Race Terminology – Bonk

The body will shut itself down.

In this article, we continue our attempt to take on some terms we believe are being misused in the community. Our first entry was about “Hypothermia” . In this entry to the “Terminology Series,” we take on “bonk”.

The Bonk
“Dude, I was having a great race at the Spartan Beast, but then I bonked at mile 10 after carrying those buckets up the hill. I had to walk the last couple of miles.”

That does suck. I’m sorry you had difficulty with your race, but I really doubt that you actually “bonked”.

To the Brits the term “bonk” may describe a pleasurable act, but to those of us in the Obstacle Racing community (or any endurance sport), bonking describes something most unpleasurable.

Although the exact definition and description of “bonk” varies, depending on your source, most doctors and sports physiologists agree that bonking is not just fatigue. It is not just cramping. It is a specific physiological experience and it is much more severe. For example, one of the symptoms of bonking is “Feeling like you are going to die” That’s not a joke. That is not an exaggeration. Athletes bonking for the first time often start to at least consider the possibility that they are dying. That sounds most unpleasurable.

Bonking is more than just being "really fatigued."
Bonking is more than just being “really fatigued.” Photo Credit: Mark Wiltshire

Here are some other symptoms:

  • Extreme irritability
  • Severe fatigue/weakness
  • Severe muscle cramping
  • Extreme tiredness/sleepiness
  • Confusion and disorientation
  • Nausea/vomiting
  • Temporary partial loss of vision or hearing
  • Fainting


Generally, bonking is caused by the depletion of the body’s glucose and glycogen. You rely on these blood sugars to produce energy (the body uses glucose first and then begins breaking down stored glycogen to produce glucose). When your glycogen stores get low during a race or training, your brain makes your body slow down. This is fatigue. If you keep pushing and your body continues to deplete its energy source, your brain starts to run low on glycogen. This is the beginning of the bonk.

Your brain doesn’t have the energy it needs to function properly. It goes into self-preservation mode. It doesn’t care about continuing your race. It doesn’t care about completing 75 miles or whatever goals you had going into an event. It only cares about survival.

The body will shut itself down.

It is no longer a case of mind over matter. It is no longer an issue of being tough enough to continue. Your body will NOT continue to perform. You can’t move more than a slow shuffle. Your thinking becomes confused. You can become disoriented. You are in trouble.

It is no longer an issue of being tough enough to continue.
It is no longer an issue of being tough enough to continue. Photo Credit: Mayra Rodriguez

Bonking is not when your calves or quads or hamstrings cramp up so bad on the trail that you cannot run. Bonking is when your legs are cramped up so bad on the trail that you cannot move, but then when race support locates you in the woods you say you don’t need a ride because your wife just called you a cab and it will be there to pick you up any minute. (yes that is an amusing story, but it really did happen to a top competitor in an OCR endurance race).

The body will shut itself down.
The body will shut itself down.

You can help stave off bonking (and many cramping and fatiguing issues) by staying on top of your nutrition. Make sure you are properly fueled heading into an event and make sure you continue to take in calories during the event. (This is not intended as medical advice, consult your doctor, results may vary, some assembly required).

The sources I used for this article are listed below, check them out for more on the science, prevention, and treatment of bonking.

“Difference between Glucose and Glycogen”, http://www.differencebetween.net/science/difference-between-glucose-and-glycogen/

“The Science Behind Bonking”, by Paul Scott, http://www.runnersworld.com/nutrition-for-runners/the-science-behind-bonking

“What You Need to Know About the Dreaded Bonk”, by Gale Bernhardt, http://www.active.com/triathlon/articles/what-you-need-to-know-about-the-dreaded-bonk

“Bonking vs. Fatigue vs. Cramping, What You Need to Know” by Jeff Gaudette, http://runnersconnect.net/running-nutrition-articles/bonk-fatigue-cramp/

J.D. Allen

Originally from Detroit, he has spent time in Chicago and Boston and now lives just outside Atlanta. He has raced in many events across North America, including Ironman Wisconsin. J.D. is a director on the Georgia Obstacle Racers and Mud Runners (GORMR) leadership committee and is an NSCA Certified Strength and Conditioning Specialist.
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