Jeff Gaudette

Written by Jeff Gaudette


Bonking vs. Fatigue vs. Cramping: What You Need to Know

The first step to fixing any problem is accurately identifying what is causing the issue.

Got a leaky faucet? Before you can do any good, you need to figure out if the leak is coming from the handle, the spout, the pipes or something else. Only then can you solve the problem.

It’s the same process when you struggle in a marathon.

The dilemma for most runners is that they don’t fully understand what’s happening to their body when they start slowing down; or they use terms interchangeably, which makes it difficult to specifically address the actual problem.

As an example, many runners use the term “bonk” when they fade at the end of the race. But bonking is different than fatigue and mistakenly tying to fix one won’t help with the other, leading to repeated marathon failures.

To help you better understand and determine the reasons for your marathon struggles, I am going to perform a case study of sorts.

My hope is that by dissecting a real-life example and then explaining the science and true meaning behind all the possible outcomes, I can help you fix your own marathon issues.

Let’s get started.

Most runners use the terms interchangeably, making it difficult to address the actual problem. We explain each, and how you can prevent in future races.

Setting the stage – meet “Jack”

Here is an email I received from a runner who just finished the Houston marathon and really struggled. Because I don’t want to call him out, I removed identifying information and we’re going to call him Jack from here on.

….Long story short: I hit the wall and bonked at mile 17. I was ok with the cardio part, and not “super” tired or fatigued at mile 23, but my legs just couldn’t go faster. I started cramping at mile 12 even though I drank lots of fluids. My training was going well (I PRd at the half marathon about 5 weeks ago) so I am really struggling with why this happened.

Do I need to eat more carbs or take salt tabs or something like magnesium to prevent the cramps? Maybe I didn’t drink enough? Please help.

Clearly, Jack had a tough race (I am sure many of you reading this can relate – I know I can). In the end, Jack took a look back at how he felt and believes the issue was related to nutrition in some way.

However, when I look at Jack’s synopsis, I don’t think nutrition played a big role.

Specifically, I think Jack is using the terms “bonk” and “cramp” to describe the feeling of fatigue and fading he experienced at the end of the race without regard for their true, scientific meaning.

The result is he’s looking in the wrong place to solve his problems.

To help you better understand the factors at play when you struggle in a marathon, I am going to breakdown these common terms into what they really mean, why it happens, how to fix, and provide specific examples of what they might actually “feel” like.

After reading this, you should be able to better diagnose your marathon struggles and train to target and eliminate them at your next race.


What is bonking and why does it happen?

Bonking, also known as hitting the wall, is a term used to describe what happens when your body runs low on glycogen to burn as a fuel source.

While your body can burn fat directly for energy, it tends to prefer glycogen, as it is easier to burn and more efficient. Thus, when running at marathon pace, some portion of your energy output is going to come from burning glycogen – there’s no way around this.

As your glycogen stores begin to run 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 will begin to slow unless you increase your effort. However, if you continue, your glycogen stores will get so low that your body will basically shut down and even jogging will be almost impossible.

This is what’s called bonking.

Bonking is not feeling tired; bonking is not an inability to move your legs faster. Bonking is when your glycogen stores get low enough that your brain shuts down your body.

What does it feel like to bonk?

A “true” bonk will almost always result in you not being able to physical run any longer.

You may be able to shuffle and probably walk, but anything that resembles running is likely out the window. More than likely you’ll feel dizzy or light-headed (a result of your brain not getting the glycogen it needs) and some runners feel nauseous.

As you can see, this feeling is a bit different than fading or getting fatigued during the latter miles.

How can you prevent a true bonk?

You have to primary ways to prevent bonking.

First, you can slowly train yourself to burn fat more efficiently as a fuel source. This will enable you to burn less glycogen per mile at your marathon pace.

It’s important to remember that you cannot race a marathon using fat alone as a fuel source. If you run easy enough, sure, but if you’re pushing yourself it’s just not scientifically possible.

You have a few ways you can train your body to be more efficient at burning fat. One is to perform “fasted long runs”, which you can learn more about here. The other is to include marathon pace training during your long runs.

The second way to prevent a true bonk is to fuel yourself adequately before and during the race.

The trick here is that it’s just as bad to over fuel as it is to under fuel.

Your body can only process a finite amount of carbohydrate per hour (30-60 grams depending on your individual efficiency). If you try to take in more carbohydrates than you can handle, the digestive system starts to shut down and you don’t absorb anything.

So, how do you figure out exactly how much to take in?

First, use this calculation to determine how much glycogen you need during the race. Then, use this formula (at the bottom of the article) to determine how to divvy up your intake between fluids and gels. (Note: If you want us to do these calculations for you and provide a personalized fueling plan, check out our Nutrition Blueprint)

Using the above calculations and implementing the fat-burning long runs should solve your bonking problems for your next race.


How is fatigue different from bonking?

Now that we understand the true definition of bonking, we can start looking at what runners often “mislabel” as bonking – getting tired.

It may seem simplistic, but I think a lot of the struggles marathoners go through when they have a bad race can be attributed to fatigue.

I think we often forget that a marathon is a grueling event, even when you’re well trained. Three, four and even five hours of pounding the pavement is tough on the body and it’s almost always going to take its toll.

And while attributing your struggles to fatigue may sound oversimplified, it’s the first step in targeting the right elements in your training to prevent it from happening again.

What causes “marathon fatigue”?

Fatigue during the marathon is the same as any other race distance. The problem is that it’s magnified by at least twice as much thanks to the distance.

First, you have muscle damage, which can be quite significant during a marathon.

  • One scientific study conducted on the calf muscles of marathon runners concluded that both the intensive training for, and the marathon itself, induce inflammation and muscle fiber necrosis that significantly impaired muscle power.
  • Another study concluded that creatinine kinase (CK) damage – a marker that indicates damage to skeletal and myocardial tissue – persisted more than 7 days post marathon while another study confirmed the presence of myoglobin in the bloodstream post marathon for 3-4 days post race.

In short, when you’re racing the marathon, you’re significantly damaging your muscle fibers (see this study for some cool muscle biopsy photos post marathon).

This damage to the muscle fibers reduces their ability to produce the powerful contractions needed to maintain marathon pace effort. It also causes that soreness and dead-leg feeling you get late in a race. If you don’t significantly increase your effort, you begin to slow down.

Second, as you begin to increase you effort to make up for the muscle damage, you begin to produce more lactate (a by-product of anaerobic respiration) which interferes with your body’s ability to clear hydrogen and results in a build-up of acid in the muscles.

Likewise, the more effort you expend, the more you have to rely on glycogen as a fuel source (since it’s the most efficient form of energy). As discussed previously, this signals the brain to slow down to ensure survival, which means the brain is now also sending signals to slow down.

As you can see, this quickly becomes a triple whammy of fatigue that sets you down a path of fading during the final miles.

How can you better prepare yourself?

The challenge of preparing yourself for marathon fatigue is that running the full marathon distance in training is not recommended (due to how long it would take to recover).

So, we need to get creative in training to simulate the fatigue and develop the muscular endurance needed. To accomplish this, we can do two things:

We can implement what coaches call the theory of “accumulated fatigue”. Basically, this means that the fatigue from one workout accumulates and transfers to the next so that you’re always starting a workout or a long run a little tired from your previous training.

This type of training helps your develop the muscular endurance without needing to run the full marathon in training. An example of accumulated fatigue would running a steady run the day before a marathon-specific long run.

Second, using what we know about how muscle fibers work, you can implement specific workouts that are designed to fatigue your legs and muscle and then have you train and run at marathon pace. Some example workouts include:

Tailoring your marathon training to include accumulated fatigue and specific workouts can make a dramatic difference in how you feel during the later stages of the race.


Thanks to the millions of dollars funneled into the sports drink market every year, most runners blame their cramps on dehydration or lack of electrolytes. But, when you look at the research, it’s very clear that only a very small percentage of muscle cramps in runners are caused by fluid of electrolyte loss.

First, as noted by physiologist and MD Tim Noakes1, exercise cramps don’t occur exclusively on hot days. Even swimmers in very cold water can suffer from muscle cramping and there’s little high-quality research linking exercise in the heat to an increased risk of cramps.

In another study2, 43 Ironman athletes who developed cramps during the course of the race were followed. The data showed that neither body weight changes (which can estimate dehydration levels) nor blood electrolyte levels were correlated with suffering cramps during a race.

Finally, A 2005 study published in the Journal of Athletic Training3 had 13 athletes perform a series of exercises done in hot, humid conditions. During one trial, the men were given large volumes of sports drink with extra salt added in, and during the other, no liquids were consumed. Nine of the 13 men still got cramps even in the sports drink trial. Moreover, in the no-liquid trial, only seven men experienced cramps.

So if it’s not dehydration or electrolytes causing cramps, what is?

The cramps you’re likely experiencing during a marathon are called “muscle overloading” or a fatigue cram.

These occur when the neural mechanisms that are supposed to inhibit muscle contraction are depressed and the chemical and electrical synapses that fire the muscle fibers is enhanced.

But why does this happen during the marathon?

As we just learned, your muscles undergo significant stress when racing. As you get further into the race, the slow twitch fibers you’ve been using start to get tired and you can no longer fire them as efficiently.
As a consequence, you start to recruit some intermediate fibers to help maintain pace. Of course, these intermediate fibers require more glycogen and are not as fatigue resistant as slow-twitch, so it won’t be long before you start compensating.

As an example, as your glute muscle fatigues (one of the most important muscles when it comes to generating power from your stride), your leg won’t simply stop working. Instead, your brain tells your muscles, “hey, this glute isn’t getting the job done, let’s fire the calves more forcefully to make up for the lack of power.”

Your calf isn’t nearly as strong or powerful as the glute. Moreover, it’s likely you’ve never trained it to handle this type of stress. As a result, the calf cramps!

How to prevent cramping during the marathon

Now that we understand how muscle fatigue can lead to marathon cramping, how do we go about addressing the issue?

Specifically, how can we simulate the fatigue you’ll experience 20 miles into a marathon?

  1. Improve form and posture

Just like you need to perform core and injury prevention work to stay healthy, it’s important you perform specific strengthening exercises that target the mechanics that commonly deteriorate late in a race.

For example, if you tend to suffer from calf or quad cramps late in a race, you’ll want to perform exercises, drills and stretches that focus on improving your hip extension and posture (two very common culprits of bad form that leads to cramping).

Not only will this reduce many of the limitations that may be preventing you from generating proper hip extension, these exercises will help you improve your muscular endurance and ability to generate proper hip extension late in a race when you’re tired.

You can download a free sample of our hip extension specific routine here. It is available in PDF and Video format.

  1. Simulate late race fatigue in the gym

Bodybuilders have long employed a technique called split training, where they train the same muscle group twice per day. The goal is to use the morning session to lift heavy and maximize the recruitment of muscle fibers and thereby fatigue the muscles. They then come back for a second evening session where they use lighter weights and higher reps to blast their fatigued muscles. This stimulates tremendous growth.

We can use this principle and tweak it to better train our running-specific muscles to withstand the rigors of the full marathon distance.

First you perform a morning session that consists of heavy weights and low repetitions. These will be mostly compound exercises that focus on the hips, hamstrings, glutes and lower back. Our goal is to recruit maximum muscle fibers and fatigue the muscle.

You’ll then come back for an evening session and perform very running-specific hip, hamstring, glute and lower back exercises designed to train those muscles while they are tired.

This specific training should simulate the fatigue you experience late in the marathon race and prepare the muscle groups most responsible for a breakdown in running form.

This exact routine and more on the science behind it is available in our Strength Training for Runners program (it’s called the Phidippides routine). We don’t currently have a sample because it’s new, but I’ll be writing more on this soon (and providing some case studies).

If you are sure your cramping is due to electrolyte loss, we recommend EnduroPacks Electrolyte Spray as you can add it to any drink (or take it with you in your next marathon to spray in your mouth), and it contains no sugar or calories.


It’s also important to remember that these causes of marathon failure don’t occur in isolation. I am very certain all runners experience some amount of each in every race they run.

That means you shouldn’t ignore any element in preparation for your next race.

However, most runners find that it’s one particular factor that’s causing the most trouble.

Now that you better understand the science, the causes and appreciate the difference between all three you can better plan your training for your next race.

Were you using the terms incorrectly? Which of these have you experienced in a marathon?

Free Marathon Nutrition Course

The Simple 3 Step Calculation To Vanquish The Marathon Bonk

Here’s what we’ve got for you

How to calculate your exact glycogen storage and carbohydrate use so you know exactly how much you have to refuel

How to calculate your sweat loss and re-hydration rate to maintain optimal performance.

How to develop your individualized fueling plan using you sweat rate and glycogen usage.


Noakes, T. D., Waterlogged: The Serious Problem of Overhydration in Endurance Sports. Human Kinetics: 2012; p 448

Jung, A. P.; Bishop, P. A.; Al-Nawwast, A.; Dale, R. B., Influence of Hydration and Electrolyte Supplementation on Incidence and Time to Onset of Exercise-Associated Muscle Cramps. Journal of Athletic Training 2005, 40 (2), 71-75.

Schwellnus, M.; Drew, N.; Collins, M., Increased running speed and previous cramps rather than dehydration or serum sodium changes predict exercise-associated muscle cramping: a prospective cohort study in 210 Ironman triathletes British Journal of Sports Medicine 2011, 45, 650-656.

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2 Responses on “Bonking vs. Fatigue vs. Cramping: What You Need to Know

  1. Great article! Have a history of cramping in longer races. Whilst I have always focused on electrolyte intake, you have given me a new perspective and new ideas for better training. Thanks.

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