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".
The term is used colloquially both as a noun ("hitting the bonk") and a verb ("to bonk halfway through the race"). The condition is also known to long-distance (marathon) runners, who usually refer to it as "hitting the wall". The British may refer to it as "hunger knock," while "hunger bonk" or "bunger honk" was used by South African cyclists in the 1960s.
It can also be referred to as "blowing up". 
Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen:
How much energy comes from either source depends on the intensity of the exercise. During intense exercise that approaches one's VO2 max, most of the energy comes from glycogen.
A typical untrained individual on an average diet is able to store about 380 grams of glycogen, or 1500 kcal, in the body, though much of that amount is spread throughout the muscular system and may not be available for any specific type of exercise. Intense cycling or running can easily consume 600-800 or more kcal per hour. Unless glycogen stores are replenished during exercise, glycogen stores in such an individual will be depleted after less than 2 hours of continuous cycling or 15 miles (24 km) of running. Training and carbohydrate loading can raise these reserves as high as 880 g (3600 kcal), correspondingly raising the potential for uninterrupted exercise.
Such fatigue can become seriously debilitating; in cycling, exhaustion can reach the point where the cyclist is unable to stand without the support provided by the bicycle. Symptoms of depletion include general weakness, fatigue, and manifestations of hypoglycemia, such as dizziness and even hallucinations. This condition will not be relieved by brief periods of rest.
The rising levels of serotonin are caused by increased delivery of tryptophan to the brain. What’s interesting, Davis says, is that the increase in free tryptophan in the blood is very much related to the increase in free fatty acids in the blood. “While many people believe that the increase in free fatty acids is very important to delaying fatigue in the muscle,” says Davis, “we think it has a negative effect in terms of central fatigue.”
To make matters worse for the marathon runner, the brain’s production of dopamine (the neurotransmitter responsible for generating feelings of excitement, reward, motivation, and pleasure) begins to drop even as serotonin levels are rising, which can be the cause of the mental voices in your head telling you "I can't do it" according to Luis Manzo, Ph.D., a sports psychologist and running coach in New York City,which he wrote in Against the wall
There are several approaches to prevent glycogen depletion:
- Carbohydrate loading is used to ensure that the initial glycogen levels are maximized, thus prolonging the exercise. This technique amounts to increasing complex carbohydrate intake during the last few days before the event.
- Consuming food or drinks containing carbohydrates during the exercise. This is an absolute must for very long distances; it is estimated that Tour de France competitors receive up to 50% of their daily caloric intake from on-the-bike supplements.
- Lowering the intensity of the exercise to the so-called 'fat max' level (Aerobic Threshold or "AeT") will lower the fraction of the energy that comes from glycogen as well as the amount of energy burned per unit of time.
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Bonk training is an exercise program designed for weight loss. It suggests the following strategy: on an empty stomach the first thing in the morning, when glycogen store levels are low, consume coffee or caffeine equivalent to 2 or 3 cups of coffee, run or cycle at a casual pace (60% of max heart rate) for 20–90 minutes, and have a normal breakfast right after exercising. Proponents claim this will force the body to "bonk" shortly into the exercise, and subsequently burn more fat to generate the energy. It is not clear how medically sound this idea is; exerting too much energy and "bonking hard," or experiencing severe hypoglycemia, can be dangerous.
- See Britain by Train (DVD) 2. British Transport Films Collection 2005, Volume Two, Disc One, Track Three. 2005. Unknown parameter
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- "The Science of Carbohydrate Loading".
- marathon and beyond
- J. Mark Davis, PhD