The Science Behind Hitting the Wall


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Have you ever suffered through the last few miles of a half or full marathon? If so, you were experiencing the runner’s bonk or what’s commonly called ‘hitting the wall.’ The fatigue, cramping, and lack of energy CAN even happen when you are properly trained. Why…? Because you probably don’t have a nutrition plan that works.

Today, in this short video, I want to teach you science behind the wall so that you can conquer it in your next long training run or race!

Let me know if you have questions!

JJ Mayo PhD, RDN

P.S. I would mean a lot to me if you would share this with someone who could use this info–


Finally…you found it! A simple explanation of the science behind hitting the wall!

Carbohydrates are going to be the key to fueling exercising muscle. It’s all about loading carbohydrate up in your muscle so that you can make it for the long haul.

When you eat the carbohydrate it’s digested into the simplest form which is glucose. We store carbohydrate in the muscle and the liver as glycogen.

Glycogen is just glucose strings attached together and glycogen is the storage form of carbohydrates in your diet

Our goal as runners is to top off glycogen stores before your run or big race. Then maintain glycogen stores during exercise and this is where often times maybe some of us do a good job of storing the glycogen before we run but then we have a hard time fueling 3 and 4 hours into our sessions.

So how do carbohydrates improve performance? And by the way when we talk about carbohydrates foods we are talking about our pancakes, pasta and other grains. Also, fruits and vegetables are primarily carbs too.

Here is an example of a one-hundred fifty-pound person with a pretty low percent body fat of 12%. The majority carbohydrates stored as glycogen is found in the muscle which is great, that’s exactly where we need it.

You have anywhere from 1500 to 2000 calories stored as muscle glycogen and then you have a little bit stored in the liver. You also must have glucose in our blood.

When you do the math you only have about 2500 calories of stored carbohydrate. This is why you hit the wall at around Mile 19-22 of a run.

When you exercise you burn about 100 calories a mile so by the time you get to mile 20 that’s about 2000 calories– you’re burning through all the stored glycogen.

Hitting the wall coincides with running out of muscle and liver glycogen and see what it looks like is this we’ve got muscle glycogen we’ve got liver glycogen and then like

As we run long, we start by breaking down muscle glycogen first. As it becomes depleted, blood glucose is going to help out the muscle. so glucose Eventually, liver glycogen will be used to support the blood.

Hitting the wall or bonking is what happens when liver glycogen runs out. Liver glycogen is the final storage depot for carbohydrate and when you run out which could happen anywhere from 19-22 miles this is where we get in trouble.
Fortunately, at every aid station we’ve got sports drinks and energy gels to maintain glycogen stores. Sports drinks are popular because they have everything you need… the simple carbohydrates, the electrolytes (if you’re going pretty long) and then the fluid.

Because it’s already in simplest form, we consume the Gatorade or Powerade or make your own aid and it goes into the bloodstream directly. This conserves liver glycogen and that’s your goal is through supplementing with sports drinks and energy gels.

Underfueling leads to hitting the wall which results in poor performance or even a DNF. Underfueling can take you from a really nice race pace effort… (you’re looking good) to a slow jog and then a walk within a matter of a mile or less.

I’m sure many of you have experienced hitting wall but you don’t have to. Now that you understand the science you can develop a fueling plan that works. It’s really your key to success!

Hope you enjoyed the video and if you’d like to learn more…hop onto my mailing list where I send out weekly tips straight to your inbox.

You can run faster by fueling better in your next marathon!

  1. Ronnie says:

    JJ this is Great information. As a triathlete I battle this every day training for the longer races. As a technical analytical person I love to understand the reason behind this in order to decide how to solve them. Where could I find more info regarding this subject and best ways to replanish glycogen, timing etc?

  2. jjmayo says:

    Ronnie right here on the blog is a great place to start! I’ve got a ton of posts on these topics. Check out the archives link at the top! Of course I could also provide you with some research articles but my goal on the blog is to use the KISS principle.

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Learn the exact steps I used to become a 9x Ironman finisher, ultramarathoner, and Boston Qualifier. Take the guesswork out of race-day nutrition.

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