Everything You Wanted to Know About Cramping And Bonking (But Were Afraid to Ask)

A lot of things can go wrong in a race, but the two we hear about most often are cramping and bonking. While they’re common runners’ problems, many runners don’t have deep knowledge of them. They can put a name to what they are feeling but don’t fully understand what’s happening to their body when they start slowing down.

Here’s what is actually happening to your body when you experience the two most frequent race day mishaps.


Thanks to the clever advertising and the millions of dollars sports drink companies funnel into research, most runners blame cramps on dehydration or lack of electrolytes. But research shows that only a tiny percentage of muscle cramps in runners are caused by a fluid or electrolyte loss.

In his book Waterlogged: The Serious Problem of Overhydration in Endurance Sports, physiologist Dr. Tim Noakes observed that exercise cramps don’t occur exclusively in heat. Swimmers in frigid water still suffer from muscle cramping, for instance. Moreover, there’s little proof linking exercise in heat to an increased risk of cramps.

Secondly, in a 2005 study published in the Journal of Athletic Training, 13 athletes performed a series of exercises in hot, humid conditions. During one trial, the men drank large volumes of sports drink with extra salt and drank nothing in a second trial. Nine of the thirteen men got cramps even in the sports drink trial. In the no-liquid trial, only seven men experienced cramps.

So What Causes Cramps?

During races, our muscles undergo significant stress. As you get further into the race, the major muscle groups start to get tired and you can no longer fire them as efficiently. Consequently, you start recruiting additional muscles to help maintain your pace or effort.

For example, if your glute muscle fatigues, your leg doesn’t simply stop working. Instead, you subconsciously recruit another muscle group, like the calves, to pick up the extra slack. The calves aren’t nearly as powerful as the glutes, and haven’t been trained to handle the additional stress. And voila, the calf starts to cramp.


Bonking, otherwise endearingly called “hitting the wall,” happens when your body runs low on the glycogen it needs to fuel your run. While your body can burn fat directly for energy, it burns glycogen more efficiently.

As your glycogen stores start running low, your body recognizes the potential danger and slows the body down gradually to conserve energy.

At this point, you can still run, but your pace starts to slow unless you work harder. But if you continue, your glycogen stores will drop so far that your body will basically shut down and even jogging will be almost impossible.

That shutting down feeling is what’s called bonking.

A true bonk will almost always stop you from running. You may be able to shuffle and probably walk, but, as I showed in the 2018 UTMB, anything close to running could leave you dizzy or light-headed, or possibly even nauseous, since your brain isn’t getting the glycogen it needs.

And while it’s always disappointing when your body tells you it needs to stop, understanding why it’s sending that message can make it easier to listen.