In endurance sports, particularly cycling and running, hitting the wall or the bonk describes the condition when an athlete suddenly loses energy and becomes fatigued, the result of glycogen stores in the liver and muscles becoming depleted.

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 recent DVD issued by the British Transport Films Collection contains several old films, one of which entitled "Cyclists Special", a colour 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".^[1]

The term can be used 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". It is also referred to as the "green grass" as pupil dilation causes vivid hues to appear.

Mechanisms

Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen:

• via fat metabolism and
• via breakdown of glycogen into glucose, followed by glycolysis.

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.

The average human body stores enough glycogen to generate 1500 to 2000 kcal of energy. Intense cycling or running can easily consume 600-800 or more kcal per hour. Unless glycogen stores are replenished during exercise, glycogen stores will be depleted after 2 hours of continuous cycling or 15 to 20 miles (24 to 32 km) of running.

Effects

Such fatigue can become seriously debilitating; in cycling, without the support provided by the bicycle, exhaustion can reach the point where the cyclist is unable to stand. Symptoms of depletion include general weakness, fatigue, and manifestations of hypoglycemia, such as dizziness and even hallucinations. This dangerous condition will not be relieved by brief periods of rest.

Avoidance

There are several approaches to prevent glycogen depletion:

1. 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.
2. 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.
3. Lowering the intensity of the exercise to the so-called 'fat loss' level (heart rate of 130 bpm for a 30-year-old athlete) will lower both the energy requirements per unit of distance and the fraction of the energy that comes from glycogen.

Bonk training

"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, 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.^[2]

See also

• Fatigue (medical)

References

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