Bonking is a term used by cyclists to describe a sudden state of extreme fatigue. It is characterized by heavy legs and a significant drop in energy levels, making pedaling feel nearly impossible. 

In this article, we will explain the symptoms of bonking, the underlying causes, and effective strategies for quick recovery and prevention during future rides.

What is Bonking in Cycling?

Bonking, commonly referred to as "hitting the wall," is a cyclist's worst nightmare. It happens when the body's glycogen stores (its primary energy source) have been depleted during prolonged or intense exercise. 

This depletion results in a significant drop in energy levels, leaving you feeling weak, shaky, and unable to maintain your performance. 

Symptoms of bonking include:

• Extreme fatigue

• Dizziness and even 

• Confusion, which can make it difficult to continue riding.

Bonking typically happens when you do not adequately fuel your body before or during a ride. 

To prevent it, you should consume a balanced mix of carbohydrates, proteins, and fats prior to starting your ride and replenish your energy levels with snacks or sports drinks during longer rides. Additionally, proper hydration is crucial for avoiding bonking.

What Bonking Feels Like

I'll never forget the day I bonked (massively) on a long 185km group ride last year through the Japanese mountains and around Lake Biwa. 

I remember pedaling hard; my legs were burning but manageable when, suddenly, it all fell apart. One moment, I'm gliding; the next, it's as if someone drained my energy in an instant. My legs felt like they weighed a ton, and each pedal stroke was a monumental effort. 

My heart pounded like a drum, and my vision blurred as if I were looking through foggy sunglasses. I couldn't think clearly, simple decisions felt impossible, and frustration built.

Hunger hit me like a freight train, an empty, hollow feeling that gnawed at my stomach. My hands trembled on the handlebars, and sweat dripped down my face, though my skin felt clammy and cold. 

Every kilometer seemed to go forever, my body begging to stop. I was trapped in a cycle of exhaustion, counting the seconds until I could rest. It felt like my body had betrayed me, leaving me utterly powerless. 

Bonking isn't just physical; it's an emotional, humbling reminder of the limits of human endurance. 

The Main Symptoms of Bonking

Extreme Fatigue

When you "bonk," your body runs out of glycogen, which is the primary fuel source for muscles during exercise. Without glycogen, muscles cannot generate enough energy, leading to an overwhelming sense of exhaustion and significantly reduced power output.

Dizziness and Lightheadedness  

Low glycogen levels often coincide with a drop in blood glucose (hypoglycemia). This deprives the brain of its primary energy source, resulting in dizziness, lightheadedness, and even a feeling of detachment or imbalance.

Muscle Weakness  

Without glycogen, your muscles struggle to contract efficiently. Your body attempts to shift to fat oxidation for energy, but this process is slower and less effective, resulting in noticeable weakness and a lack of coordination.

Irritability and Confusion  

Your brain relies heavily on glucose to function. When blood sugar drops, cognitive abilities are impaired. You may (as I did) experience mood swings, irritability, and difficulty focusing or making decisions. This "brain fog" can make even simple tasks seem daunting.

Rapid Heart Rate  

In response to energy depletion, the body activates the sympathetic nervous system to compensate. This leads to an increased heart rate as the cardiovascular system works harder to deliver oxygen and attempts to mobilize fat stores for energy.

Sweating and Cold Skin  

Bonking can disrupt your body's ability to regulate temperature. The combination of low blood sugar and stress hormones, such as adrenaline, may cause sweating and clammy skin, often accompanied by chills or a sensation of being cold.

The science behind bonking

Bonking happens when your body runs out of glycogen stores, which are essential sources of energy found in the liver and muscles. Glycogen acts as a quick-access fuel for high-intensity physical activity. Once these stores are depleted, your body switches to fat oxidation, a slower process that cannot meet the immediate energy demands of intense exercise.

As glycogen levels drop, blood glucose levels also decrease significantly, leading to hypoglycemia. The brain, which relies almost solely on glucose for energy, starts to malfunction. 

This can result in cognitive issues such as:

• Confusion

• Irritability, and

• Poor decision-making.

Hormonal changes also occur during bonking, triggering a series of stress responses. The adrenal glands release cortisol and adrenaline to mobilize energy from fat and protein stores. However, this response is often inadequate to sustain the high energy output needed for activities like cycling, which can intensify the symptoms.

Moreover, the body struggles to maintain homeostasis. Reduced energy availability impacts everything from muscle contractions to temperature regulation, resulting in symptoms such as cold sweats and shaking. 

How Long Does a Bonk Last

A cycling bonk can last anywhere from 30 minutes to several hours, depending on how quickly you refuel. Consuming fast-digesting carbs like gels, bananas, or sports drinks can help you recover faster, but full recovery may take longer if glycogen stores are severely depleted.

How to Recover

Following the steps below will help you recover efficiently and prevent prolonged fatigue or reduced performance in future rides.

1. Stop and Rest: As soon as you experience symptoms of a bonk, such as extreme fatigue, dizziness, or mental fog, stop riding immediately. Continuing will only worsen your condition and delay recovery.

   
   

2. Refuel Quickly: Your body is in urgent need of energy, so focus on consuming fast-digesting carbohydrates that can quickly replenish your blood sugar levels. Ideal options include energy gels, bananas, or sports drinks that provide easily absorbed glucose. Aim to consume 20-40 grams of carbs right away.

   
   

3. Hydrate: Dehydration often accompanies a bonk. Drink water or a sports drink with electrolytes to replace lost fluids and essential minerals like sodium and potassium, which are crucial for muscle function and recovery. 

   
   

4. Eat a Balanced Post-Ride Meal: After your immediate refueling, have a meal rich in carbohydrates, protein, and healthy fats. Carbohydrates help restore glycogen stores, protein aids in muscle repair, and fats provide longer-lasting energy. A good example is rice or pasta with lean protein and vegetables.

   
   

5. Rest and Recover: Your body requires time to fully recover, especially after severely depleting glycogen. Aim to rest for at least 24-48 hours before attempting another intense workout, and prioritize sleep to quicken your recovery.

Can You Bonk while Training?

Bonking, or "hitting the wall," can occur during training, particularly during prolonged or intense workouts when energy expenditures surpass fuel intake. This phenomenon is more likely to happen if you start your training with depleted glycogen levels, miss meals, or do not sufficiently refuel throughout the session. To prevent bonking, it is crucial to prioritize proper nutrition and hydration.

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