If you are a casual runner, you may have heard people use the term “bonk” when talking about their run. But what does this term mean, exactly? And is the myth that bonking is bad for you really true?

What Is Bonking?

Athletes who focus on endurance will sometimes exercise to the point where they experience a sudden depletion of energy. Their legs will feel like they are made of lead, and the unexpected fatigue forces the runner to stop. This is known by the slang term of “bonking” or “hitting the wall.”

The first time an athlete used the term was in a 1950s British film in which cyclists stated if they did not eat and rest, they would “bonk.” It was their way of saying they hit the limit of what their bodies could endure and could not bypass it with willpower alone. The feeling was similar to being bonked on the head and, as a result, knocked out of the competition.

What Happens When I’m Bonking?

Runners get their energy from sugar reserves stored by the body in the liver and muscles. This fuel, called glycogen, is preferred over burning fat directly because it is easier to process and more efficient to use. As the runner continues with their exercise, glycogen stores will eventually begin to run low. The body recognizes the possible danger and makes the runner slow down as a way of conserving energy. The person can still run, but their pace will slow if they do not put in more effort to continue.

By pressing on, glycogen reserves eventually deplete to the point that the body all but shuts down. The person may also feel light headed or nauseous because the brain is not able to get the glycogen it requires, and the runner just needs to stop and rest for a while. The runner might still be able to walk, but a true bonk means running is out of the question.

How Bad Is Bonking?

At first glance, there may appear to be some truth to the common belief that bonking is bad for you. However, many running coaches actually recommend otherwise. It is widely believed that runners, especially those who participate in marathons, can use it as a way of improving their endurance levels. Essentially, bonking can help train the body to more efficiently manage glycogen reserves without experiencing the crash and burn before the end of the race.

That said, while bonking may be helpful to marathon participants, casual runners will not benefit much if they are trying to increase their distance or speed. It is not a dangerous practice when done correctly, but it does little for most people other than being an unpleasant experience.