What You Need to Know About the Dreaded Bonk

You may have heard people say, "I totally bonked on that long ride" or "I got the bonk on my run."

What does that mean, exactly? What is a bonk?

Sensation and History

Often, people associate the word "bonk" with "hitting the wall" during endurance events. For endurance athletes it is a sudden and overwhelming feeling of running out of energy. You were running or riding along at what seemed like a manageable pace, then seemingly without warning your legs turned to cement. With heavy legs, a body-wide feeling of fatigue and sometimes dizziness, you are forced to stop.

One of the first instances of the athletic term "bonk", came from a film produced by British Transport Films in the mid-1950s in which cyclists noted that if they didn't rest and eat, they would bonk. In other words, they would hit a limit (the proverbial wall) governed by the body and uncontrolled by the mind and sheer willpower. It is said that the feeling was similar to getting hit on the head (bonked on the head) and knocked out of competition.

What Causes a Bonk?

Perhaps a complete bonk can be described as total glycogen depletion from the muscles and liver. Glycogen is the primary fuel source for endurance athletes. This severe glycogen depletion does not occur during short duration, high intensity efforts, rather it occurs during continuous exercise at some 70- to 85-percent of VO2 max that is sustained for periods of more than about two hours.  

When you are forced to stop exercising after completing very high intensity bouts of less than an hour, it is typically not glycogen depletion that is limiting your exercise.

Without going into detail here, shorter, high-intensity efforts use a different combination of energy systems in the body. These energy system waste products ultimately interfere with muscle contractions and force you to stop or slow down. Even though you're forced to stop after a high intensity effort, your body still has plenty of glycogen stores to continue at a slower pace.

Can a Bonk be Prevented?

Yes, you can prevent a bonk with several strategies:

  • Eat a diet adequate in carbohydrates. Experts argue over what value is seen as "adequate" and the optimal value is likely individual. Consume a diet that is some 40- to 65-percent carbohydrates.
  • Eat a diet adequate in overall caloric density for your body weight. Chronic under-eating can deplete glycogen stores.
  • Consume adequate fuels during training and racing.
  • Replenish glycogen stores after long workouts, particularly if you have not consumed fuel during the last hour or so of a long workout. You can use home-made recovery drinks or the commercial variety.
  • Eat before training and racing to "top-off" your tank, particularly if it has been several hours since your last meal.

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