In endurance sports such as cycling and running, hitting the wall (or the bonk) describes a condition caused by the depletion of glycogen stores in the liver and muscles, which manifests itself by sudden fatigue and loss of energy. Milder instances can be remedied by brief rest and the ingestion of food or drinks containing carbohydrates. The condition can usually be avoided by ensuring that glycogen levels are high when the exercise begins, maintaining glycogen levels during exercise by eating or drinking carbohydrate-rich substances, or by reducing exercise intensity.
The term bonk for cycling fatigue is presumably derived from the original meaning ‘to hit,’ and dates back at least half a century. The term is used colloquially both as a noun (‘hitting the bonk’) and a verb (‘to bonk halfway through the race’). The British may refer to it as ‘hunger knock,’ while ‘hunger bonk’ or ‘bunger honk’ was used by South African cyclists in the 1960s.
The Daily Omnivore 
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