Theories of Fatigue and Training Implications

A few posts ago, we summarized the history of lactate research and explained how thinking about lactate and fatigue has evolved. In short, lactate is no longer thought to be a cause of fatigue. But if lactate doesn’t cause fatigue, what does?

We found a thoughtful consolidation of theories in Steve Magness’s 2010 grad school research project. While his entire paper delves into the science of performance and fatigue for distance running, Steve has consolidated modern fatigue theories into a practical framework that appears to apply well for all endurance athletes. The framework identifies manifestations of fatigue and their potential causes as follows.

Theories of Fatigue

Steve’s framework holds that two main manifestations of fatigue are 1. reduced energy production and 2. reduced muscular force output. These manifestations are thought to be caused by one or more of the following:

1. Build-up: Build-up theory is based on your body’s production and accumulation of byproducts during sustained exertion. Excess production and accumulation of these byproducts may interfere directly with your ability to generate energy and produce muscular force. Essentially, your system starts to choke because the build-up of these byproducts is too great (almost like an engine that stalls when you flood it with too much gas). Among the byproducts thought to build-up are hydrogen ions, ammonia, potassium, phosphate, calcium, ADP, and core temperature. Though not discussed directly in Steve’s paper, examples of evidence for this theory of build-up include:

2. Depletion: This is the exact opposite of build-up theory. Accordingly, you may get tired from sustained exertion because certain substances run low. Here, Steve focuses mainly on fuel sources. “Whenever a vital fuel source is running low, then fatigue is going to occur because the runner will need to slow down and switch fuel sources to make sure that total depletion does not occur.” He indicates that stored ATP, stored glycogen, blood glucose, and branch chain amino acids are among the main culprits that may directly result in fatigue when depleted. Though not discussed directly in Steve’s paper, examples of evidence for this theory of depletion include:

3. Regulation: While the theories above hold that build-up and depletion directly cause fatigue due to an actual limitation or catastrophe, regulation theory treats build-up and depletion as mere warning signals. These signals may cause your brain to slow your exertion well-before you reach a physical limit. In other words, your body may be capable of much more while your brain wants to shut the mother down. In theory, your brain acts this way out of caution to prevent any real physical damage. In support of this theory, Steve mentions the Central Governor theory espoused by Tim Noakes as an example of subconscious regulation (“when the brain is receiving signals that fatigue is building up, it decreases muscle fiber recruitment to protect the body”) and the “You’re Just A Quitter” theory by Samuel Marcora as an example of conscious regulation (“as these products build up or decrease, the body creates the sensation of pain, so that then the runner slows down consciously because of the high pain levels.”). Though not discussed directly in Steve’s paper, examples of evidence for these theories of regulation include:

While there may be other frameworks for understanding fatigue, we like Steve’s because it is comprehensive yet simple. It may also be especially useful if it helps create training and racing strategies that improve performance.  As Steve summarizes so well:

This basic understanding on how and why fatigue develops is critical in understanding the development of the scientific models of endurance performance, and in figuring out what limits performance.  Using the concepts expressed above, to prevent fatigue we must reduce the rate of by product build up or fuel source depletion, or alternatively increase the level of build up or depletion that we can withstand before we consciously or subconsciously start reducing performance.

Thus we can design training with these goals in mind, and Steve proposes how we might do just that.

Training Implications

Steve uses the theories above to propose examples of training designed to achieve particular fatigue-reducing adaptations. They include:

1. Lactate-based Training. The build-up theory of fatigue has close ties to the build-up of lactate. Though lactate is not thought to cause fatigue, its accumulation during sustained exertion seems to correspond with the accumulation of suspected fatigue-causing byproducts. Thus we think that, if lactate build-up can be reduced or managed, corresponding fatigue-causing byproducts can simultaneously be reduced or managed. The adaptations sought by this training include:

  • Decrease your production of lactate or fatigue byproducts
  • Decrease your accumulation of lactate or fatigue byproducts at race pace (by increasing utilization of these byproducts)
  • Increase your ability to tolerate higher levels of lactate and byproduct production/accumulation

2. Muscle-fiber-based Training. Since fatigue manifests itself in part as the reduction in muscular force output, training can be designed to improve force, limit force reductions, or both. This training category focuses on muscle fibers. The adaptations sought by this training include:

  • Increase the maximum amount of muscle fibers you can recruit
  • Increase the percentage of those fibers you can use over course of a performance
  • Increase the stamina of those fibers (or the duration that those fibers can last)

3. Integrated training. Finally he suggests an integrated training approach that is based largely on affecting the regulation model of fatigue by influencing the neural/muscular feedback loop. The adaptations sought by this training include:

  • Delay or reduce central down-regulation in exertion by delaying or reducing byproduct build-up or substance depletion
  • Delay or over-ride the signal from your brain that down-regulates exertion by increasing your tolerance for byproduct build-up or depletion

The first and third approach are designed to influence causes of fatigue (build-up and regulation) while the second approach is designed to influence a manifestation of fatigue (reduced muscular force). Though Steve proposes these and other training ideas in his paper, he points out that isolation of any of these variables is, at best, fodder for the lab:

While studies on training are interesting, they should be taken with a grain of salt.  In research we have to isolate one variable to see its effect.  In the real world, variables are never done in isolation.  This creates a situation where training in the lab is completely different than training in the real world.  The training effect is the result of a combination of all of the training stimuli applied to the athlete.

Implications For High-Intensity and Strength-Based Endurance Training

In light of the theories above, it becomes easier to understand how high-intensity and strength-based endurance protocols may successfully influence endurance performance. For example:

  • High intensity may achieve the adaptations sought by Magness’s lactate-based training by decreasing the accumulation of fatigue-related byproducts and increasing your ability to tolerate higher levels of byproduct production and accumulation.
  • Strength work may achieve the adaptations sought by Magness’s muscle-fiber-based training by increasing the recruitment, utilization, and stamina of muscle fibers.
  • Combined high-intensity and strength-based training may achieve the adaptations sought by Magness’s integrated training by reducing your CNS’s cautionary response to premature warning signals and giving you more confidence to consciously override your otherwise default reactions to those signals.

These are just brief examples. In future posts, we’ll return to this framework of fatigue as we explore in further detail how high-intensity and strength-based protocols can influence endurance.


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