What does it mean to bonk or hit the wall during a run or a bike ride?
In endurance sports such as cycling and running, "hitting the wall" or the "bonk" is a condition of sudden fatigue and loss of energy which is caused by the depletion of glycogen stores in the liver and muscles.
Can this can be a fairly serious thing, so what are the effects of this that makes it so serious?
From a logistical perspective, on a bicycle you are at most risk. You could lose balance, swerve into the road, or fail to be able to un-clip from your pedals. In this case the risks vary from a bruised ego to death by oncoming lorry.
From a scholar perspective you will have a lower power output:
In one study, "reduction in preexercise muscle glycogen from 59.1 to
17.1 mumol X g-1 (n = 3) was associated with a 14% reduction in maximum power output but no change in maximum O2 intake; at any given
power output O2 intake, heart rate, and ventilation (VE) were
significantly higher, CO2 output (VCO2) was similar, and the
respiratory exchange ratio was lower during glycogen depletion
compared with control." (1)
Is bonking the same as hitting a wall?
Yes. The term bonk for fatigue is presumably derived from the original meaning "to hit".
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" was used by South African cyclists in the 1960s. It can also be referred to as "blowing up".
Please explain what causes a bonk to occur, what things you would experience before, during, and after.
Athletes engaged in exercise over a long period of time produce energy via two mechanisms, both facilitated by oxygen:
- via fat metabolism
- via breakdown of glycogen into glucose, followed by glycolysis.
When you run out of glycogen you are operating under fat metabolism only and your power output will decrease. You will feel like you have rapidly lost a lot of energy over a short period (1 mile running or so, varies by individual).
Afterwards you still feel like you have no energy but this is where the mental ruggedness/determination of endurance running becomes more important.
Are there benefits to bonking?
Technically yes, you are more effective at burning fat after bonking because it is all your have left.
However, generally no. Your body is not designed to operate without glycogen for an extended period and thus you should not aim to bonk during exercise.
How can I prevent bonking?
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.
(1): Heigenhauser, G. J.; Sutton, J. R.; Jones, N. L. "Effect of glycogen depletion on the ventialatory response to exercise". Journal of Applied Physiology. American Physiological Society. 54 (2): 470–474. ISSN 1522-1601.
