What is Tired?

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What is Tired?

Written by Richard Ferguson, Ph.D. on 30 November 2015.

It might be a very strange question to ask, but just what is “tired”? How many times in a day you here others, or yourself, say, “I’m tired”? You know what it feels like to be tired, but just exactly what leads to the sensation that we label as tired? Of course running can make you feel tired, both during and after running. You know the feeling of fatigue late in a long workout or race. The heavy legs, the feelings that your legs simply won’t move, that dull ache in your muscles, and the labored breathing are all outward signs of being fatigued or “tired”. There is also the mental struggle where your mind may be trying to tell you to stop or slowdown and you just feel mentally fuzzy, such as not being able to think clearly or experiencing strange thinking patterns and visual distortions. In other words, late in that marathon or long training you feel just plain tired.

                  Most of the research on fatigue in runners has centered on what is known as peripheral fatigue, or the fatigue that may occur within the muscles themselves. Low muscle glycogen leads to muscular fatigue, lactic acid production leads to muscular fatigue, dehydration can lean to muscular fatigue. All of these causes of fatigue are pretty well established through research.

                  But what about the fatigue that is occurring in your central nervous system, or as it is commonly referred to, central fatigue? The question remains, does fatigue occur in your muscles or in your brain? Is the reason your legs won’t move late in a marathon due to something going on in your brain and not necessarily in your muscles? The role of central fatigue and running performance has garnered more attention over past several years. Famous South African researcher and author, Timothy Nokes, feels central fatigue is actually the way the central nervous system protects the body from being damaged from intense physical effort. In other words, he feels the brain may actually limit our performance and, as such, fatigue can be reduced if you are prepared psychologically to deal with the sensations of fatigue. So just maybe feeling tired is a perception rather than an actual physical phenomenon.


But is there some physiological root to central fatigue and those psychological feelings of being tired? Disturbances of neurotransmitters in the brain during exercise may lead to perceptions of central fatigue. Exercise may raise brain levels of the neurotransmitter serotonin and too much serotonin is related to increased perceptions of fatigue. Long term exercise may also lower the neurotransmitter norepinephrine, which acts as a nerve stimulant, thereby increasing perceptions of fatigue as well.

                  Serotonin has received the most attention on its effect on fatigue. Late in a long run, such as a marathon, glycogen (carbohydrate) stores get low. In an effort to maintain blood glucose levels, some protein in our muscle tissue is actually broken down for energy. Of course protein is formed from amino acids and there are three particular amino acids that seem to be utilized the most. These three are commonly referred to as the Branched Chain Amino Acids and consist of leucine, isoleucine and valine. Well, in a nutshell, if the Branched Chain Amino Acids get low in your blood stream it allows another amino acid, tryptophan, to leave the blood and enter the brain in large amounts. Increased tryptophan in the brain may cause more of the neurotransmitter serotonin to be produced, thereby causing increased perceptions of fatigue. (You may have heard that turkey contains tryptophan and that’s why you get sleepy after eating a meal with turkey, such as on Thanksgiving!)

                  So if feeling tired is something central that is occurring in the brain, is there anything you can do about it, especially in running? You may not be able to totally prevent fatigue, but there are a number of things you can do to delay the onset of fatigue. From a physiological standpoint, your training actually serves to help you delay fatigue. So a sound training program can actually help you delay fatigue by serving to increase your energy stores (glycogen), increased energy system enzymes, and better energy metabolism, all of which can improve your ability to run faster for a longer period of time.

                  Psychologically, you can train your mind to tolerate the stress and discomfort that comes long term endurance events. If fatigue is indeed a central nervous system event, then mental training techniques such as positive self-talk, concentration training, relaxation and/or arousal training and even hypnosis can help you overcome the mental perceptions of fatigue.(See many of my previous articles on these topics). By working on your psychological strength, you will be able to understand the feedback your body is giving you and increase your ability to tolerate greater intensities and durations of running effort.

                  Much of the research on running fatigue is actually nutritionally focused. For example, in moderate to heavy aerobic exercise, like running, the human body burns carbohydrate as the main energy source, so glycogen storage becomes a concern. If you run out of glycogen you will begin to burn total fat and fat is much less efficient as a fuel source. When you burn fat you will have to reduce the pace of your running and you will feel more fatigued. That’s exactly what hitting the wall in a marathon is! So being sure to ingest a diet with adequate carbohydrate is important. Also taking a form of carbohydrate during exercise, such as a sports drink, can help you maintain blood glucose levels and spare some muscle glycogen for latter in the run. Taking a carbohydrate source during running can also reduce your perception of central fatigue, or in other words, your running will seem easier. Maintaining blood glucose levels can also help prevent protein breakdown and the Branched Chain Amino Acid imbalances that have been hypothesized to increase brain serotonin and central fatigue.

                  Finally, ingesting a carbohydrate sports drink which also contains Branched Chain Amino Acids during exercise may also serve to reduce central fatigue. While the research is not totally clear, the BCAA’s consumed during running may serve to keep the BCAA/tryptophan ratio in balance during running, thereby keeping brain serotonin levels lower and reducing central fatigue and the perceived difficulty of running late in an event. Research is continuing in this area, so stay tuned!

                  So yes, being tired is a real human condition, and while the mechanisms of fatigue are not totally understood, fatigue, or as most of us say being tired, does occur during running. But the good news is we may be able to actually reduce fatigue through mental techniques, training and nutrition. A little less tired is always a good thing in running!

Richard Ferguson is Chair of the Physical Education, Wellness, and Sport Science Department at Averett University and is an AASP Certified Sport Psychology Consultant. He may be reached via e-mail at This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Read more of Dr. Ferguson articles on - http://runohio.com

The Reason We Run - http://runohio.com/index.php/features/1027-the-reason-we-run

Some New Recruits - http://runohio.com/index.php/features/956-some-new-recruits?   

Are You a RUNNER? -  http://runohio.com/index.php/features/928-are-you-a-runner

A Different View -  http://runohio.com/index.php/features/892-a-different-view

The Components of Peak Performance - http://runohio.com/index.php/features/871--the-components-of-peak-performance

Running and Sleep -  http://runohio.com/index.php/features/845-running-and-sleep

Don’t Panic! -  http://runohio.com/index.php/features/824-dont-panic#:

The Mental Maximization of Training - http://runohio.com/index.php/features/773-the-mental-maximization-of-training

With the Help of a Friend -  http://runohio.com/index.php/features/723-with-the-help-of-a-friend

Beating the Winter Blues- http://runohio.com/index.php/features/718-beating-the-winter-blues

Go For It - http://runohio.com/index.php/features/702-go-for-it

Beating Burnout - http://runohio.com/index.php/features/658-beating-burnout

A New Outlook - http://www.runohio.com/index.php/features/611-a-new-outlook

Expect the Unexpected - http://www.runohio.com/index.php/features/560-expect-the-unexpected

Pain or Discomfort ? - http://www.runohio.com/index.php/features/529-pain-or-discomfort

Keep Your Eye on the Prize - http://www.runohio.com/index.php/features/512-keep-your-eye-on-the-prize

 Running Free - http://www.runohio.com/index.php/features/507-running-free

Running and Role Models - http://www.runohio.com/index.php/features/196-running-and-role-

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