What Is Bonking In Cycling – And How Do You Avoid It?

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It’s happened to most cyclists at one time or another.

Your ride is going great, you’re enjoying the miles in the saddle, your legs feel strong, and your heart rate is under control.

But within minutes, everything changes. The wheels fall off – metaphorically speaking.

Suddenly, every pedal stroke feels ten times harder than the last. You feel tired, sluggish, weak, and mentally disengaged.

This is bonking.

But what is bonking scientifically speaking, and what causes it? And most importantly, how do you avoid bonking?

In this article, we’ll be covering: 

  • What Is Bonking In Cycling?
  • Why Am I Bonking On the Bike?
  • How to Avoid Bonking On the Bike

Ready to get to grips with this bonking meaning?

Let’s get started!

What Is Bonking Cycling: Title Image

What Is Bonking In Cycling?

Put simply, bonking in cycling refers to the experience of being suddenly and dramatically more fatigued.

Also referred to as “hitting the wall,” bonking in cycling is a phenomenon that occurs during endurance rides or long workouts as a result of depleting your muscle and liver glycogen stores. 

Essentially, bonking on the bike occurs because your body has run out of carbohydrates for fuel.

Why Am I Bonking On the Bike?

So, why does bonking happen during cycling?

To answer this question, let’s take a step back and take a look at the basic biochemistry that’s going on in your body during cycling. 

Whenever you do any form of exercise, including cycling, the muscles and heart need energy to contract and do work.

This energy, called adenosine triphosphate (ATP) is generated by breaking down stored fuel (primarily carbohydrates and fats) from the foods you’ve eaten and converting these nutrients into usable energy.

Glycogen is the storage form for carbohydrates in the body.

A cyclist pauses after bonking cycling.

This means that when you eat any food that contains carbohydrates, such as fruits, whole grains, legumes, vegetables, and so on, the carbohydrates are broken down into glucose and other simple sugars.

These simple sugars are released into the bloodstream for use.

Your brain, liver, skeletal muscles, and heart, for example, can take up glucose through receptors and metabolize it for energy.

Any glucose that is not needed right away by your tissues and organs can be shuttled to the muscles and liver for storage. There, the small glucose molecules are synthesized into larger molecules known as glycogen.

According to the Academy of Nutrition and Dietetics, an endurance-trained athlete, such as a long-distance cyclist, has the capacity to store up to 1800 to 2000 calories of energy as glycogen in their skeletal muscles and liver. 

Smaller cyclists might store closer to 1,500 calories worth of glycogen fuel.

Depending on your body size and workout intensity, this amount of stored glycogen may be able to fuel about 90 to 120 minutes of vigorous cycling.

A cyclist in a white jersey is passed out on an exercise bike.

This is often enough energy to support daily training rides and shorter workouts, especially if you’re doing a good job fueling during your workout and taking in the recommended 30 to 60 grams of carbohydrates per hour on the bike.

However, if you are doing a long training ride, competing in an Ironman triathlon, or testing yourself with a century ride, just having 90-120 minutes of carbohydrate-based energy isn’t going to suffice.

As your ride approaches these limits, you are essentially exercising in a glycogen-depleted state since these storage “silos” are basically tapped out.

Even if you try to stay on top of your fueling strategy during your ride by refueling with simple carbohydrates to top off your blood sugar every 15-30 minutes, you can find yourself bonking cycling when your glycogen stores are fully depleted.

Bonking is experienced as the dreaded feeling of sudden fatigue and heavy legs. Pedal strokes become much harder and your power output and speed tend to drop precipitously, or at least you have to fight way harder to keep up your power.

An exhausted cyclist leans on their exercise bike after bonking.

So, why does bonking occur in cycling? What is going on in the body?

As discussed, your muscles can generate the ATP (cellular energy) they need to allow you to keep cycling from carbohydrates and fats (and to a lesser degree, proteins).

Ultimately, the matter of burning either fuel comes down to the intensity of your workout. The harder you ride, the faster your heart and muscles need to produce energy to keep up with the need for enough ATP for your higher workload.

During exercise at intensities greater than 60% of your VO2 max (maximum oxygen consumption), blood glucose and muscle glycogen are the primary fuels being used to produce the necessary energy for your muscles because carbohydrates can be broken down for energy much faster than fats. 

This means that when burning glycogen for fuel, you are able to sustain a higher workload or higher intensity of cycling because the ATP is being produced fast enough to meet high demand from the heart and muscles.

In contrast, fat is oxidized more slowly, so your muscles can only keep up with generating energy at the rate they need it when you are cycling at a slower speed, or low intensity. 

This is why your cycling speed and power output can drop drastically once your glycogen stores are empty – your muscles are forced to switch to burning fat for energy and this is a slower process. 

What Is Bonking In Cycling - And How Do You Avoid It? 1

How to Avoid Bonking On the Bike

Some amount of glycogen depletion is going to occur while doing long endurance rides because your muscles are going to be burning glycogen for fuel.

However, there are a few strategies you can use to help avoid bonking while cycling.

#1: Take In Carbohydrates On the Bike

The most important thing you should do to prevent bonking while cycling is to try and keep up with your carbohydrate needs on the bike.

This will help keep your blood glucose levels topped off, so that your muscles don’t have to dig into the glycogen stores as readily, enabling the glycogen to last longer during your ride before becoming fully depleted.

According to the American College of Sports Medicine (ACSM), you should aim to ingest 30–60 grams of carbohydrates per hour during endurance exercise lasting 1-3 hours or more. 

More isn’t necessarily better. Evidence suggests that the maximum rate of carbohydrate absorption during exercise is 60 grams per hour. Therefore, sticking to the 30-60 grams/hour recommendation is a good idea.

As there are 4 calories per gram of carbohydrates, 30-60 grams works out to 120-240 calories of carbohydrates per hour.

#2: Try Carb Loading

Carb loading – also called “glycogen supercompensation” – is a dietary protocol that involves first depleting your glycogen stores by following a low-carbohydrate diet for 2-3 days, and then following a very high-carbohydrate diet for 2-3 days leading up to a big endurance event.

The purpose of carb loading is to increase your muscle glycogen storage above your baseline levels so that you have a bigger glycogen reserve to work with (greater than that 90-120 minute range) before bonking.

According to the Mayo Clinic, you should aim for 8-12 grams of carbohydrates per kilogram of body weight for 1-3 days before the event.

A cyclist on a white road bike climbs a steep hill.

#3: Train Your Body to Become More Fat Adapted

It’s also possible to train your body to become better at oxidizing fat at higher intensities of exercise.

Muscles are “metabolically flexible”, meaning they can use carbohydrates or fat for fuel. While carbohydrates are the preferred fuel source at high-intensity exercise, you can try to improve the ability of the muscles to burn fat more efficiently so that fat can supply energy at higher workloads.

This primarily involves performing glycogen-depleted rides. 

To do a glycogen depletion ride, the day before your targeted ride, perform a moderately-difficult workout. Eat a low-carb dinner the night before your glycogen-depletion workout.

Do not eat carbohydrates before or during your long ride. You should fuel with fats such as nut butter and/or protein and drink plenty of water.

Expect to feel tired and sluggish. The idea is that doing these types of workouts occasionally can help your body become better able to oxidize fat at higher intensities of exercise.

It’s important to refuel afterward as soon as possible with plenty of carbohydrates, protein, and fluids.

Overall, though, you should be able to avoid bonking on the bike by following a well-balanced diet, fueling well before and during your workouts, and training your body to become better at burning fat by doing long rides.

Found this bonking meaning guide helpful? Check out more from the BikeTips experts below!

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Amber is a fitness and nutrition writer and editor, and contributes to several fitness, health, and cycling websites and publications. She holds two Masters's degrees - one in Exercise Science and one in Prosthetics and Orthotics. As a certified personal trainer for 12 years, Amber enjoys staying active and helping others do so as well.

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