Dear Runners,

As the long runs are getting longer, remember that they need to serve as practice for race days.  So pay attention to your pre, during and post race routines to make sure you have everything practiced before race day.   You need to think deliberately about what you are eating, drinking, wearing before during and after your runs.  Also pay attention to the temperature changes when you are running. I know that it is sooooo hot right now that there aren’t many changes, but remember that there are thermal differences between a 6 Am run and a 6 PM run.  Hopefully the fall races will feel more like the 6AM runs and you need to make sure you are used to all temperatures to perform your best. 

We are entering the phase of our training where our long runs are getting longer.  As the distances in those long runs increases the requirements of our bodies also change.  As those changes occur, we are also more inclined to experience a level of fatigue that may not have been experienced at the shorter mileages.  To help us have a better understanding of both the physical and mental components of fatigue this week’s and next week’s e-mail will focus on the psychological and physiological elements of fatigue and how we can impact those to make us better runners.  Then as we go into next week’s note we will look at specific mental techniques dealing with association and disassociation and how to use those in your training.  And of course, at the end of the note, I have a YouTube video. 

So let’s jump in…

What is Fatigue?

Fast runs (tempos, intervals, fartleks) require speed but long runs require endurance.  By definition, speed is expressed in the equation, distance traveled divided by time.  Technically speed is the absolute value of that equation, but that is another point.  However, endurance refers to the capacity to withstand unpleasant, difficult, or wearing situations without giving way. 

I read a quote recently from Will Smith (the actor) who talked about this concept.  Here is the quote:

“The only thing that I see that is distinctly different about me is I’m not afraid to die on a treadmill. I will not be out-worked, period. You might have more talent than me, you might be smarter than me, you might be sexier than me, you might be all of those things, you got it on me in nine categories. But if we get on the treadmill together, there’s two things: You’re getting off first, or I’m going to die. It’s really that simple, right?”

–        Will Smith

The human body can be trained and improved across both the dimensions of speed and endurance but is generally much more trainable for endurance than speed.  There are a number of equations to determine the ability to increase a runner’s speed based on his/her level of training, strength, previous performance and a number of other factors.  Similarly, there are equations to predict ability to train for endurance.  However, when the equations are solved, a person’s endurance can be increased by over 10,000 percent and that type of improvement just can’t be replicated in speed training. 

But even as endurance increases it stands to reason that runners will become fatigued…they will get tired.  This is particularly true at the end of long runs.  So the question becomes, what exactly is fatigue? Even though we have all felt fatigued and can recall the sensations it makes sense to look at the actual definition. 

Fatigue can be defined as “a mental and/or physical sensation of increasing difficulty in performing a given work load while maintaining previous efficiency.”  Running fatigue can be defined colloquially as “mental and/or physical pain that causes you to work harder to maintain the same speed or go slower with the same effort.”  Some believe that fatigue is a protection mechanism that shuts down or slows down the body to prevent injury or permanent damage.  Pain is thought to serve a similar purpose and can be a component of fatigue. 

There are several types of fatigue and understanding them helps to address their causes. 

Types of Fatigue

Glycogen Depletion – if you remember from the e-mail on energy systems, glycogen is the primary source of fuel for the muscles.  Glycogen from carbohydrate reserves combined with oxygen is converted to create ATPs. When the body runs low on carbohydrate then it must turn to other sources such as fat.  While fat is actually a richer source of ATPs, the conversion process is harder.  As a result, when glycogen reserves run low, the muscles can enter a state of fuel deficiency.  Of course a lot of this depends on how well your aerobic infrastructure has been trained over these last weeks and months because that determines how efficient your body is at converting fat to fuel.  Again…check the energy system note for the details.

So the short answer would seem to be that our bodies just run out of glycogen and fat.  But that doesn’t make sense really.  If you think about pure glycogen levels, our bodies can hold fuel for about two hours of intense exercise.  That is why carb loading and not taking fuel on a half marathon can work.  You may not produce your best time, but the physiology works.  However, what about the fat?  Well, it turns out that even the skinniest person reading this note has enough fat in their body to fuel several marathons. 

However, there is no way any of us could go out and run or walk the distance of several marathons without getting fatigued.  So there must be something more. 

Muscle Fatigue – The fibers in your muscle – called myofibrils – are continuously firing during running to contract and extend our muscles.  Specifically a muscle contraction is caused by an interaction between potassium inside your cells and sodium outside of your cells.  We know there are several by-products produced as part of the fuel conversion process used during running, including hydrogen ions, lactate and pyruvate.    While some of these components can be shuttled to the liver or other muscle cells for conversion or use this is not possible with all of them.  As these levels as well as the levels of calcium, potassium, and sodium change we can lose our physiological homeostasis.  As our bodies and muscles move farther away from that homeostatic state it stands to reason that the muscular abilities will erode.  Furthermore, we know that exercising muscles introduces micro-trauma and there is some evidence that this could also be a factor of muscle fatigue. 

This is also the least understood component of fatigue.  Even if you factor out all of these components of muscle fatigue it still doesn’t completely explain why muscles get fatigued. 

Also, if fatigue is thought of as a protection mechanism for the body and if fatigue was entirely physiological then we would never be able to sprint across the finish line after a long race.  Physiologically it just wouldn’t be possible.  So what else is there??? 

Mental Fatigue – It is no surprise that our brains control all of these components and our brains are capable of some remarkable things.  However, when we run, we spend a good deal of our time in a state of cognitive dissonance.  During this dissonance period our brain is receiving conflicting signals.  One signal says we want the mental and physical rewards that come from conquering a long run or race.  However, there is a conflicting physiological signal being sent by our bodies which says that we don’t feel good, and we want to stop. 

The Central Governor Model, introduced in the 1930’s argues that the reason we slow down and feel fatigued is because our brain is shutting off signals to different muscles or slowing down those signals to make us run slower. 

Our brain is essentially trying to resolve the dissonance and keep our bodies in a state of homeostasis.  If energy production is low, the brain tells us to slow down or get more energy, if a muscle is overused, the brain tells it to slow down, or tells another muscle to compensate often altering our gait.  So even though the signals may be physiological the response and governing mechanism is psychological. 

What gets me excited about the mental fatigue component and also the Central Governor Model is that we have a lot of control over what we think.  While we can’t change everything our brains do (nor would we want to) we can choose to push ourselves harder when things get tough.  This is why we can sprint at the end of a race, even though we are tired.  And also why that sprint feels so good. 

Brining it all Together

The bottom line is that there is ample evidence to support all of these explanations for fatigue but there are also a lot of unknowns.

The research says that fatigue is both mental and physical.  As a result – and this is really important – to be strong, you need to train the body and you also need to train the mind to tell the body to keep pushing and keep moving. 

Previous notes have touched on the physiological components of training and those will always be important.  You need to eat properly; the right foods at the right times.  You need to train your body properly to make it stronger, increase endurance and improve the efficiency of your energy systems.   

Next week we will talk about some specific tools including association and disassociation and how those can be used in training.  By the way, the most familiar form of disassociation I see in runners today is listening to music.  There are also other tools and they all have their strengths and weaknesses. 

For now, just know that when you feel fatigued it is important to look within to identify where that signal is coming from.  Are you low on fuel? Eat. Are your muscles sore or worn? Rest.  Are you just wanting a break and dealing with cognitive dissonance? Carry on. 


This video is titled “the most inspiring video you will ever watch!”  Clearly this guy pushed through a lot of fatigue to meet his goals…and if he can do it…you can too!   Enjoy…

Run well…


Shawn Herbig is an RRCA Certified Running Coach.  In his day job he owns a market research and data analytics firm called IQS Research. 

© 2013 Shawn Herbig