Central fatigue

Although fatigue is common and everyone knows what it is, it is difficult to define. One definition is the inability to maintain the required power output . Fatigue can be divided into two classes, peripheral and central. Peripheral fatigue arises within the muscle whereas central fatigue arises within the brain or the motor nerves. Although fatigue affects most people at some time in their life, there is no acceptable biochemical mechanism(s) to explain it. There are, however, several hypotheses. 

Overcoming or reducing central fatigue may be part of the basis of what sports psychologists recognise as the will to win . 

Events within the brain can limit power output, a phenomenon known as central fatigue. It is demonstrated experimentally when the maximal effort that can be achieved voluntarily is less than that which... 

Three hypotheses have been put forward to account for central fatigue the hypoglycaemic hypothesis the dopamine hypothesis and the 5-hydroxytryptamine hypothesis. 

Figure 13.27 A possible mechanism by which a low blood glucose level could give rise to central fatigue. A low blood glucose level reduces the rate of glucose utilisation in the brain which decreases the ATP/ADP concentration ratio in the presunaptic neurone. This reduces the energy available for synthesis of neurotransmitters, packaging of neurotransmitter molecules into vesicles and exocytosis of neurotransmitter into synaptic cleft. This decreases electrical activity in postsynaptic neurones and hence in the motor pathway.

On the basis of the proposed mechanisms for peripheral fatigue and for central fatigue, it is possible to combine the mechanisms to provide an overall summary of all the... 

Figure 13.29 A diagram summarising three possible mechanisms for central fatigue. It is possible that all three mechanisms could occur simultaneously, giving rise to severe fatigue.

Figure 13.30 A summaiy diagram illustrating the mechanisms by which central and peripheral fatigue can cause fatigue. Central fatigue could be caused by changes in concentration of blood glucose, 5-HT or lumaine concn. in presynaptic neurones.

Fatigue accompanies viral infection, sepsis, trauma or major surgery. The cause of this fatigue is not known it may be peripheral, central or both. Studies on biopsy samples of patients with trauma show a reduction in the muscle ATP concentration, which could be responsible for peripheral fatigue, as explained above (Chapter 18 Table 13.3). Central fatigue has been identified in three different clinical conditions, post-polio syndrome, multiple sclerosis and after spinal cord injury, but has not been investigated in other conditions. 

Yamamoto, T., Castell, L. M., Botella, J., Powell, FT, Flail, G. M., Young, A., and Newsholme, E. A., Changes in the albumin binding of tryptophan during postoperative recovery a possible link with central fatigue Brain Res. Bull., 43, 43, 1997. 

Fatigue (defined as a decline in force following repeated contractions) can occur as a result of a failure to activate motoneurons (commonly referred to as central fatigue) or from failiire in neuromuscular transmission, excitation-contraction coupling, or from the mismatching of energy... 

The central fatigue hypothesis also predicts that ingestion of BCAAs will raise the plasma BCAA concentration, and hence reduce transport of fTRP into the brain. Subsequent reduced formation of serotonin may alleviate sensations of fatigue (Figure 13.5b) and, in turn, improve endurance exercise performance. 

If the central fatigue hypothesis is correct and the ingestion of BCAAs reduces the exercise-induced increase of brain fTRP uptake and thereby delays fatigue, the opposite must also be true that is, ingestion of tryptophan before exCTdse should reduce the time to exhaustion. A few studies have included supplemental tryptophan... 

Blomstrand E, Perrett D, Parry-Billings M, Newsholme EA. Effect of sustained exercise on plasma amino acid concentrations and on 5-hydroxytryptamine metabolism in six different brain regions in the rat. Acta Physiol Scand 1989 136 473 181. Blomstrand E. Amino acids and central fatigue. Amino Acids 2001 20 25-34. Fernstrom JD, Fernstrom MH. Exercise, serum free tryptophan, and central fatigue. J Nutr 2006 136 553S-559S. 

Saldanha A, Ekblom N, and Thorstensson A (2008) Central fatigue affects plantar flexor strength afta- prolonged running. Scandinavian J Med Sci in Sports 18(3) 383-388. 

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