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What causes fatigue? Why is it different in CrossFit?

What causes fatigue? And why is it sometimes so hard to push through it?

Few people enjoy the sensations of fatigue and pain that accompany intense exercise.  While endurance athletes may get a “runners’ high” that feeling of elation is not common during a CrossFit WOD or in other forms of intense physical output.  The runners high is thought to be caused by feel-good chemicals produced by the brain that blunt pain and allow people (and animals) to run for long distances.  You can read up on these chemicals (endocannabinoids) here.  Without them you will feel very differently.  There are several thoughts on what causes fatigue.  Possibilities are:

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  • Build-up of metabolic by-products
  • Signals produced by the brain that try to tell you “that’s enough”
  • Production of inflammatory cytokines
  • Signals from the body that enter the brain to tell it “stop.”

Is buildup of Ammonia a factor in Fatigue?

It may be.  Ammonia builds up in blood just a little during moderate intensity exercise.  But it increases rapidly when levels of effort are heavily ramped up.  And it increases during prolonged (over an hour) sub-maximal efforts.  The buildup of ammonia may be caused by the breakdown of Branched Chain Amino Acids (BCAA’s).  When ammonia builds up in the blood it can enter the brain and cause problems.  Including stupor.  Maybe this is part of what happens to marathon runners.

Caffeine, Amino Acids and Escaping Fatigue

Caffeine has been shown to reduce fatigue and improve athletic performance.  Caffeine also changes the manner in which amino acids (and BCAA’s) are metabolized during exercise.  Perhaps one of the ways caffeine helps workouts is by reducing the amount of ammonia build up.   Meanwhile, it seems that supplementation with glutamine suppresses fatigue and ammonia build-up.  Glutamin is considered a non-essential amino acid, but there is growing evidence that it might be particularly important under intense exercise.  Good sources of glutamine include beef, chicken etc. as well as wheat.  This is not the same thing as MSG (mono-sodium glutamate) which can trigger nasty headaches in many people.  Supplementation with branched-chain amino acids may also suppress build-up of ammonia as well.  We’ll see what comes up in the next few years.

 

Bassini A, Magalhães-Neto AM, Sweet E, Bottino A, Veiga C, Tozzi MB, Pickard MB, & Cameron LC (2013). Caffeine Decreases Systemic Urea in Elite Soccer Players during Intermittent Exercise. Medicine and science in sports and exercise, 45 (4), 683-690 PMID: 23135367
Wilkinson DJ, Smeeton NJ, & Watt PW (2010). Ammonia metabolism, the brain and fatigue; revisiting the link. Progress in neurobiology, 91 (3), 200-19 PMID: 20138956

Bassini-Cameron, A., Monteiro, A., Gomes, A., Werneck-de-Castro, J., & Cameron, L. (2008). Glutamine protects against increases in blood ammonia in football players in an exercise intensity-dependent way British Journal of Sports Medicine, 42 (4), 260-266 DOI: 10.1136/bjsm.2007.040378

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