The Elements of Fatigue – Running Fatigue Made Simple

By Rick Morris

Most runners really hate fatigue. They consider it the equivalent of a big, ugly, four headed, slime coated monster that jumps onto their back at a most inconvenient time and ruins their race or training run. In their opinion, fatigue has absolutely no redeeming values. Maybe I’m crazy, but I look at fatigue a little bit differently. I consider fatigue one of my best friends. I think that fatigue serves a valuable and necessary service to me both as a runner and a biological entity. Before you call the men in the white coats and have me placed in a straight jacket, let’s really look at what fatigue is.

A common dictionary definition of fatigue is: “A temporary loss of strength and energy resulting from hard physical or mental work.” That sounds like a reasonable description of fatigue. A definition more specific to running, from “Better Training for Distance Runners” by David Martin and Peter Coe, is “…a sensation of increasing difficulty in performing at a given work load while maintaining previous efficiency”.

I think Martin and Coe nailed the definition of running fatigue with that description. The only change I would make to their definition is to add the words mental and/or physical to the phrase, making it “A mental and/or physical sensation of increasing difficulty in performing at a given work load while maintaining previous efficiency”.

There’s a reason I would add the term mental to the definition. It’s the same reason I referred to fatigue as a four headed monster. Fatigue is more than just physical muscle weariness. There are really four separate but highly interrelated types of fatigue – muscle fatigue, metabolic fatigue, energy depletion and central nervous system (CNS) related mental fatigue. There’s also a reason I said I consider fatigue my friend. No – I not a masochist, it’s just that I have learned the purpose and value of running fatigue. Part of the value of fatigue is philosophical and part is physical.

Philosophically speaking, I believe that fatigue is critical for most runners and all competitive runners. Without fatigue there would be very little reason or motivation for running. My primary reason for running is to push myself to do something that is difficult to accomplish. That is the goal in nearly any endurance event. Distance running is hard, and it should be. After all, if running a marathon were easy, everyone would do it. Then, what would be the point.

So without fatigue, would there be any sport of running? Sure – there would still be fitness runners, who run only for weight loss and fitness, but would there be any actual sport of running. Would the marathon even be an event? I sincerely doubt that it would.

Physically speaking, our bodies are remarkably efficient. There are no processes without purpose – no wasted energy or motion. There is a reason for every action our bodies take, including the process of fatigue. Fatigue is a sensation that is very similar to physical pain. Pain is in fact, part of fatigue. Imagine what would happen if you didn’t feel pain.

You could place your hand in a fire and never know anything is amiss until your skin burned away and fell off. Pain is a signal that something is taking place with your body that needs your immediate attention. The more severe the pain, the more critical the situation is. That is the purpose of pain. It’s a signal that you simply can’t ignore. Fatigue is the same type of process. It is your body letting you know that something is going on that is out of the ordinary. Something that your body does not particularly care for and it would like you to do something about it. Just as with pure pain, more severe levels of fatigue indicate a more serious condition.

That is essentially why I consider fatigue to be my friend. It’s a signal to let me know how my body is reacting to the race or training run I am participating in. Fatigue gives me advance warning if my body is losing its metabolic balance or “homeostasis”. If my muscles are beginning to breakdown or weaken, I will know ahead of time. It allows me to make any adjustments to my pace, stride or mental techniques that are to my advantage. Without the warning system of fatigue I could end up having a complete physical or mental breakdown before the end of my race or may even have to endure the long term consequences of a serious injury.

As I mentioned a bit earlier, fatigue is a four-headed monster. Each of the causes of running fatigue are separate, but they work together to make sure your body maintains sufficient strength and balance to insure both your safety and the successful completion of your goal.

Muscle Fatigue

The most basic type of fatigue is a weariness and loss of power in your muscles. This type of fatigue affects all runners from sprinter to ultra marathon runners. Muscle fatigue has a number of causes. The most basic is a decrease in the ability of your muscles to contract powerfully. Your muscles are composed, in part, of many structures called myofibrils.

These myofibrils shorten, with the help of a storehouse of calcium, which causes your muscles to contract. During exercise your myofibrils can lose their ability to contract. That is most likely because your muscles begin to accumulate phosphate ions, especially during sprinting events, which depresses both the sensitivity of the calcium and your muscles ability to produce force. Micro trauma and damage to your muscle fibers themselves can also be the cause of muscle fatigue.

Metabolic Fatigue

While muscle fatigue may be the most basic type of fatigue, metabolic fatigue is the most familiar to distance runners. Metabolic fatigue can rear its ugly head during any race distance from 800 meters to 10 kilometers or more. During those mid to long distance races you are running at a pace that produces a lot of metabolic substances including: hydrogen ions, potassium and phosphate. Each of these metabolites makes a contribution to running fatigue. As your running intensity increases, a number of things happen.

First of all, you begin to produce more lactic acid. The lactic acid breaks down into hydrogen ions and lactate. The lactate is shuttled into nearby muscle cells where it is processed to produce additional energy in the form of ATP. The hydrogen ions continue to build up, raising the acidity of your blood. At the same time, potassium also begins to build up.

The contraction of your muscles depends upon an electrical charge that is caused by potassium inside your cells switching places with sodium outside your cells. High running speeds leads to a lot of potassium building up outside your cells, which depresses the ability of your cells to create that electrical charge. So your muscles begin to fatigue.

At one time it was thought that the rise in the acidity of your blood, caused by hydrogen ion accumulation, was one of the primary causes of running fatigue. Recent research is beginning to call that belief into question. Now it is thought that the lactic acid actually helps diminish the effects of the accumulating potassium and helps restore muscle function. The blood acidity may still be an indirect cause of running fatigue by irritating nerve endings and causing a burn or feeling of fatigue in your muscles.

Energy Depletion

Metabolic fatigue plays a large role during high intensity pace, but what about the longer marathon or ultra marathon distance. During very long distance running your running intensity is low enough that your body is able to process the metabolites and keep your body in a state of homeostasis. So – metabolic fatigue isn’t usually a problem at those paces. The primary cause of fatigue during long distance events is energy depletion. You burn a combination of oxygen, fat and carbohydrates to produce energy.

Most runners have enough carbohydrate stores to last somewhere around 2 hours of running at a moderate pace. When you run out of carbohydrates you reach a debilitating level of fatigue that many runners refer to as “the wall”. Fat is also used to produce energy and even the slimmest runners have enough fat stores to fuel many marathons. The problem is that your body prefers carbohydrates to produce energy quickly.

Think of it in terms of your gas fired furnace. The gas is like your fat stores and the pilot light is like your carbohydrates. The furnace will have a hard time firing without the pilot light. You will be able to continue even with low carbohydrate stores but you’ll be forced to slow down or walk.

Central Nervous System Fatigue

The final but by no means the least important cause of fatigue is your central nervous system (CNS). Your CNS controls your body. It tells your muscles what to do. Every muscle contraction starts with a signal sent from your CNS. That signal travels through your vast network of nerves and ends at your muscle motor units. A motor unit is a nerve receptor and the group of muscle fibers that the receptor innervates. A more powerful contraction requires the solicitation of more motor units. For example, picking up a piece of paper only requires a few motor units. Running full speed up a steep hill requires many motor units.

Your CNS is very good at its job. One of its duties is to make sure your body maintains a state of balance. If a build up in metabolites causes an imbalance your CNS steps in to correct it. If you are running out of carbohydrates, your CNS tries to fix that. If your muscles are becoming damaged your CNS comes to the rescue. How does it do that? Really simple – it starts to cut off signals to your motor units, which make you slow down.

In other words, it causes fatigue. When you slow down your metabolites get a chance to regain equilibrium. Your damaged muscles get a break. Your carbohydrate starved body burns more fats and conserves what carbohydrate stores it has left. Your CNS is protecting you against yourself.

Your CNS is in charge of your body, but you are in charge of your CNS. You can override the protective mechanisms of your CNS. That’s how you’re able to increase to nearly sprint pace for the final 200 meters of a marathon despite being almost completely carbohydrate depleted. Extreme excitement or danger produces adrenaline which plays a role in your ability to override your CNS.

As you can see there are several mechanisms involved in running fatigue. They all work together to send you the proper signals and do their bit to protect your body and maintain its homeostasis. So don’t hate fatigue- embrace it and use it. It’s there to help you not hinder you.


Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause?, News Physiol.Sci. Vo 17, Feb 2002

Is fatigue all in your head? A critical review of the central governor model, Br J Sports Med 2006;40:573-586

Effects of lactic acid and catecholamines on contractility in fast twitch muscles exposed to hyperkalemia, Am J. Physiol Cell Physiol, 2005 Jul;289(1)

From catastrophe to complexity: a novel model of integrative central neural regulation of effort and fatigue during exercise in humans: summary and conclusions, Br J. Sports Med 2005;39;120-124

Better Training for Distance Runners, David E. Martin, Peter N. Coe, Human Kinetics