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Bonking: This is why you hit the wall

Bonking or hitting the wall during a marathon is a major problem for a majority of marathon runners, maybe you too? 

In a survey on marathon runners from 2008, 43 percent of all runners replied, that they had bonked during the marathon race.

In this two-part blog series we'll first explore the main causes for bonking. What do recent scientific studies have to say. 

Then in part-two you will get specific tips on how to avoid hitting the wall in the future. You'll get 5 specific tips on how to keep your pace high and consistent through the whole marathon.

Bonking: The 3 classic causes

It is essential that we know which psychological and physiological mechanisms, that causes marathon fatigue.

Only by knowing these mechanisms, we are able to make the correct decisions on how to fight bonking.

The 3 causes of fatigue most frequently mentioned are: 

  1. Glycogen (sugar) depletion in our muscles
  2. Low levels of blood sugar
  3. Dehydration 

The real question is:

Can modern research support these claims? Or have they become entrenched myths without solid scientific evidence? 

1: Sugar or glycogen depletion in our muscles

For many years bonking has been synonymous with depleted glycogen stores in our muscles. Especially the muscles we use during running.

When emptying our glycogen stores, the body must switch to fat and protein burning. Both of these energy processes influence our running pace negatively. We simply have to slow down.  

However, this view has been challenged quite consistently in more recent research. 

Essentially two things decide whether or not depletion of our glycogen stores can be a reason for bonking. 

  1. The size of your glycogen stores
  2. The use of glycogen during running 

In a theoretical review by Rapoport, the researcher concludes that depleted glycogen stores can be a cause for hitting the wall. The reason being that too much glycogen is being used compared to the size of the stores. 

However, the chance of that happening is incredibly small, and for most marathon runners such a scenario is unlikely. 

It requires that we run a marathon at 85 percent of our max heart rate, and at the same time "forget" to carbo-load before the race. 

There are few non-elite runners  who are capable of running a whole marathon at 85 percent of max heart rate.

With all the focus on carbo-loading strategies, we can practically rule out that failing to eat enough carbohydrate is a real problem for marathon runners. 

Now theory is one thing, what practically happens is another. 

In our study from 2007 on danish marathon runners (2:18 to 2:54 in finishing times), we found, that even after they had finished the marathon, 60 percent of their energy came from carbohydrates. 

As you might have guessed, the runners weren´t exactly full of energy. In fact, only one runner managed to finish the 8 minute trial at marathon pace.  

In conclusion, the feeling of bonking does not seem to come from alarmingly low glycogen stores in our bodies.  

2: Low blood sugar levels

In order to function properly our brain needs a little bit of sugar. This bit of sugar comes from our blood.

A low blood sugar level means, that our brain and nervous system are unable to drive muscle function due to inefficient signalling. 

The blood sugar concentration is held constant through our liver glycogen store. This store contains only 100-140 gram of glycogen.

During stressful exercise like a marathon, the liver can release approximately 1 gram of glycogen pr. minute, enough for 1½ to 2 hours of work.

It is therefore theoretically possible to hit the wall due to low blood sugar levels.

But is this a realistic real-life scenario?

In a Spanish study from 2013, the researches found no link between the blood sugar level and a loss in tempo over a marathon. 

On the contrary, the blood sugar level was higher post-race than pre-race. Why? Probably due to an excessive intake of energy products during the race.

Since energy products are available at all major marathon races, and there is a lot of attention on energy intake alltogether, we must assume, that low blood sugar levels is not a primarily cause of bonking for a majority of runners.

3: Dehydration

For many years experts in marathon running have been saying that it is important for you to drink in order to avoid detrimental health and performance issues. 

The reason being that a two percent loss in body weight leads to a loss in performance by 20 to 30 percent. 

The advice is based on a number of elder studies that showed a significant reduction in running performance due to dehydration.

Fallowfield found a 33 percent reduction in time to exhaustion at 70 percent of VO2max in runners with a 2 percent body weight loss. 

There is just one issue. No marathon or no race for that matter is run as an open trial. The marathon is a fixed distance.

Thus, recent studies with a more realistic study design have indeed come to different conclusions.

New research on marathon and half-marathon runners does not show any link between dehydration and a loss in performance. 

Furthermore, a study on elite runners showed that the winners of these races had lost more than 8 percent of their pre-race body weight. 

It doesn´t mean you should stop drinking or that hydration has no impact on your marathon performance. 

But, provided you have hydrated fully prior to the marathon, overdrinking during the marathon isn´t going to prevent you from bonking.  

Bonking: 3 alternative (new) causes

So, even if we have fueled and eaten well before and during our marathon, it doesn´t necessarily prevent us from bonking. 

There must be something else, that slows us down. 

Fatigue in our nervous system including our brain is one such cause. 

Our brain and nervous system can fatigue due to 3 reasons: 

  1. A reduction in brain signaling to our muscles and tendons
  2. A reduction in the ability of our muscles and tendons to receive and react on brain signals
  3. A reduction in the efficiency of our stretch reflex

If one or more of these causes are met, it will make us bonk during a marathon. 

Let's take a look at modern scientific research in order to see, whether or not you should take neuromuscular fatigue seriously. 

1: A reduction in brain signalling to our muscles and tendons 

By combining voluntary force output with electrical stimulation of our muscles (The superimposed twitch technique), it is possible to test for brain fatigue after prolonged exercise like the marathon.

One study by Nicolas Place from 2004 showed, that a 5 hour run lead to a significant reduction in force output in well-trained runners and triathletes. 

This reduction in force output could be a result of reduced brain signalling to the muscles, because the muscles were able to produce significantly more force, when electrically stimulated.  

In an older but similar study from 1986 by Guillaume Millet, a similar result was seen over a 30 km trial on runners.

Interestingly, studies on non-impact sports like nordic cross country skiers and cyclists, doing the same amount of endurance work, do not show the same results. 

Therefore, it seems that it is the nature and length of the exercise more than solely the length of the exercise that decides whether the brain decides to shut down. 

One possible explanation is that the muscle pain we experience in the latter part of a marathon tells the brain to reduce signalling in order to protect the muscles.

It does seem that a reduction in nerve signalling to the muscles could be a possible cause for bonking, especially if our muscles are not well prepared for the stress. 

However, it should be said, that no studies (to my knowledge) have found a direct link between bonking and a reduction in brain signalling.     

2: A reduction in the ability to receive brain signalling  

During a marathon run our body is heavily loaded due to the many ground impacts. In fact, a 75 kilo runner needs to withstand an amazing 5-6 million kilos during a marathon.

This load can damage our muscles significantly.  

When a muscle is damaged to a smaller or greater extent, it will impair its ability to function properly e.g. receive and act on brain signalling.   

In a Finnish study from 2000, blood creatine kinase values increased after a marathon - a strong indicator of muscle damage. 

But it wasn't until 2013, that a study by Juan del Coso showed a direct link between muscle damage and bonking. 

40 marathon runners were divided into two groups.

The first group of 22 were able to keep a fairly constant pace from the first 5 k to the end of the marathon. 

The second group of 18 had to reduce their pace by more than 15 percent from the first 5 k to the finish.

The group, that bonked, had a higher concentration of markers for muscle damage in their blood. This was the only significant difference between the two groups.

Now, research on muscle damage as a potential cause for bonking is relatively new and concluding based on a few studies in the area is premature. 

Together with my previous point on reduced brain signalling, it does make sense that muscle damage/pain is somehow involved in marathon bonking.

3: A reduction in stretch reflex efficiency

During the landing phase of our running strides elastic energy we accumulate so-called elastic energy that is realised when our foot leaves the ground.

This cycle is called the stretch-shortening cycle (SSC) and can be compared to the actions of a rubber-band.  

As elastic energy equals free energy, the more elastic energy we can get out of a running stride the better. 

The SSC is not a constant. It can be improved through training e.g. plyometrics. But it can also acutely worsen due to fatigue.  

When that happens, we need to use more of our biological engine to make sure we keep our pace. 

No scientific studies have yet to establish a direct link between hitting the wall and a reduced SSC. However, there is indirect evidens that a deteriorated SSC can contribute to bonking.

In our study from 2007, the marathon runners had to use more oxygen to keep the pre-race pace after the marathon, presumably because the SSC had worsened. 

Likewise, muscle damage can also impact the SSC negatively. When we know that muscle damage occurs in runners who hit the wall, the link between bonking and an impaired SSC is evident.

Summary: How to fit the pieces of the puzzle

Recent scientific studies have shown, that it is time to expand our understand of why bonking in a marathon occurs. 

While it is still strongly advised to eat well and drink in order to perform at our best, correct nutrition and water intake alone is not going to prevent us from hitting the wall. 

If our focus is solely on nutrition, there is a great risk that we might miss out on other more prominent causes of bonking such as our neuromuscular system.

This is key, because if we do not attribute bonking to the correct causes, how can we prevent it from happening in the future? 

Based on new research, I have formulated specific tips on how to minimize the risk of hitting the wall in the future. 

You can read these tips in part II of this series on bonking in a marathon. 

Kim Petersen
 

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