You’re Not Really Tired – It’s All in Your Head! A New Look at Running Fatigue


The next time you start gasping for air in the last mile of a 5K or try to keep running on legs that feel like over cooked noodles near the end of a marathon, keep one thing in mind. You really aren’t fatigued – it’s all in your head! OK – I may be exaggerating a little bit. There always has been and there always will be physical reasons for fatigue but some of today’s running coaches and researchers think that one of the main causes for fatigue are not physical, but mental in nature.

The Central Governor Model

A new theory of fatigue has been advanced recently that places at least part of the blame on your brain. This recent theory called “The Central Governor Model” (CGM) suggests that your brain subconsciously doles out energy to your exercising muscles with the goals of preserving a state of homoeostasis and preventing any possible catastrophic physiological failure such as rigor or heart problems. Homoeostasis is constancy in the normal operating conditions of your body. Think of it in terms of your car. Your car is in a state of homoeostasis when the operating temperature, oil levels and fuel levels are within normal ranges.

If you run low in fuel or if your engine temperature rises your car loses its homoeostasis. Your body is very similar. If your levels of ATP (fuel for your body), oxygen usage, potassium levels and blood acidity are all within normal levels your body is in homoeostasis. If your body runs out of ATP, cannot deliver enough oxygen to fuel your heart or your potassium levels rise you have lost homoeostasis. Rigor is a complete stiffness caused by an inability of your muscles to relax. ATP – a substance that your muscles use to contract and relax – is needed to allow your muscle to relax. If your body were to be completely depleted of ATP, rigor would take place. Your heart is dependant upon an adequate supply of oxygen to keep pumping. If your heart were required to keep contracting without enough oxygen, angina (heart pain) would be the result.

High intensity running or prolonged running at moderate paces places high energy demands upon your body. The results of those energy demands can cause a loss of homoeostasis. The CGM proposes that your brain will subconsciously decrease the recruitment of your muscle fibers in order to slow you down and preserve homoeostasis.

The Strengths of CGM

This new theory helps explain a number of problems that the more commonly accepted cardiovascular/anaerobic/catastrophe (CAC) theory of fatigue cannot support. The CAC theory is the one that you probably have already heard of that suggests that running fatigue is caused by either a lack of sufficient oxygen at your exercising muscles, a build up in blood acidity at your muscle cells or a combination of the two. There are a couple of discrepancies with this theory. First – if the fatigue is caused by a lack of sufficient oxygen delivery to your muscles then it follows that your heart muscle should also be starved for oxygen.

If that were to happen you should be suffering from angina (heart pain). The other interesting problem with this common theory is the ability of a runner to not only continue a quality pace but actually increase their pace in the final stages of races from a 5K to a full marathon. If the cause of fatigue were purely physical in nature it should be impossible to perform that feat. The CGM helps explain those discrepancies. If the fatigue is caused by your brain cutting off the recruitment of your muscles you would still able to override that subconscious impulse with a conscious effort, much like the flight or flight response of our ancestors.

Is CGM Cause of all Fatigue?

The new CGM theory does a good job of explaining the discrepancies with the accepted CAC theory, but does it explain all causes of fatigue? Probably not – studies have shown that there are a number of changes in your body that can have an effect on fatigue. Here are just some of the other possible causes of running fatigue.

  • Potassium levels can increase which impair the ability of your muscle to contract
  • The capacity of the myofibrils in your muscle fibers are reduced during fatigue
  • Hydrogen ion accumulation may inhibit energy production

The role of your mind and central nervous system in running fatigue is a relatively new and developing theory. It explains problems with other theories and almost certainly plays a major role in running fatigue, but it probably does not act alone. The most current research seems to show that running fatigue is caused by a combination of central processes such as CGM and peripheral fatigue explained by the CAC, potassium build up and acidosis. Fatigue has also been shown to be task dependant. The main cause of long distance running fatigue is probably more closely related to CGM while shorter running distance fatigue is closely associated with peripheral causes of fatigue.

More research and studies are on going in this field and new discoveries will be made so stay tuned for more recent findings in the field of running fatigue.