Fatigue – Is it just in your Head?

When you feel tired, the source of that feeling may be coming from your brain. A popular theory of fatigue is that intense training causes central fatigue, a pattern of tiredness coming from the upper centers of the nervous system, or the brain. Central fatigue is getting a lot of interest with coaches and athletes, because it may provide a way to guide training by measuring it. Scientists have believed that central fatigue is a major player in causing poor performance and even be a culprit to injuries. In this blog we will cover topics such as central nervous system fatigue, Electroencephalography (EEG), and muscle inflammation. In addition to those subjects, we will go head to head with EEG approaches and share why we created CHECK™ to get a better way to gage fatigue of the neuromuscular system.

The Peripheral Nervous System vs The Central Nervous System

Attempts to artificially classify fatigue into different systems of the body are employed by coaches at times, but the body is one integrated organism and should be seen as a cohesive unit. The smartest approach is not to try to measure one system of the body and detect changes in fatigue, but to tap into the way the body communicates between systems for more accuracy. After heavy training, the body - specifically the muscles and other tissues - release pro-inflammatory messengers through the body. Eventually the brain will interpret the information and decide if the body is working too hard and may be endangering itself or creating harm. One hypothesis proposed by noted researcher Dr. Smith was that the CNS and peripheral network communicates back and forth with the peripheral system (1), and she states:

“It seems clear that the brain and peripheral immune/inflammatory cells form a bidirectional communication network. In particular, products of the immune system that are external to the CNS, communicate with the brain. Cytokines appear to be the major messenger molecules, in particular the pro-inflammatory IL-1β, IL-6, and TNF-α.”

From the short summary above, the early inflammatory responses may be one of the first of a series of communications to the brain, and may be a powerful control switch to have the brain project fatigue to the individual. The CHECK™ system was tested and compared to not only blood measurements of pro-inflammatory markers, but compared Heart Rate Variability (HRV) and neuromuscular assessment. Mentioned earlier in the blog previous, the Frequency Analysis Method or FAM that CHECK™ uses is able to detect neuromuscular impairment and pro-inflammatory changes: 

“The results of this study shows significant changes is leukocytes, lymphocytes, neutrophils, IL-6, TNF-alpha and cortisol especially immediately after and two hours after the exercise. FAM-variables were found to correlate strongly with some of the measured blood variables, in particular with lymphocytes and neutrophils which are inflammation markers suggesting that the changes observed with FAM may be sensitive to these markers. Serum CK after eccentric exercise was also found to be correlating with the FAM -method indicating that FAM may also be sensitive to muscle damage.”

The pattern of muscle breakdown and interaction of fatigue centrally is not completely understood, but enough evidence supports monitoring peripheral changes for athletes (2).

Is Electroencephalography the Gold Standard for Measuring Fatigue?

The use of brain waves is not new, but using Electroencephalography or EEG is starting to increase in popularity because of the smartphone market. Presently the EEG technology is more user friendly with athletes, cheaper, and access to sensors more available. The question many coaches and athletes have is EEG the best option to evaluate fatigue, specifically the Central Nervous System (CNS)? Perhaps a better question is central fatigue the ideal measurement since the voluntary component is highly variable. 

When we created the CHECK™ system years ago we realized the problem is not accessing the perception of fatigue, but the physiological cost of intense training to the body. Instead of using the brain’s interpretation of the peripheral cost of training, going directly to the PNS with an objective neuromuscular assessment through muscle stimulation was the best approach. (3) Removing the motivational element of testing with the use of our peripheral assessment the CHECK™ system only looks at what the body is truly capable of, not what the athlete may perceive in the brain.

“This phase synchrony, or coherence of activity in different brain regions is also found to occur in EEG and electromyogram activity during motor tasks, generally in the gamma rhythm range (25–70Hz),indicating that central and peripheral neural systems may communicate using synchronisation processes or activity.”

It is not that the use of EEG is not useful or invalid for a fatigue assessment, just that it is not necessary and likely to be just a secondary option. Going directly to the peripheral system and getting a physical assessment of the body in our experience is not only accurate, but removes the variable emotional and motivational changes athletes have psychologically. We believe that coaches need to know when to push their athletes based on their bodies, not just their current motivation that can be influenced by different factors unrelated to physical readiness. Athletes want to know what their physical limits are for both injury and proper planning, and not let the mind fool them into taking a different direction.

Mental Burnout and Staleness

Athletes and even coaches will burnout from the lack of balance somewhere, be it training or social life. Training monotony from the lack of variety may interfere with motivation and create staleness. While being concerned about staleness and strain from monotony is logical, using EEG for body fatigue could spell trouble.  Central fatigue is in the mind and not the muscle, and although we don’t like separating the two, some delineation is necessary to ensure what we are measuring is truly what are trying to assess with athletes.

An easy but very powerful approach to training is to create a plan and manage the daily effects of training with the CHECK™ system. When directly assessing the neuromuscular status of the body and compare it to the training done, a clear relationship is visible to both the coach and the athlete. If one is training on their own, workouts are easily guided if one elects to follow their own program. The sport science community is excited about the brain being more accessible to measurement, but interpretation of the brain requires a myriad of additional measurements besides a short EEG sampling, and interactions of an athlete’s emotional status can’t be summarized in one brainwave reading each day. Using the CHECK™ -system to directly evaluate the neuromuscular status is an excellent option alone, or a powerful guide if used with other measures.

Central Fatigue and Further Suggestions

The use of EEG and Central Fatigue is a noble venture but it’s too early to use brain waves to dictate what the readiness is with the body. The brain is more than an array of neurons but a very deep organ full of many unknowns. The complexity of the brain is a fascinating one but we believe that summarizing one brain wave or even several metrics is not enough to tell if the body is ready.  Surveys and other exchanges can be recorded and coaches and athletes need to communicate more with traditional exchanges such as a simple talk after practice when needed.  A line graph in the morning or at the stadium can’t fully represent the athlete’s thoughts and mindset, and while conventional profile of mood states (POMS) or similar weekly screens are limited, written input is still of value and can help bridge the gap between the perceptions of fatigue and what the body is experiencing.


  1. Smith, L. L. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med. Sci. Sports Exerc., Vol. 32, No. 2, pp. 317–331, 2000.
  1. Linnamo et al., FAM frequency response method as a tool to measure neuromuscular and physiological recovery after eccentric and concentric exercise. Unpublished Study, 2010.
  2. .K. Measurement of human muscle fatigue. Journal of Neuroscience Methods., Vol. 74, pp. 219–227, 1997.

Ville Simola
Ville Simola


Leave a comment

Comments will be approved before showing up.