The Kalenjin, who occupy much of the Rift Valley, are Kenya’s third largest tribe. Despite only making up approximately 12% of Kenya’s population, they represent more than 75% of their country’s top runners. If you think Kenyan running couldn’t be more impressive, think Kalenjin running.

To put Kalenjin running success into perspective. Consider that 17 American men in history have run faster than 2:10 in the marathon. 32 Kalenjin men did that last October…” – David Epstein, 2014.

There are many physiological factors that are believed to influence elite Kenyan runners’ success, from specialized morphological features to high aerobic capacities. More recently, a new physiological mechanism was brought to light, which could be another important underlining factor for their unparalleled success.

A study, conducted by Jordan Santos-Concejero and colleagues, looked into the cerebral oxygenation (oxygen reaching the brain) of a group of elite Kalejin Kenyan runners during a maximal self-paced 5km time trial. Previous studies have shown that a decline in cerebral oxygenation corresponds with a declined performance output. This happens because a reduced oxygen delivery compromises the ability of the cerebral cortex to recruit muscle, which slows running speeds.

Santos-Concejero et al. (2014) found that the group of 15 Kalenjins were able to maintain cerebral oxygenation within stable ranges over the course of the 5km time trial, which is favorable in reducing the onset of central fatigue. These findings were significant in that the cerebral oxygenation response differed from previous studies which tested elite European runners. Santos-Concejero concluded that: “elite Kenyan athletes have greater brain oxygenation during periods of maximum physical effort, which contributes to their success in long-distance races”.

Protection mechanism

Our central nervous system,  specifically our brain , plays an important regulatory (and anticipatory) role in exercise, preventing the body from consciously harming itself. Its main mechanism of regulating the body’s exertion is by moderating the amount of muscle which is recruited to work. In the case of down regulating exercise, by simply recruiting less muscle, the brain immediately amends parameters which set off alarms. For example, decreased muscle recruitment = less muscle work (which results in slower running speeds) = less heat production, more oxygen available for brain and heart, glycogen sparing etc.

Our bodies have receptors which monitor all kinds of parameters, as with the 3 above-mentioned. Their duty is to ensure that our body’s parameters are kept within their healthy (life-sustaining) range. These receptors act as our bodies ‘alarm system’ and are triggered to ‘go-off’ if systems appear to be disturbed. Consequently, they signal our central nervous system to respond to particular disruption. If this system wasn’t in place, exercise could be catastrophic, as there would be very little preventing our bodies from severely violating homeostasis.

Both homeostasis and the minimizing of energy expenditure are, from a scientific perspective, the 2 most important biological goals of our body. These 2 characteristics are likely evolutionary adaptations that predispose us to a state of biochemical laziness and which once allowed our ancestors a more favorable chance at survival.

Therefore, for training to lead to adaptations (that correlate to improved performance), we need to disrupt our body’s state of homeostasis (which it craves) to a particular magnitude/frequency. If we present our body with a stressor, it’s in the body best interest to become more capable of handling the same stress in the future, so that homeostasis is more easily maintained. However, in doing so, we’re required to expend energy (going against the  biological goal) to build a body with greater energetic needs. This is a large factor as to why our body detrains and ‘undoes’ previously built adaptations when we don’t train, since there’s no incentive as to why our body should maintain ‘unused machinery’ (adaptations) with a costly energetic need.

Possible explanation for high cerebral oxygenation

There are several possible causes for this, the strongest explanation being prenatal exposure to high altitude conditions.

High altitude is associated with a lower oxygen partial pressure, which subsequently results in blood being less saturated with oxygen than what it would be at sea-level. To compensate for the decreased oxygen presence per oxygen carrier, the fetus receives increased blood flow, which is a protective measure to ensure that the fetus’ oxygen needs are cared for. It is believed that an early exposure to increased blood flow could, according to the researchers, result in: “increased cardiopulmonary capacity [in an athlete’s] adulthood and consequently less incidence of arterial desaturation.”

Though the effect of training on improving cerebral oxygenation is not well-known, childhood exercise could also be a notable factor. The researchers note that a high exposure to exercise throughout childhood could “stimulate  trophic (hormonal) factors and neuronal growth”, and increase blood circulation in the brain through “vascularization of the brain”.

Further reading:

We recommend reading Ross Tucker’s article:  The Kenyan success genetic controversy, which talks about whether superior running ability is unique or just more frequent among Kenyans.


Billaut, F., Davis, J.M., Smith, K.J., Marino, F.E. and Noakes, T.D., ‘Cerebral Oxygenation Decreases But Does Not Impair Performance During Self-Paced, Strenuous Exercise’ (2010) 198 Acta Physiologica

Fan, J. and Kayser, B., ‘Fatigue And Exhaustion In Hypoxia: The Role Of Cerebral Oxygenation’ (2016) 17 High Altitude Medicine & Biology

Santos-Concejero, J., Billaut, J.F., Grobler, L., Oliván. J.,  Noakes T. D. and Tucker R., ‘Maintained Cerebral Oxygenation During Maximal Self-Paced Exercise In Elite Kenyan Runners’ (2014) 118 Journal of Applied Physiology.

Santos-Concejero, J., Billaut, J.F., Grobler, L., Oliván. J.,  Noakes T. D. and Tucker R., ‘Brain Oxygenation Declines In Elite Kenyan Runners During A Maximal Interval Training Session’ (2017) 117 European Journal of Applied Physiology.


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