Hitting the wall
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 glucose levels during exercise by eating or drinking carbohydrate-rich substances, or by reducing exercise intensity.
Etymology, usage, and synonyms
The term bonk for cycling fatigue is presumably derived from the original meaning "to hit", and dates back at least half a century. A 2005 video issued by the British Transport Films Collection contains several old films, one of which, entitled "Cyclists Special", a color film produced in 1955, tells the story of a party of cyclists touring the English countryside. At one point they stop for refreshments and the film's commentator states that if they didn't rest and eat they would get "the bonk".