You have heard it mentioned multiple times but still probably have no idea what it really means. Simply explained bonking or ‘hitting the wall’ is a phenomenon that occurs mainly in endurance sports or intense exercise, where glycogen (energy) stores in the liver and muscles are depleted causing exhaustion and loss of energy.

Basically glycogen is stored in the liver and muscles. During exercise glycogen in the muscles is burned directly for energy, and when this runs out the liver steps in producing blood sugar for energy. At most your liver can produce enough sugar to last 12 hours at rest, so during intense exercise when muscles are drawing sugar from your bloodstream at a rapid rate, the liver runs out of its stored sugar and your blood sugar level drops and you bonk.

Once the body has exhausted glycogen stores, it turns to fat as its next source, followed by protein. However these sources need oxygen to be utilised, and no athlete wants an increased demand on their oxygen supplies during exercise. It means you can’t generate the same level of energy, and you have to slow down. Also, the brain can only use glucose as an energy source, so once the glucose is gone you start feeling mentally fatigued, which is when you begin to doubt your ability to continue. Basically things start going rapidly downhill from a physical and mental perspective.

Whether you run, cycle, box or swim, bonking can be beaten! In the days before an event carb load, which packs muscle with glycogen especially if consumed straight after training. During an event or intense exercise consume carbs in the form of sports drinks/gels (remember liquids are absorbed quicker than solids), as this has been shown to replenish glycogen stores, allowing you to continue your exercise without the fatigue! And most of all, keep training - the fitter your body becomes, the more it uses fat for moderate tasks, leaving glycogen ready for use when you really need it!