The Anatomy of a Runner: the Wall

Hitting the Wall: In endurance sports such as cycling and running, hitting the wall or the bonk is a condition of sudden fatigue and loss of energy which is caused by the depletion of glycogen stores in the liver and muscles. (Wikipedia)

Alternative Definition: a collapse of the entire system: body and form, brains and soul.

Etymology, usage, and synonyms: The term bonk for fatigue is presumably derived from the original meaning “to hit”, and dates back at least half a century. Its earliest citation in the Oxford English Dictionary is a 1952 article in the Daily Mail.

The term is used colloquially both as a noun (“hitting the bonk”) and a verb (“to bonk halfway through the race”). The condition is also known to long-distance (marathon) runners, who usually refer to it as “hitting the wall”. The British may refer to it as “hunger knock,” while “hunger bonk” was used by South African cyclists in the 1960s.

It can also be referred to as “blowing up”.

Possible Symptoms: a fast or pounding heartbeat, cranky/irritable, headache, dizziness, light headedness, nausea, hunger, extreme and debilitating fatigue, shaking or trembling, unclear thinking, poor coordination, possibly hallucinations and/or mental confusion, a complete absence of energy.


Endurance athletes experience indescribable ‘pain’ as a result of glycogen depletion: a complete or a near-total depletion of glycogen, the storage form of glucose which is the source of human energy derived from carbohydrates consumed through food.

Glycogen is a molecule derived from the carbohydrates in our diet, which is stored in the body and used as a source of energy. The amount of glycogen stored in the body mostly depends on physical training, basal metabolic rate and eating habits, but the average body stores approximately 2,000 kilocalories of glycogen at any given time – just about enough glycogen to support the average adult for 12-14 hours, or up to about 2 hours of exercise.

Why do marathon runners ‘Hit the Wall’? (Yahoo Answers)


Athletes engaged in endurance exercises produce energy via fat metabolism and the breakdown of glycogen into glucose – both facilitated by oxygen. How much energy comes from each source depends on the intensity of the exercise.

Activating the fat-burning process takes longer (and more oxygen) than burning sugar, which is why the body generally uses glucose unless it has a compelling reason not to. (In extreme cases, such as severe hunger, the body can also use protein for energy.)

Note: Carbohydrates – foods such as rice, potatoes, bread, tortillas, cereal, fruit, vegetables, and milk – are the body’s main source of glucose. If you eat more glucose than you need, your body will store it in your liver and muscles or change it into fat so it can be used for energy when it’s needed later.

IMG_3386With intense exercise that pushes the limits of VO2 max, most energy comes from glycogen and is burned off quickly. Lowering the intensity will also lower the amount of energy burned per unit of time, which is why runners learn not to start a race too fast. Elite runners practice maintaining their ideal race pace with an even effort to extend their glycogen stores for as long as possible – hopefully to the end of the race.

The brain uses glucose exclusively – in fact, it is said that seventy-five percent of the glucose available to the body is used to service the energy requirements of the brain and the central nervous system. Stored glycogen is also used to regulate blood sugar levels in the body between meals.

Once the glycogen is stored in the muscle for fuel, however, it is not flexible in terms of its deployment in the body since muscle-stored glycogen isn’t capable of being shared with or transported to other areas that might require fuel. It must be used at the point of storage.

If the body were to rely solely on glycogen for energy, 31 kg (67.5 lbs) of stored glycogen would be required as compared to 4.6 kg (10 lbs) of fat.

As people become more fit, it has been thought the muscles shift from burning carbohydrates (glucose) to burning fat; in fact, this shift in substrate utilization from glucose toward fat has been a traditional hallmark of trained muscle. In other words, it has been long believed that training improves endurance because it allows the muscles to more effectively burn fat as an energy source.

There are several issues with using fat as the predominant source of energy, however:

  • Fat is slow to digest and be converted into a usable form of energy (it can take up to 6 hours).
  • Converting stored body fat into energy takes time. The body needs to break down fat and transport it to the working muscles before it can be used as energy.
  • Converting stored body fat into energy takes a great deal of oxygen, so exercise intensity must decrease for this process to occur.

Hitting the Wall

Findings in a study released in May 2017 (PPARδ Promotes Running Endurance by Preserving Glucose) shows that rather than training the body to use fat more efficiently as previously believed, training actually teaches the body to burn less glucose (remember, the body simultaneously burns a combination of glucose and fat depending on the level of intensity).

In other words, the body prioritized energy derived from fat (FA catabolism) thereby lowering the conversion of glucose to energy (glycolysis) with the net effect of preserving systemic glucose reserves in the body. This new thinking is that training progressively re-programs muscle to burn less glucose, preserving it as an energy source for the brain.

In the top part of this image, muscle consumes glucose and fatty acids during exercise. In the bottom part, muscle that’s been stimulated by PPARδ consumes mostly fatty acids, leaving more glucose for the brain. (Courtesy:

A focus of the study was to better understand ‘hitting the wall’ – their results show that when the brain runs out of glucose, the body shuts down. The wall is made of sugar.

In endurance sports, ‘hitting the wall’ is a dramatic demonstration of sudden and complete exhaustion. You become disoriented, maybe dizzy, overtaken by sudden and overwhelming fatigue – the symptoms of hypoglycemia, or low blood sugar.

The surprise of the science here is that the longer the brain stays active, the longer you will be able to run. And so it’s more about the brain and sugar than we previously thought.”

Ronald Evans, co-author of the study, American professor and biologist at the Salk Institute for Biological Studies in La Jolla, California and a Howard Hughes Medical Institute Investigator.

Note: The next logical question for the scientist of this latest study was whether a drug could trigger the body’s exercise gene to instruct the muscles to burn fat and preserve glucose. The chemicals (mimetics) given to the mice in their study improved performance by 70%. Their goal now is to quickly transition this into a drug for people (currently known as the ‘exercise pill’), which has been taken on by a Boston-based company, Mitobridge. The study’s abstract concludes by saying, “Collectively, these results . . . highlight the potential of PPARδ-targeted exercise mimetics in the treatment of metabolic disease, dystrophies, and, unavoidably, the enhancement of athletic performance.”

Critical Power

A more recent study confirms that the runner’s physiology changes as we become tired – metabolism, breathing, even running form is affected by exceeding our critical power threshold. A runner’s critical power is the highest level of effort that can be sustained for a specified time. If you’ve run more than a few races, you likely know your personal threshold. It helps us determine the pace we can most successfully maintain throughout the race.

This latest study found that critical power does not change in the first 40 to 80 minutes, but after 120 minutes it drops by 9 percent. For a majority of recreational runners, this puts us somewhere between halfway and the 20-mile mark. And crossing this critical power threshold can happen whether runners have or have not fueled properly throughout the race. Training to maintain race pace close to but still less than critical power is a desirable strategy (elite runners compete at around 96 percent).

It’s important to remember, however, that almost every runner slows down over the course of the marathon. The most successful runner is the one that learns to resist the fatigue and associated physiological changes associated with hitting the wall.

Moving the Wall

It may not be that you have simply hit the wall, maybe there’s a blister on your foot, you’re dehydrated, or worse, your stomach is having its own moment of distress, and no matter how hard your mind wills your legs to move, they refuse. How endurance athletes convince themselves to keep putting one foot in front of the other despite the pain requires that we understand ourselves, and how committed we are to what motivated us to be here in the first place.

There was a good article about pushing through the wall in RunnersWorld a few years ago where one expert was quoted as saying the biggest challenge in pushing through the pain is to understand that this kind of pain won’t kill us. “After a lifetime of avoiding pain or being protected from it, the reason a runner won’t push through a wall of exhaustion could be good old-fashioned self-preservation.” Instead of pushing through the pain, our natural instinct is to stop.

Dealing with pain can be learned; it’s one of the objectives of our marathon training programs. There’s a few different approaches to consider – marathon pace runs help prepare our minds for race day, as will speeding up for the last third of a long run, for example. Another option is to eliminate carbs the day before a long run, or go all the way with a ketogenic diet – there are drawbacks to be considered before adopting the approach, however.

If you typically run with music, try turning it off completely for your next long run, or, as some coaches recommend, go for a run when you’re hungry, or tired – something that may not require a lot of planning for some of us. (Thoroughly research the strategies you deploy to understand risks and recovery.)

Whatever the approach, the intent is to prepare our minds for the inevitable end-of-race push we’ll need to reach the finish line.


Elite runners tend to use the attentional strategy of ‘association’ to deal with pain, which involves focusing their attention internally on the body’s sensations, such as muscular strain, breathing, etc. They literally embrace the pain and see it as a necessary part of their eventual success.

Non-elite runners, on the other hand, are more likely to direct their attention away from the body’s physiological signs of distress by distracting themselves and concentrating on other things, or ‘disassociation’.

Although research findings seem to be mixed, association is generally related to faster running times than dissociation, although runners of all levels are more likely to associate during competition and disassociate during training runs.

It is the state of association that holds a greater risk of injury by continuing the effort despite the body’s warning signals. When used properly, however, association can allow runners to successfully ride the thin line between pushing hard and overdoing it.

The Carbo Load

Carbohydrate loading is the technique of gradually increasing carbohydrate and fluid intake each day, beginning anywhere from a week to 24 hours before competition, while exercise is tapered downward, to maximize glycogen storage.  Some strategies also called for depleting carbs by exercising intensely and lowering carb intake. This approach has proven dangerous and did not necessarily optimize glycogen stores (Reference: Advanced Sports Nutrition by Dan Benardot).

Each subsequent study seems to tweak the original carbo-loading approach ever so slightly, which is why I’m going to stick with my favorite advice on this one: do what works best for you. However, there’s some interesting information out there in regards to pre-race nutrition and refueling options during the race:

  • Maybe it’s the taper, not the carbo-loading: a 1992 study at McMaster University reported that a seven-day taper increased glycogen stores in middle-distance runners significantly and also resulted in a 22 percent increase in running time to exhaustion. Diet was not manipulated in this study.

By sharply reducing mileage, as with the pre-marathon taper, the amount of carbohydrate the muscles burn daily also sharply decreases, causing glycogen stores to increase without any change in the diet.

  • A study of 257 male and female runners in the 2009 London Marathon revealed that carbohydrates eaten at breakfast on race day, during the race itself, or on days earlier in the week were relatively unimportant. It was primarily what people ate on the day before the race that mattered. The runners who consumed more carbohydrates the day before the race maintained their pace past the 18-mile mark where the others did not.
  • It’s worth repeating that glycogen stores are used up faster if your pace fluctuates above and below a certain average than if your pace holds steady at that average.
  • On the other hand, if you feel your glycogen stores won’t last, a walking break will help preserve glycogen. You might also try Jeff Galloway’s run-walk-run approach.
  • Instead of increasing food volume or calories the day before a race, replace some fats or proteins with carbohydrates.
  • A 2016 study found that carbo-loading may be harmful to the heart by reducing the production of atrial natriuretic peptide (ANP), a hormone that helps the body get rid of excess salt and reduces blood pressure. The principal driver for this acute reduction in ANP appears to be the increase in glucose. A word of caution for Clydesdale runners or runners with pre-existing health issues.
  • Immediately following an intense workout is an ideal time to consume simple carbohydrates (some say this window of time is within the first hour while others say within 2 hours). There’s a great post by Rob Sulaver on Arnold Schwarzenegger’s website explaining the science behind this (Exercise & Carbs: A Game-Changer). This science shows that consuming carbs during this window of time feeds the muscles without getting stored as fat.

The Carbo RE-Load

Refueling during the race is a science of its own, and the options seem limitless when you consider energy drinks, energy gels, caffeinated options, and homemade varieties. Studies continue to find carbohydrate refueling an important component of maintaining exercise durations longer than 90 minutes.

A Human Kinetics post, Carbohydrate Intake During Exercise, included an excerpt from a study (as documented in the book, Sport Nutrition) that showed glucose concentrations dropping during exercise (at 70% of V.02max) after 1 hour, and reaching extremely low concentrations at exhaustion after 3 hours. With carbohydrate feeding, glucose concentrations were maintained and the test subjects continued to exercise for 4 hours at the same intensity.

The very best article I’ve found on the subject of refueling is at This article, “A comparison of the best energy gels”, explains which gels work best, for what reason, a definition of gel’s ingredients, as well as a comparison of the ingredients between major brands of gels, and when to use them. There’s also dozens of links to additional resources from the original article. Read more at site.



Featured Image: Statue of the “Tired Man” (Megfáradt ember in Hungarian), referring to the poem of Attila József. The statue is the work of József Somogyi.

Read more:

Exercise-in-a-pill’ boosts athletic endurance by 70 percent (Science Daily)

“Exercise-in-a-pill” boosts athletic endurance by 70 percent (

The physchology of sports injury: is ‘no-pain no-gain’ the path to sports injury? Sports Injury Bulletin

A Winner Runs Through It  RunnersWorld

The Evolving Art of Carbo-Loading,

How and when to carbohydrate load (AKA carbo loading),

When to eat Energy Gels in the Marathon,


This post is meant for informational purposes only. It is not intended to provide medical advice. Please consult a physician to discuss your specific pain/injury, the treatment options most appropriate for you, and to ensure there are no contraindications in the treatment options you may adopt.



2 thoughts on “The Anatomy of a Runner: the Wall

  1. Marcia – I commented last night via my iPhone and then it didn’t post! Should have saved it. Wanted to tell you how impressive this researched and well-documented teaching about energy, strenuous exertion and ‘the wall’ is. You should be coaching runners or writing for Runners World! What have your personal experiences been like with this? Mike has a buddy who used to do the carbo-loading routine(Pasta) the day before a marathon and swore by it. then he switched to a vegetarian diet and attributed his good results to that. Like you mention, what works for oneself is the operative paradigm. Trial and error. A lot of what you write about would be also helpful to cyclists like those in the current Tour de France. Good job, Marcia. I know that took a lot of time and thought!


    1. Thanks Maria! And thanks for reading through the whole post. It became quite the effort in an attempt to cover all aspects of the topic. I’ve been reluctant to use gels when I run/race so I’ve experienced a good bit of ‘crashing’ as a result. This research may have changed my approach actually – at least I’m going to try consuming more carbs during a race. This was half of what started out as that post on pain I had mentioned, but then I realized there was really too much material to combine the two. Hitting the wall is just about the most pain I’ve ever experienced though. 🙂 Thanks as always for your encouragement.


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