Understanding Bonking in Endurance Cycling: Scientific Insights

"Bonking," colloquially known as "hitting the wall," is a well-recognized phenomenon in endurance sports such as cycling. Scientifically, it refers to a state of acute glycogen depletion resulting in central and peripheral fatigue. The sudden onset of extreme exhaustion, impaired cognitive function, and muscular dysfunction marks a physiological failure to maintain metabolic demands. This article dissects the biological underpinnings of bonking, clinical symptoms, and evidence-based recovery strategies.

What is Bonking?

Bonking is a metabolic crisis that occurs when glycogen stores in the liver and skeletal muscles are depleted. Glycogen, a polysaccharide of glucose, serves as the primary and rapidly mobilized energy source during prolonged moderate to high-intensity exercise. Once these stores are exhausted, the body becomes reliant on slower, less efficient metabolic substrates such as free fatty acids, leading to a dramatic decline in power output and cognitive performance (Cermak & van Loon, 2013).

Key Symptoms of Bonking

1. Extreme Fatigue: Depletion of muscle glycogen impairs ATP synthesis, reducing contractile force (Bergström et al., 1967).
2. Dizziness and Lightheadedness: Associated with hypoglycemia—a drop in circulating blood glucose—which impairs cerebral function.
3. Cognitive Dysfunction: The brain's heavy reliance on glucose renders it vulnerable; symptoms may include confusion, irritability, and impaired decision-making (Nybo, 2003).
4. Muscle Weakness: With glycogen gone, muscles struggle to oxidize fats at the rate required for intense efforts.
5. Elevated Heart Rate: Compensatory sympathetic activation results in tachycardia as the body attempts to sustain oxygen delivery.
6. Cold Sweats and Chills: Catecholamine release and impaired thermoregulation contribute to clammy skin and subjective chills.

The Science Behind Bonking

As glycogen levels decline:

• Muscle ATP production drops, compromising performance.
• Blood glucose falls, impairing central nervous system output.
• Hormonal responses shift, with increased cortisol and epinephrine attempting to mobilize alternate fuels (Hackney, 2006).
• Fat oxidation becomes dominant, but this system is inherently slower and less oxygen-efficient.

Fatigue is both central (brain) and peripheral (muscle), and while fats offer abundant energy, their slow kinetics cannot match the immediate demands of high-intensity cycling (Romijn et al., 1993).

Recovery Protocol After Bonking

Immediate and deliberate action is essential to halt further physiological deterioration and promote recovery.

Step 1: Cease Exercise

• Continuing activity exacerbates the energy deficit and delays glycogen resynthesis.
• Find shelter or stop safely to rest and allow the autonomic nervous system to recalibrate.

Step 2: Rapid Carbohydrate Refeeding

• Ingest 20–40 g of high-GI carbohydrates immediately (e.g., glucose gels, sports drinks, or ripe bananas).
• Liquids may be more tolerable during nausea or dizziness and are absorbed faster.

Step 3: Hydration with Electrolytes

• Include sodium (300–600 mg/L) and potassium (100–200 mg/L) to restore plasma volume and facilitate glucose transport.
• Avoid plain water, which may dilute electrolytes further and contribute to hyponatremia if overconsumed.

Step 4: Structured Post-Ride Meal

• Within 30–60 minutes: consume 1–1.2 g/kg carbohydrates plus 0.3 g/kg protein (Ivy et al., 2002).
• Meal examples: rice or pasta with lean meat and vegetables, or a smoothie with banana, protein, and oats.

Step 5: Recovery Time and Sleep

• Glycogen supercompensation can take 24–48 hours. Avoid intense sessions the next day.
• Sleep supports hormonal recovery, glycogen restoration, and cognitive recalibration.

Prevention Strategies

Preventing bonking involves proactive fueling:

• Pre-load carbohydrates: 1–4 g/kg carbohydrate 3–4 hours before long rides (Burke et al., 2011).
• During Exercise: Consume 30–90 g/h of carbohydrates depending on duration and intensity. Use glucose-fructose blends for higher absorption capacity (Jeukendrup, 2010).
• Gut Training: Gradually increase carb intake during training to adapt gastrointestinal tolerance (Costa et al., 2017).

Can You Bonk in Training?

Yes. Training in a fasted or low-glycogen state, common in "train-low" strategies, increases the likelihood of bonking. While this method may have some benefits for mitochondrial biogenesis, it compromises performance and increases mental fatigue risk if used improperly (Impey et al., 2016). Only experienced athletes with sound nutritional support should incorporate such sessions under professional guidance.

Conclusion

Bonking is not simply running out of energy—it is a complex metabolic collapse with systemic consequences. Understanding its mechanisms and implementing sound nutritional planning is essential for any endurance cyclist. Prevention remains the best strategy, but when bonking strikes, rapid intervention is key to recovery.

References

1. Bergström, J., et al. (1967). Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica, 71(2–3), 140–150.
2. Burke, L.M., et al. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(S1), S17–S27.
3. Cermak, N.M., & van Loon, L.J.C. (2013). The use of carbohydrates during exercise as an ergogenic aid. Sports Medicine, 43(11), 1139–1155.
4. Costa, R.J.S., et al. (2017). Gut training: the missing link in nutrition strategies to optimize endurance performance? Sports Medicine, 47(1), 1–12.
5. Hackney, A.C. (2006). Stress and the neuroendocrine system: the role of exercise as a stressor and modulator of stress. Expert Review of Endocrinology & Metabolism, 1(6), 783–792.
6. Impey, S.G., et al. (2016). Fuel for the work required: A theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Medicine, 46(6), 961–970.
7. Ivy, J.L., et al. (2002). Early post-exercise muscle glycogen recovery is enhanced with a carbohydrate–protein supplement. Journal of Applied Physiology, 93(4), 1337–1344.
8. Jeukendrup, A.E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452–457.
9. Nybo, L. (2003). CNS fatigue and prolonged exercise: effect of glucose supplementation. Medicine & Science in Sports & Exercise, 35(4), 589–594.
10. Romijn, J.A., et al. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology-Endocrinology and Metabolism, 265(3), E380–E391.