Why Do Cyclists Fatigue During a Hard Rides And Races?

Here's a crash course on why cyclists become fatigued during training and races, with a heightened view on ‘central fatigue’ (Tornero-Aguilera et al., 2022):

1) Fatigue is the impairment of several physiological processes that are responsible for muscle contraction. I.e. a disruption of homeostasis.

2) The central nervous system (CNS) becomes fatigued through Central Fatigue, which originates in the brain. This is most likely explained by the biochemical changes in the brain during exercise. Specifically, the changes in serotonin, GABA, glutamate, dopamine and a reduction in glucose availability.

3)Serotonin increases lethargy and tiredness and reduces neural drive. During exercise, the amino acid tryptophan crosses the blood-brain barrier, then converted to serotonin, thereby increasing fatigue.

4) Notably, under fatigue, the branch chain amino acid (BCAA)/Tryptophan ratio decreases, favouring tryptophan to cross the blood-brain barrier. This raises the thought that increasing BCAA levels through the diet may compete with tryptophan’s entry into the brain, decreasing serotonin production and reducing fatigue.

5) Dopamine, a neurotransmitter that delays fatigue and exhaustion, also plays a pivotal role in mental and central fatigue management. The amino acid Tyrosine crosses the blood-brain barrier, where it’s used for dopamine production. Therefore, does this highlight dietary Tyrosine considerations?

6) Mental performance can decrease through the accumulation of the neurotransmitter adenosine, therefore increasing perceived effort and impairing motivation. Most likely through reducing the release of dopamine. Caffeine is an adenosine-antagonist and combats mental fatigue by blocking adenosine’s interaction with the CNS.

7) Fatigue also comes from the peripheral nervous system. This is known as peripheral fatigue, which is localised to the muscle and is primarily caused by metabolic changes. I.e. glycogen depletion, dehydration, acidic environment etc.

Additional factors beyond that of exercise further increase CNS fatigue, such as sleep deprivation and psychological strain.

- Coach Chris