nervous systems.
It seems we are looking at a very complex phenomenon here, dependent on many factors.
Of course, fatigue is an important performance factor in sports, but in many other fields, fatigue is important too. e.g. in ergonomics, but also in the military field and don’t forget the medical field. In clinical physical sense, fatigue can be a symptom for many different diseases, from anaemia to malignancy. But let us also not neglect mental fatigue like in chronic fatigue syndrome, in whiplash, in burnout and in depression.
It’s not my intention to write an extensive or complete book about fatigue instead of a blog, so I’ll have to skip a lot.
One of the early studies of fatigue has been done by Angelo Mosso in the end of the 19th century. Mosso studied fatigue with his Ergograph, measuring the effects of fatigue on the strength of repeated finger flexions.
But in his book he is did not neglect the central component of fatigue. He spends chapters of his book on “intellectual fatigue” and on “mental overpressure”.
In the end of the nineteenth and the beginning of the twentieth century, in Western Europe ( Great Britain, France, Italy and Germany) and the USA, fatigue was studied in order to improve the productivity of the laborers  in manufacturing plants.                Harvard University’s Graduate School of Business set up a Fatigue Laboratory in 1927.   But also to improve the toughness of soldiers. From the start of the Soviet-Union fatigue was studied for the same purposes. After the second world war Scandinavia became another source for the study of fatigue, but this time physical fitness and health, began to take important reasons as well.

Until now, many different possible factors of fatigue have been proposed : (in rather random order)
+decrease in metabolic substrates like:
-ATP and CP
+increase in metabolic breakdown products like:
-lactic acid and hydrogen-ions
-potassium-ions or inorganic phosphate
-reactive oxygen species
+increase in immune cytokines and inflammatory factors
+changes in calcium release
+changes in neuro-muscular junction e.g. acetyl-choline
+decrease in neural drive
+increase in core temperature
+ central factors:
-dysregulation of neurotransmitters e.g serotonin
-dominance of central inhibition processes
+fatigue cause by the autonomic nervous system
+fatigue cause by the adrenal system

This list isn’t complete, but it already shows us the complexity of fatigue.

But what do we use in daily practice: most of the time we start measuring the results of fatigue in training or competition like decrease in movement velocity, in strength or power output, increases in split time as the distance progresses, decrease in motor activity and efficiency,  impaired coordination and accuracy of movements.
We also found ways to detect singular fatigue-factors, which understandably will never cover the whole story. We tested for lactic acid, ammonia and neurotransmitters in blood. We took muscle biopsies and looked at glycogen. We measured EMG and looked at the shift in median frequency. We tested HRV, EEG and slow cortical potential. But the conclusion is that every specific activity has its own causes of fatigue, there is no singular, overall cause of fatigue.  Any factor can be useful, but only if it is related to this specific activity! Glycogen doesn’t tell you much about the decline in top speed at 100 meters. Lactic acid won’t tell you much if you walk or march 50 kilometers.

Yes, it is complex, but for me: complexity means opportunity…..

A few good basic books and articles:

  • Mosso, A: Fatigue:  Putnam, New York,1904.
  • Folbort, G.V.(Ed.): Problems of the physiology of the processes of fatigue and recovery; Kiev, 1958.
  • Schmidtke, H: Die Ermüdung; Huber, Bern, 1965.
  • Heiss, F; Franke, K.(Eds.): Die vorzeitig verbrauchte Mensch; Enke, Stuttgart, 1964.
  • FIR: Ermüdung und vorzeitiges Altern; Barth Verlag, Leipzig, 1973.
  • Viru, A: Defense reaction theory of  fatigue; Schweizer.Zeitschr. Sportmedizin; Vol.23, No.4, 1975, pg. 171-186.
  • Sologub, J.B: Elektroenzephalography im Sport; Barth Verlag, Leipzig, 1976.
  • Hoberman, J.M: Mortal Engines. Macmillan, New York, 1992.
  • Watanabe, Y; Evengard, B; Natelson, B.H; Jason, L.A; Kuratsune, H: Fatigue science for human health; Springer, Japan, 2008


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